Health: Low-Fat Diet Does Not Cut Health Risks

Scientists Learn How Food Affects The Brain: Omega 3 Especially Important

July 11, 2008
ScienceDaily

In addition to helping protect us from heart disease and cancer, a balanced diet and regular exercise can also protect the brain and ward off mental disorders.

“Food is like a pharmaceutical compound that affects the brain,” said Fernando Gómez-Pinilla, a UCLA professor of neurosurgery and physiological science who has spent years studying the effects of food, exercise and sleep on the brain. “Diet, exercise and sleep have the potential to alter our brain health and mental function. This raises the exciting possibility that changes in diet are a viable strategy for enhancing cognitive abilities, protecting the brain from damage and counteracting the effects of aging.”

Gómez-Pinilla analyzed more than 160 studies about food’s affect on the brain; the results of his analysis appear in the journal Nature Reviews Neuroscience.

Omega-3 fatty acids — found in salmon, walnuts and kiwi fruit — provide many benefits, including improving learning and memory and helping to fight against such mental disorders as depression and mood disorders, schizophrenia, and dementia, said Gómez-Pinilla, a member of UCLA’s Brain Research Institute and Brain Injury Research Center.

Synapses in the brain connect neurons and provide critical functions; much learning and memory occurs at the synapses, Gómez-Pinilla said.

“Omega-3 fatty acids support synaptic plasticity and seem to positively affect the expression of several molecules related to learning and memory that are found on synapses,” Gómez-Pinilla said. “Omega-3 fatty acids are essential for normal brain function.

“Dietary deficiency of omega-3 fatty acids in humans has been associated with increased risk of several mental disorders, including attention-deficit disorder, dyslexia, dementia, depression, bipolar disorder and schizophrenia,” he said. “A deficiency of omega-3 fatty acids in rodents results in impaired learning and memory.”

Children who had increased amounts of omega-3 fatty acids performed better in school, in reading and in spelling and had fewer behavioral problems, he said.

Preliminary results from a study in England show that school performance improved among a group of students receiving omega-3 fatty acids. In an Australian study, 396 children between the ages 6 and 12 who were given a drink with omega-3 fatty acids and other nutrients (iron, zinc, folic acid and vitamins A, B6, B12 and C) showed higher scores on tests measuring verbal intelligence and learning and memory after six months and one year than a control group of students who did not receive the nutritional drink. This study was also conducted with 394 children in Indonesia. The results showed higher test scores for boys and girls in Australia, but only for girls in Indonesia.

Getting omega-3 fatty acids from food rather than from capsule supplements can be more beneficial, providing additional nutrients, Gómez-Pinilla said.

Scientists are learning which omega-3 fatty acids seem to be especially important. One is docosahexaenoic acid, or DHA, which is abundant in salmon. DHA, which reduces oxidative stress and enhances synaptic plasticity and learning and memory, is the most abundant omega-3 fatty acid in cell membranes in the brain.

“The brain and the body are deficient in the machinery to make DHA; it has to come through our diet,” said Gómez-Pinilla, who was born and raised in salmon-rich Chile and eats salmon three times a week, along with a balanced diet. “Omega-3 fatty acids are essential.”

A healthy diet and exercise can also reduce the effect of brain injury and lead to a better recovery, he said.

Recent research also supports the hypothesis that health can be passed down through generations, and a number of innovative studies point to the possibility that the effects of diet on mental health can be transmitted across generations, Gómez-Pinilla said.

A long-term study that included more than 100 years of birth, death, health and genealogical records for 300 Swedish families in an isolated village showed that an individual’s risk for diabetes and early death increased if his or her paternal grandparents grew up in times of food abundance rather than food shortage.

“Evidence indicates that what you eat can affect your grandchildren’s brain molecules and synapses,” Gómez-Pinilla said. “We are trying to find the molecular basis to explain this.”

Controlled meal-skipping or intermittent caloric restriction might provide health benefits, he said.

Excess calories can reduce the flexibility of synapses and increase the vulnerability of cells to damage by causing the formation of free radicals. Moderate caloric restriction could protect the brain by reducing oxidative damage to cellular proteins, lipids and nucleic acids, Gómez-Pinilla said.

The brain is highly susceptible to oxidative damage. Blueberries have been shown to have a strong antioxidant capacity, he noted.

In contrast to the healthy effects of diets that are rich in omega-3 fatty acids, diets high in trans fats and saturated fats adversely affect cognition, studies indicate.

Junk food and fast food negatively affect the brain’s synapses, said Gómez-Pinilla, who eats fast food less often since conducting this research. Brain synapses and several molecules related to learning and memory are adversely affected by unhealthy diets, he said.

Emerging research indicates that the effects of diet on the brain, combined with the effects of exercise and a good night’s sleep, can strengthen synapses and provide other cognitive benefits, he added.

In Okinawa, an island in Japan where people frequently eat fish and exercise, the lifespan is one of the world’s longest, and the population has a very low rate of mental disorders, Gómez-Pinilla noted.

Folic acid is found in various foods, including spinach, orange juice and yeast. Adequate levels of folic acid are essential for brain function, and folate deficiency can lead to neurological disorders such as depression and cognitive impairment. Folate supplementation, either by itself or in conjunction with other B vitamins, has been shown to be effective in preventing cognitive decline and dementia during aging and enhancing the effects of antidepressants. The results of a recent randomized clinical trial indicate that a three-year folic acid supplementation can help reduce the age-related decline in cognitive function.

In patients with major depression and schizophrenia, levels of a signaling molecule known as brain-derived neurotrophic factor, or BDNF, are reduced. Antidepressants elevate BDNF levels, and most treatments for depression and schizophrenia stimulate BDNF. Here, too, omega-3 fatty acids are beneficial, as is the curry spice curcumin, which has been shown to reduce memory deficits in animal models of Alzheimer’s disease and brain trauma. BDNF is most abundant in the hippocampus and the hypothalamus — brain areas associated with cognitive and metabolic regulation.

The high consumption of curcumin in India may contribute to the low prevalence of Alzheimer’s disease on the subcontinent.

In humans, a mutation in a BDNF receptor has been linked to obesity and impairments in learning and memory.

“BDNF is reduced in the hippocampus, in various cortical areas and in the serum of patients with schizophrenia,” Gómez-Pinilla said. “BDNF levels are reduced in the plasma of patients with major depression.”

Smaller food portions with the appropriate nutrients seem to be beneficial for the brain’s molecules, such as BDNF, he said.

Gómez-Pinilla showed in 1995 that exercise can have an effect on the brain by elevating levels of BDNF.

He noted that while some people have extremely good genes, most of us are not so lucky and need a balanced diet, regular exercise and a good night’s sleep.

The research was funded by the National Institutes of Health’s National Institute of Neurological Disorders and Stroke.

24 Feb 2006
By John Luik

Of all the beautiful hypotheses in the temple of preventive medicine, the claim that low-fat diets could prevent cancer and heart disease is perhaps the most central. But the last few weeks have been more than a little unkind to the beautiful hypotheses of the lifestyle medicine crowd. In fact the ugly facts have been piling up fairly quickly.

In early January, a study published in the Journal of the American Medical Association reported that low fat diets produced only temporary, moderate weight loss.

Then along came another JAMA study early this year that showed that the consumption of omega-3 fatty acids did not significantly reduce the risk of cancer.

And finally, again in JAMA, there were the three attention grabbing studies which reported that low-fat diets failed to reduce the risks of heart disease, colorectal cancer and breast cancer.

There have been numerous attempts to spin these studies into something that they weren’t — clear, if weak, endorsements of low fat diets as protective for cancer and cardiovascular disease. But that evidence simply isn’t there.

For example, in the breast cancer study, incidence rates between the intervention and comparison group did not show a statistically significant difference, meaning that any differences between the groups were indistinguishable from chance. Similarly, in the cardiovascular disease study, the “diet had no significant effects on incidence of CHD (coronary heart disease), … stroke … or CVD (cardiovascular disease).”

So whatever the hype about these studies showing “trends” toward the benefits of low-fat diets, the scientific results do not support those claims.

What’s difficult to know is just which bit of the story about the new studies is most extraordinary. Is it the fact that the advice provided to Americans to eat diets low in fat in order to reduce the risk of heart disease and cancer is not supported by current science? Is it the fact that this advice hasn’t had strong scientific support for the last 30 years? Is it the fact that this doesn’t seem to disturb the public health community? Or is the fact that that community isn’t going to change the “lifestyle” advice they provide?

The scientific evidence about the link between low-fat diets and heart disease has been tenuous for the last three decades.

In the Multiple Risk Factor Intervention Trial (MRFIT) reported on in JAMA back in 1982, 12,866 U.S. men who were at increased risk from heart disease were divided into two groups — usual care and special intervention. Members of the special intervention group were given drugs to reduce their hypertension, told to stop smoking and encouraged to reduce their fat and cholesterol intake. Despite these factors, there was not a statistically significant difference in mortality rates from coronary heart disease or any other cause between the two groups, and there was a substantial increase in lung cancer in the intervention group, hardly an optimal outcome.

Or consider the Masai, a nomadic group of Kenyans who consider vegetables and fiber fit only for cows and whose diet consists of milk, meat and blood. Despite this high fat diet the Masai have some of the lowest levels of cholesterol ever measured, with rates about 50% less than the average American and extremely low levels of heart disease. (see G. Mann et al, ” Cardiovascular disease in the Masai,” Journal of Atherosclerosis Research, 4: 289-312, 1964)

This pattern wasn’t confined to the lesser developed world. As Petre Skrabanek reported in his 1995 National Review article ” Fat Heads,” the Seven Countries Study also fails to support the notion that low fat=lower cardiovascular disease. Crete, with 40% of its diet made up of fat, had one of the lowest incidences of heart disease.

Or consider the results of the famous study from Finland — the North Karelia Project (Puska et al, “Changes in coronary risk factors during comprehensive five-year community programme to control CVD,” BMJ 2: 1173-1178, 1979) Five thousand subjects were recruited for a study that ran for five years and focused on reducing CVD risk factors such as dietary fat. Yet in both women and men there were no significant differences in cardiovascular mortalities.

A little closer to home, there are the results of two long and large studies.

The Framingham Heart Study, which started in 1958 by following 5,200 men and women, has resulted in more than 1,000 published analyses. Here, the data failed to provide any confirmation of a benefit from a low-fat diet in terms of reduced coronary mortality. Indeed, according to a 1987 study in JAMA those with higher levels of cholesterol over the age of 50 — supposedly the product of a high fat diet — had lower rates of coronary and total mortality than those with lower cholesterol levels, effectively turning the low-fat hypothesis on its head.

Meanwhile, in the massive Nurses’ Health Study involving almost 99,000 nurses aged 30-55 and run by researchers from Harvard since 1976, an analysis in 1997 (Hu et al “Fat intake and the risk of CHD in women,” 33: 1491, New England Journal of Medicine, 1997) of CHD risks and dietary fat found some association between 939 cases of developing coronary heart disease among 80,200 of these women reported over 14 years and animal and saturated fat intake, but nothing significant. If there was any effect, it was “consistent with the small-to-negligible effect predicted by metabolic studies.” Another way of putting it: No big deal. On total fat and cholesterol intake there was no significant relation to intake and CHD.

Indeed, a 1998 review in the Journal of Clinical Epidemiology of the low fat-cholesterol-CHD thesis found that out of a total of 27 studies involving more than 150,000 subjects, in only three cohorts did patients with CHD consume more fat than the controls. In one cohort, those with cardiovascular disease actually consumed less fat than the controls. In all of the remaining cohorts, there was no statistically significant difference in fat consumption between those who had CVD and those who did not.

And a very large study of 28,000 Swedes — the Malmö Diet and Cancer Study — published just last year in the Journal of Internal Medicine noted: “Over the last decades evidence from large-scale epidemiological studies has been emerging, partly defying the previously believed hypotheses, with voices rejecting the fat-disease hypotheses becoming more prominent.”

The aim of the Malmö study was to test the basis for the recommendation by governments and health authorities that people limit the percentage of the calories in their diets due to fat to less than 30%. What the study found was that with one exception “individuals receiving more than 30% of their total daily energy from fat (saturated, unsaturated, monounsaturated, polyunsaturated ) did not have increased mortality.” Indeed, “receiving more than 10% of total energy intake from saturated fat did not have a significant effect on all-cause, cardiovascular or cancer mortality for men or women.” The one exception was women who consumed the highest amount of total fat. They had a relative increased risk (RR) of 1.46 for cancer mortality, though even this result was barely statistically significant as it fell within the a 95% confidence interval of 1.04-2.04 indicating a small risk for subjects of the study dying from cancer in the first place.

These results have all been published in major journals and were certainly known by health professionals. As J. Bruce German and Cora J. Dillard commented in The American Journal of Clinical Nutrition in 2004, “studies on the long-term health benefits of consuming a low-fat diet … are lacking, and low fat diets have been shown to exert a potentially deleterious effect on lipoprotein profiles in some persons.” And as Lawrence Kushi and Edward Giovannucci observed in the American Journal of Medicine back in 2002: “Based on current epidemiologic knowledge, public health recommendations to decrease total fat intake for the prevention of cancer appear largely unwarranted.”

As even Barbara Howard, one of the principal investigators for the low-fat studies, admitted to Gina Kolata of The New York Times for her story this month ” Low-Fat Diet Does Not Cut Health Risks, Study Finds,” ”We are not going to reverse any of the chronic diseases in this country by changing the composition of the diet.” Except for smoking, all of the advice about healthy lifestyle is based largely on indirect evidence, she noted.

Indirect evidence in science, though, is often no evidence at all. With the JAMA studies this month we have, in what Michael Thun of the American Cancer Society called the “Rolls-Royce of studies,” studies that should have found evidence if there was evidence to be found.

Yet, these facts never appear to disturb the public health community. And the strong advice about low-fat diets based on very flimsy evidence continues, with reporters paying more attention to the alleged flaws in the studies as opined by the high priests of the low-fat faith than to what the studies actually say.

In a JAMA editorial accompanying the study, authors Cheryl Anderson and Lawrence Appeal from Johns Hopkins wrote: “Despite null findings … dietary changes can have powerful, beneficial effects on CVD risk factors and outcomes. To reduce the risk of CVD, individuals should maintain a desirable body weight, be physically active, avoid tobacco exposure, and eat a diet consistent with national guidelines.”

Like what? After all, these studies found that significant increases in fruit, vegetables and grains, along with reductions in fat, did not change CVD and cancer outcomes. Eating a diet consistent with national guidelines is eating a low-fat diet — the same diet that eight years of study and almost half a billion bucks found did not work.

The attitude of we-know-best-no-matter-what-the-studies-say discredits both the scientific process and the requirement that health advice be evidence-based. It risks turning off a population already weary and wary of the lifestyle solutions offered up by the medical community. And it ignores a fundamental responsibility that attaches to providing risk advice: If the science changes, amend your advice. To do otherwise changes the enterprise from advice-giving to propaganda-making. However beautiful the hypothesis, the “ugly” facts are always preferable.  

Warning: Food Scares May Be Hazardous To Your Health

February 24, 2006        

“Last week’s reports that low-fat diets may not reduce the risk of heart disease and cancer have left Americans more confused than ever about what to eat,” begins a Tuesday New York Times op-ed by writer Harriet Brown. “I’d like to make a radical suggestion: instead of wringing our hands over fat grams and calories, let’s resolve to enjoy whatever food we eat.”

Brown does not base her suggestion on love for gastronomy alone but also on science. Her article goes on to describe a nutrition study conducted in the 1970’s in which researchers fed spicy Thai meals to a group of Thai women and a group of Swedish women. The Thai women absorbed about 50 percent more iron from the meal than did their Swedish counterparts. Furthermore, when that same meal was run through a food processor before serving, the Thai women absorbed 70 percent less iron than before — even though the meal’s nutritional content did not change. So why did the Thai women absorb so much less iron from the minced food? Brown continues:

The researchers concluded that food that’s unfamiliar (Thai food to Swedish women) or unappetizing (mush rather than solid food) winds up being less nutritious than food that looks, smells and tastes good to you. The explanation can be found in the digestive process itself, in the relationship between the “second brain” – the gut – and the brain in your head.

Imagine sitting in your favorite Japanese restaurant before a plate of sushi, chopsticks poised. You take in its fragrance and the beautiful cut of the fish, the shapely rice and nori rolls. Those delectable smells and sights tell your brain that the meal will be enjoyable, and the brain responds by pushing your salivary glands into high gear and ordering your stomach to secrete more gastric juices.

Result: you get more nutritional bang for your buck than you would, say, faced with a platter of lutefisk. In that case, your brain might send fewer messages to your mouth and stomach, causing the food to be less thoroughly digested and metabolized.

This study (available here from The American Journal of Nutrition) is hardly the final word in nutrition science. We don’t have the final word, and neither do the food police at places like the Center for Science in the Public Interest.

What’s important to keep in mind is that scaremongering comes at a price. Sometimes it’s an obvious one — as when inflated mercury fears frighten pregnant women away from fish — but the endless series of scares can easily taint a conscientious consumer’s every meal with anxiety. In 1989 Julia Child warned: “What’s dangerous and discouraging about this era is that people really are afraid of their food … Sitting down to dinner is a trap, not something to enjoy.” It’s not the only thing we have to fear, but fear itself needs to get a lot more attention from joyless public “health” advocates who seem to have forgotten about actually enjoying what you eat.           

February 14, 2006

As we discussed last week, the National Institutes of Health has released the results of its massive $415 million study on diet and women’s health, casting serious doubts on low-fat diet dogmas. Low-fat diets, according to this study, don’t prevent heart disease, colorectal cancer, or breast cancer.

Already, the usual gang of ideologues has pooh-poohed the study, saying that the scientific community has moved on from the low-fat craze. While we agree that it’s important not to put too much stock in any one study, it’s hard to give much credibility to the food cops here, since their response amounts to: “Of course the old food fad is not good science — it’s the new food fad that is good science.”

On Monday The New York Times surveyed the scientific trends surrounding childhood obesity and discovered that dietary interventions — things like banning whole milk or soft drinks — in our schools don’t tend to stand up in the face of the science that those same schools are supposed to be teaching. As the Times points out:

When many [nutrition policies] were subjected to the cold light of rigorous scientific study, they have fallen short. If nothing else, when it comes to fighting obesity, science teaches humility.

Childhood obesity is certainly worth fighting, but not all weapons are created equal. Rockefeller University obesity researcher Dr. Jeffrey Friedman told the Times that measures to fight childhood obesity can be okay, but “the burden should be on those who want to impose them to show they work. To my knowledge, no one has ever done this.” Columbia University obesity researcher Dr. Rudolph Leibel agreed, saying that “those interventions should not be imposed without first showing they have an effect.”

So why do we keep letting feel-good policies like soda bans into our schools, even when the best evidence says they make no difference? Berkeley statistician Dr. David Freedman pointed out to the Times that the public, not to mention scientists, “have this wonderful capacity for ignoring negative evidence.”

Dr. Freedman illustrated his point with a revealing anecdote:

In the 1830’s, Dr. [Pierre-Charles-Alexandre] Louis studied the effect of bloodletting, or bleeding — the standard treatment of the time — on pneumonia.
“The data showed that bleeding didn’t work,” Dr. Freedman said. But, he said, “Dr. Louis rejected this as terrifying and absurd.”

So, he made a recommendation: bleed earlier and bleed harder.

By Jeff Jacoby, Globe Columnist
February 12, 2006

I DON’T usually follow nutrition stories, but it was hard to miss last week’s shocker about low-fat diets. The Globe put it on Page 1, high above the fold: ”Study finds no major benefits of low-fat diet.” The study, a project of the National Institutes of Health, had taken eight years, cost $415 million, and involved nearly 49,000 older women, 40 percent of whom were assigned to a diet that kept their intake of calories from fat significantly below that of the other 60 percent. Researchers had expected to confirm what earlier studies and conventional medical wisdom had long suggested — that consuming less fat is good for your health.

What they learned instead was that the women who dutifully cut back on fried foods, ice cream, and pizza ended up no better off than the women who ate whatever they wanted. Both groups developed breast cancer, colon cancer, heart attacks, and strokes at the same rates. Millions of Americans have tried for years to reduce the fat in their diet — eating bread without butter, salads without dressing, chicken without skin — and now the largest study of the subject ever conducted says it has all been for naught. You could have had those fries after all.

And so once again we are reminded, as The New York Times sighed in an editorial on Thursday, that ”the more we learn about nutrition, the less we seem to know.” Does oat bran reduce cholesterol? Can dietary fiber prevent colon cancer? Are high doses of Vitamin E good for your heart? Once, the experts said yes. Then the experts said no. It sometimes seems that for every study that makes a nutritional claim, another study inevitably makes an equal and opposite claim.

Researchers can’t even agree on whether eating less fat is the way to lose weight. Some insist that obesity is caused by ingesting too much fat. Others claim that it’s carbohydrates, not fat, that make people gain weight. Which theory did the massive new study confirm? Neither. Apparently there is still no clear-cut answer — not even for $415 million.

But clear-cut answers are just what so many Americans want, and what so many of them think science ought to be able to provide. There is a seemingly inexhaustible willingness to believe that the voice of science is the voice of truth — impartial, incorruptible, and unambiguous. It isn’t, of course. Scientists are no less vulnerable to error or bias or ego than the rest of the human race. Scientists too can blunder or act from ulterior motives or convince themselves of things that aren’t so. And yet on the whole they enjoy a level of deference and public trust that people in most other fields can only envy.

Which is probably not a good thing. Scientific pronouncements should be subjected to the same level of healthy skepticism as the promises of politicians or the claims of advertisers — or the views of newspaper columnists. With the best of intentions (and otherwise), scientists sometimes peddle claptrap. Just because a statement begins with ”A new study shows . . .” or ”Researchers have found . . .” doesn’t mean that what follows is true. ”We in the scientific community often give strong advice based on flimsy evidence,” Berkeley statistician David Freedman said last week in a comment on the low-fat diet study. ”That’s why we have to do experiments.” And why the rest of us have to remember that contradiction, confusion, and changing opinions have always been a part of the scientific process.

One day after last week’s low-fat story, the New England Journal of Medicine was out with a study concluding that saw palmetto extract, an herbal product, has no effect on the symptoms caused by an enlarged prostate. Earlier studies had found just the opposite, and more than 2 million American men take saw palmetto for their prostate condition. So does it work or doesn’t it? Whichever answer you choose, there’s a study to back it up.

In Newsweek last month, Dr. Harvey Simon of the Harvard Medical School recanted a view he had preached for years: that the only way to benefit from exercise was through intense aerobic activity, complete with pounding heart and rivers of sweat. Now, citing the latest research, he says he was dead wrong, and that gentle, no-sweat exercise — even walking or gardening — is also highly effective.

From cardiac health to climate change, it’s worth keeping in mind that what the experts say today they may not be saying tomorrow. As that noted scientist Emily Litella used to put it in the old ”Saturday Night Live” skits: Never mind.

Jeff Jacoby’s e-mail address is jacoby@globe.com.

Low-Fat Diet Does Not Cut Health Risks, Study Finds

February 8, 2006
By GINA KOLATA

The largest study ever to ask whether a low-fat diet reduces the risk of getting cancer or heart disease has found that the diet has no effect.

The $415 million federal study involved nearly 49,000 women ages 50 to 79 who were followed for eight years. In the end, those assigned to a low-fat diet had the same rates of breast cancer, colon cancer, heart attacks and strokes as those who ate whatever they pleased, researchers are reporting today.

“These studies are revolutionary,” said Dr. Jules Hirsch, physician in chief emeritus at Rockefeller University in New York City, who has spent a lifetime studying the effects of diets on weight and health. “They should put a stop to this era of thinking that we have all the information we need to change the whole national diet and make everybody healthy.”

The study, published in today’s issue of The Journal of the American Medical Association, was not just an ordinary study, said Dr. Michael Thun, who directs epidemiological research for the American Cancer Society. It was so large and so expensive, Dr. Thun said, that it was “the Rolls-Royce of studies.” As such, he added, it is likely to be the final word.

“We usually have only one shot at a very large-scale trial on a particular issue,” he said.

The results, the study investigators agreed, do not justify recommending low-fat diets to the public to reduce their heart disease and cancer risk. Given the lack of benefit found in the study, many medical researchers said that the best dietary advice, for now, was to follow federal guidelines for healthy eating, with less saturated and trans fats, more grains, and more fruits and vegetables.

Not everyone was convinced. Some, like Dr. Dean Ornish, a longtime promoter of low-fat diets and president of the Preventive Medicine Research Institute in Sausalito, Calif., said that the women did not reduce their fat to low enough levels or eat enough fruits and vegetables, and that the study, even at eight years, did not give the diets enough time.

Others said that diet could still make a difference, at least with heart disease, if people were to eat the so-called Mediterranean diet, low in saturated fats like butter and high in oils like olive oil. The women in the study reduced all kinds of fat.

The diets studied “had an antique patina,” said Dr. Peter Libby, a cardiologist and professor at Harvard Medical School. These days, Dr. Libby said, most people have moved on from the idea of controlling total fat to the idea that people should eat different kinds of fat.

But the Mediterranean diet has not been subjected to a study of this scope, researchers said.

And Barbara V. Howard, an epidemiologist at MedStar Research Institute, a nonprofit hospital group, and a principle investigator in the study, said people should realize that diet alone was not enough to stay healthy.

“We are not going to reverse any of the chronic diseases in this country by changing the composition of the diet,” Dr. Howard said. “People are always thinking it’s what they ate. They are not looking at how much they ate or that they smoke or that they are sedentary.”

Except for not smoking, the advice for a healthy lifestyle is based largely on indirect evidence, Dr. Howard said, but most medical researchers agree that it makes sense to eat well, control weight and get regular exercise.

That is also what the cancer society recommends. Dr. Thun, who described the study’s results as “completely null over the eight-year follow-up for both cancers and heart disease,” said his group had no plans to suggest that low-fat diets were going to protect against cancer.

Others cautioned against being too certain that a particular diet would markedly improve health, and said that whether someone developed a chronic disease might not be entirely under their control – genetics also plays a role.

David A. Freedman, a statistician at the University of California, Berkeley, who is not connected with the study but has written books on the design and analysis of clinical trials, said the results should be taken seriously.

“The studies were well designed,” Dr. Freedman said, “and the investigators tried to confirm popular hypotheses about the protective effect of diet against three major diseases in women.”

“But,” he added, “the diet studied here turned out not to be protective after all.”

The study was part of the Women’s Health Initiative of the National Institutes of Health, the same program that showed that hormone therapy after menopause might have more risks than benefits.

In this case, the study addressed a tricky problem. For decades, many scientists have said, and many members of the public have believed, that what people eat – the composition of the diet – determines how likely they are to get a chronic disease. But that has been hard to prove. Studies of dietary fiber and colon cancer failed to find that fiber was protective, and studies of vitamins thought to protect against cancer failed to show an effect.

Many cancer researchers have questioned large parts of the diet-cancer hypothesis, but it has kept a hold on the public imagination. “Nothing fascinates the American public so much as the notion that what you eat rather than how much you eat affects your health,” said Dr. Libby, the Harvard professor.

The study found that women who were randomly assigned to follow a low-fat diet ate significantly less fat over the next eight years. But they had just as much breast and colon cancer and just as much heart disease. The women were not trying to lose weight, and their weights remained fairly steady. But their experiences with the diets allowed researchers to question some popular notions about diet and obesity.

There is a common belief that Americans get fat because they eat too many carbohydrates. The idea is that a high-carbohydrate, low-fat diet leads to weight gain, higher insulin and blood glucose levels, and more diabetes, even if the calories are the same as in a higher-fat diet. That did not happen here.

Others have said the opposite: that low-fat diets enable people to lose weight naturally. But that belief was not supported by this study.

As for heart disease risk factors, the only one affected was LDL cholesterol, which increases heart disease risk. The levels were slightly higher in women eating the higher-fat diet, but not high enough to make a noticeable difference in their risk of heart disease.

Although all the study participants were women, the colon cancer and heart disease results should also apply to men, said Dr. Jacques Rossouw, the project officer for the Women’s Health Initiative.

Dr. Rossouw said the observational studies that led to the hypothesis about colon cancer and dietary fat included men and women. With heart disease, he said, researchers have found that women and men respond in the same way to dietary fat.

The most recent study follows a smaller one, reported last year, on low-fat diets for women who had breast cancer. That study hinted that eating less fat might help prevent a recurrence. But the current study, asking if a low-fat diet could protect women from breast cancer in the first place, had findings that fell short of statistical significance, meaning they could have occurred by chance.

Dr. Rossouw said he was still intrigued by the breast cancer data, even though it was not statistically significant. The women on low-fat diets had a 9 percent lower rate of breast cancer; the incidence was 42 per thousand per year in women in the low-fat diet group, compared with 45 per thousand per year in women consuming their regular diet.

That could mean that fat in the diet may have a small effect, Dr. Rossouw said, perhaps in some subgroups of women or over a longer period of time. He added that the study investigators would continue to follow the women to see if the effect became more pronounced.

While cancer researchers said they were disappointed by the results, heart disease researchers said they were not surprised that simply reducing total fat had no effect, because they had moved on from that hypothesis.

Of course, Dr. Libby acknowledged, the latest advice, to follow a Mediterranean diet and get regular exercise, has never been tested in a large randomized clinical trial. “If they did a study like that and it was negative,” he said, “then I’d have to give up my cherished hypotheses for data.”

The low-fat diet was not easy to follow, said Dr. Rowan T. Chlebowski, a medical oncologist at Harbor-U.C.L.A. Medical Center and one of the study’s principal investigators. Women were told to aim for a diet that had just 20 percent of its calories as fat, and most fell short.

The diet they were told to follow “is different than the way most people eat,” Dr. Chlebowski said. It meant, for example, no butter on bread, no cream cheese on bagels, no oil in salad dressings.

“If a physician told a patient to eat less fat, that will do nothing,” he said. “If you send someone to a dietitian one time, that will do next to nothing.” The women in the study had 18 sessions in small groups with a trained nutritionist in the first year and four sessions a year after that.

In the first year, the women on the low-fat diets reduced the percentage of fat in their diet to 24 percent of daily calories, and by the end of the study their diets had 29 percent of their calories as fat. In the first year, the women in the control group were eating 35 percent of their calories as fat, and by the end of the study their dietary fat content was 37 percent. The two groups consumed about the same number of calories.

Some medical specialists emphasized that the study did not mean people should abandon low-fat diets.

“What we are saying is that a modest reduction of fat and a substitution with fruits and vegetables did not do anything for heart disease and stroke or breast cancer or colorectal cancer,” said Dr. Nanette K. Wenger, a cardiologist and professor of medicine at Emory University School of Medicine in Atlanta. “It doesn’t say that this diet is not beneficial.”

But Dr. Freedman, the Berkeley statistician, said the overall lesson was clear.

“We, in the scientific community, often give strong advice based on flimsy evidence,” he said. “That’s why we have to do experiments.”