PLENTY OF PROOF FOR THE BENEFITS OF UNSATURATED FATS
The popular view of science is that if you assemble the facts, they will give you clear, definite answers. The reality of science usually isn’t anything like that, especially when it comes to human nutrition and its connection with disease. We have an ocean of existing facts and a steady deluge of new data, but only a few solid answers. That’s because studies are always open to criticism and their results to interpretation. It is standard operating procedure to conclude that “more research is needed,” a phrase I have often used in reporting the results of a study or writing an editorial or review on specific aspects of diet and disease.
I don’t feel that way at all about what people should be doing with dietary fat. Cutting back on all types of fat and eating more carbs does nothing to protect against heart disease and will ultimately harm some people. Instead, replacing saturated fats with unsaturated fats is a safe, proven, and delicious way to cut the rates of heart disease.
CONFUSING NEWS ABOUT SATURATED FAT
An article published in 2014 set off a maelstrom of misinformation about saturated fat. This Annals of Internal Medicine meta-analysis of previously published studies suggested that the prevailing recommendation to eat less saturated fat—abundant in red meat, cheese, and buttery pastries—doesn’t lower the risk for heart disease.26
The media had a field day with what it called another gigantic flip-flop in dietary advice. Mark Bittman, writing in the New York Times, wrote: “Butter is back. Julia Child, goddess of fat, is beaming somewhere.” Other headlines were equally effusive. Consumers sighed, blindsided again by confusing and contradictory results from nutrition research.
The Great Butter Battle
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Beginning in the 1970s, many people accepted the demise of butter and gave it up, ruing the loss of its savory flavor but agreeing that the amount of saturated fat it contained might be bad for the heart. They dutifully switched to margarine, as researchers and nutritionists suggested. When later reports highlighted the hazards of margarine, though, many people felt duped.
There never was any good evidence that switching from butter to margarine cut the chances of having a heart attack or developing heart disease. Making the switch was a well-intentioned guess, given that margarine had less saturated fat than butter. But recommendations to switch from butter to margarine overlooked the large amounts of trans fats in many margarines.
Today, the butter-versus-margarine argument doesn’t make sense. From the standpoint of heart disease, butter is on the list of foods to use sparingly, mostly because it contains so much of the kind of saturated fat that raises levels of harmful LDL. Many margarines now for sale are low in saturated fat, high in unsaturated fat, and free of trans fats. As long as you don’t use too much, you should be fine. (They are still rich in calories.) But before you reach for butter or margarine, think about whether you could use olive oil or another liquid vegetable oil instead.
If you can’t do without the taste of butter when sautéing vegetables, scallops, or eggs, melt a small pat of butter and add olive oil.
The media attention was unfortunate, because the analysis was flawed in many ways. The authors used incorrect numbers from some of the original studies, including the Nurses’ Health Study, and omitted important data. Most seriously, the authors did not actually compare the effects of saturated fat with those of polyunsaturated fat; this would have required the original data from the studies that were summarized. These problems called into question a key conclusion of the Annals of Internal Medicine study: that replacing saturated fat with polyunsaturated fat does not reduce the risk for heart disease. (In response to the dozens of letters to the journal criticizing this paper, the authors posted several revised versions online. Although some errors were corrected, the most serious limitations could not be addressed.)
Good Sources of Essential Fats
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Many delicious foods deliver essential unsaturated fats that are good for the heart and the rest of the body. Try to eat at least one source of these every day.
Foods Rich in Omega-3 Fats
• cold-water fish such as salmon, sardines, mackerel, and trout
• canola and soybean oil (as long as they aren’t partially hydrogenated)
• flaxseeds and flaxseed oil
• walnuts
• dark green leafy vegetables such as kale, spinach, mustard greens, and collards
• omega-3 enhanced eggs
Foods Rich in Omega-6 Fats
• soybean, safflower, sunflower, corn, and other vegetable oils
• sunflower seeds, walnuts, and pine nuts
A 2016 meta-analysis of butter and heart disease caused a similar ruckus.27 It concluded that eating a tablespoon or so of butter a day wasn’t linked to dying from any cause or having a heart attack or stroke. Although the authors clearly pointed out the benefit of using unsaturated vegetable oils instead of butter, this got lost in the headlines.
Following the publication of those two studies, my team directly compared the effect on risk of heart disease of butter fat compared to other types of foods. When compared to eating refined grains, eating butter fat was associated with a similar risk of heart disease. When compared to eating whole grains or unsaturated vegetable oils, however, butter was associated with a higher risk.28
Here’s the key issue: what you choose to eat instead of foods rich in saturated fat makes a huge difference. Eating a bowl of white pasta with a low-fat sauce instead of a juicy, high-fat steak doesn’t do much for your health because both choices have harmful effects on risk factors for cardiovascular disease. But swapping that steak for salmon on a bed of greens with an olive oil and vinegar dressing is a very positive switch, because those foods improve risk factors for cardiovascular disease.
OMEGA ADVICE
Given the wide-ranging importance and benefits of omega-3 fatty acids, try to eat at least one good source of them a day. Daily doses of omega-3 fatty acids are especially important for women who are pregnant or hoping to become pregnant. From conception onward, a developing child needs a steady supply of omega-3 fatty acids to form the brain and other parts of the nervous system.
Unfortunately, omega-3 fatty acids aren’t as plentiful in the average diet as they once were. Food companies purposely destroy them in vegetable oils to make foods last longer on the shelf without turning rancid. Meat from cows and chickens now contains fewer omega-3 fatty acids. Why? Most animals used for food once foraged on wild plants and seeds, which are rich in omega-3 fatty acids. Today they are fed grains that are low in them.
The best source of omega-3s is fish, especially fatty fish such as salmon, tuna (including canned tuna), mackerel, herring, and sardines. Unfortunately, the relatively high price of fish often puts it beyond the budget of many households. Canned salmon gives the best buy for omega-3 fats and can be used in countless ways.
Eating fish two to three times a week is a good target for almost everyone. Eating more than that adds little extra protection against heart disease. Young children, and women in their childbearing years, should focus on types low in mercury (see “Fish, Mercury, and Fish Oil” on page 147). As research in this area continues to unfold, including in your diet a good source of alpha-linolenic acid (ALA) on most days of the week ensures an adequate intake of omega-3 fats. Good ways to do this include eating walnuts, flaxseeds, or foods made with or cooked in canola or soybean oil.
You don’t really need to think about getting more omega-6 fats in your diet because they are found in many common foods. Use plant oils for cooking and you’ll get all the omega-6s you need.
DIETARY FAT AND CANCER: A WEAK CONNECTION
The same kind of international comparisons that sparked the dietary fat–heart disease hypothesis have also generated strongly held beliefs about a connection between dietary fat and cancer. Countries with lower average fat intakes—mostly developing or less affluent nations—tend to have lower
rates of breast, colon, and prostate cancer than countries with higher average fat intakes. But better, more direct evidence linking diet and cancer has greatly weakened support for this connection.
Breast Cancer
Based on a few retrospective studies, the U.S. National Research Council concluded back in 1982 that reducing the fat content of the diet from 40 to 30 percent of calories would reduce the number of women diagnosed with breast cancer. Two years later, the National Cancer Institute made this the focus of a major health promotion campaign. These efforts have done little to prevent breast cancer.
Since then, larger cohort studies of cancer have not supported a connection between dietary fat and breast cancer. In the Nurses’ Health Study, more than 8,000 of the participants have developed breast cancer since 1980. With more than twenty years of follow-up, we didn’t see any increase in breast cancer among women with a higher intake of dietary fat. An analysis of all the large cohort studies from around the world also found no connection between dietary fat and breast cancer, except for an unexpected increase among the small number of women with the lowest fat intake.29 And combining data from the Nurses’ Health Study and Nurses’ Health Study II, with up to thirty years of follow-up, we looked to see whether the amount of fat in the diet either before or after a diagnosis of breast cancer increased the risk of dying from it. It did not. If anything, higher fat intake before diagnosis was related to a slightly lower risk of dying from breast cancer.30 (In fact, in the PREDIMED trial described on page 96, women who followed a higher-fat Mediterranean-type diet were less likely to have developed breast cancer over the five-year trial than those following a low-fat diet.31 Although the number of cases was small, the findings were promising.)
Most studies into the connection between dietary fat and breast cancer have focused on women in midlife or later. That makes sense, because breast cancer is more common in older women. But it can also strike younger women, and breast tissue is particularly vulnerable to carcinogenic influences at younger ages. In the Nurses’ Health Study II, which studied women who were ages twenty-six to forty-four in 1989, 714 of the more than 90,000 participants developed breast cancer over an eight-year period. High intake of animal fat, especially fat from red meat, increased the chances of having breast cancer. High intake of vegetable fat did not.32 That finding has persisted through 20 years of follow-up.33
As discussed in chapter three, randomized trials are thought to be the best way to test a hypothesis. Because of the strong belief that dietary fat was linked to the development of breast cancer, the Women’s Health Initiative was launched in 1991 with dietary fat as a prime focus. It was the most comprehensive and expensive clinical trial ever conducted. More than 48,000 women were randomized to follow either a low-fat diet or their usual diet. After seven years there was no significant difference in rates of breast cancer or, for that matter, heart disease or any other disease.34 Normally, a finding like that would put an end to the belief that a low-fat diet prevented breast cancer. But questions flew about whether the study had lasted long enough. In a follow-up publication, the authors reported that there were no differences at any time during the trial in blood levels of triglycerides or protective HDL between the two groups of women. We know that on a low-fat diet, triglycerides go up and HDL goes down. The lack of any change in these two indicated there was little if any difference in fat intakes between the study groups.
In another randomized trial conducted in Canada, women at higher risk of breast cancer because of changes in their mammograms were randomly assigned to a low-fat diet or their usual diet for an average of ten years. Levels of protective HDL were lower among women on the low-fat diet, indicating that there was a difference in fat intake between the two groups. Those on the low-fat diet had a 19 percent higher risk of breast cancer. Although this increase was not statistically significant, it was certainly headed in the wrong direction.35
The relationship between fat intake and breast cancer risk has been studied intensively over the last three decades. Although it’s impossible to prove that dietary fat has absolutely no effect on breast cancer—there could always be a tiny effect too small to detect—we can be confident that low-fat diets don’t play an important role in reducing the risk of breast cancer.
There are, however, some hints that replacing fats from animal with those from plants may provide some benefit. The clearest and most consistent finding from both animal and human studies is that too many calories during adulthood, regardless of the foods they come from, are far more important to the development of breast cancer than dietary fat.
Colon Cancer
Early studies suggested a link between dietary fat and colon cancer, the third leading cause of cancer deaths in the United States. But that, too, hasn’t been bolstered by more detailed work. There is good evidence that eating a lot of red meat increases the risk of colon cancer. This could stem from the types of fats in red meat or the cancer-causing chemicals generated by cooking red meat at high temperatures. The World Health Organization has warned that regularly eating red meat, especially processed meat, is linked to the development of colorectal cancer.36 Intake of fats from fish, chicken, and plants has not been associated with risk of colon or rectal cancer.
As is the case for breast cancer, the strongest dietary link with colon cancer is the imbalance between calories consumed and calories burned: people who are overweight are more likely to develop this cancer than people who aren’t. Protection from colon cancer comes from getting regular physical activity, not smoking, and getting adequate amounts of folic acid, one of the B vitamins (see chapter eleven).
Prostate Cancer
The situation with prostate cancer is murkier, partly because there have been relatively few studies in this area. International comparisons show that Asian men, who follow relatively low-fat diets, have substantially lower rates of prostate cancer than their Western counterparts. Although Asian men do experience some increases in prostate cancer when they move to the United States, rates in this group always stay lower than among Caucasians, suggesting that some genetic factors play an important role. If there is a connection between dietary fat and prostate cancer, it seems to be mainly related to animal fat or some other component of red meat. That’s good news, because it means that olive oil and other unsaturated fats that decrease the risk of heart disease would not increase the risk of prostate cancer.
Research on prostate cancer raises questions about a different kind of balance. Results from the Health Professionals Follow-Up Study and others have shown that men whose diets are rich in omega-3s from seafood—EPA and DHA—are less likely to develop prostate cancer. The connection between omega-3s from plants—alpha-linolenic acid (ALA)—and prostate cancer has been a bit worrisome. Some studies suggest an increase in prostate cancer and advanced prostate cancer among men with high intakes of ALA,37 although we have not seen this in our most recent follow-up in the Health Professional’s Follow-Up Study.
Why might a seemingly healthful oil be implicated in prostate cancer? One possibility is that, until recently, much of the ALA in the American diet was actually a harmful trans fat (see “Trans Fats—A Special Concern” on page 83). For years, oils rich in ALA routinely underwent partial hydrogenation to keep them from spoiling too quickly. But that also added trans fats to them. Partial hydrogenation has been greatly curtailed, which means we will be getting natural, untransformed ALA going forward.
Walnuts offer a clue that this may explain the trend. Walnuts are an important source of ALA. They’ve never been partially hydrogenated, and consumption of walnuts has never been associated with higher risk of prostate cancer. While scientists will continue to monitor the relationship of ALA with prostate disease, I think men can enjoy walnuts, canola oil, and other foods rich in ALA without worrying about their prostates.
The Bottom Line on Dietary Fats and Cancer
It is impossible to prove that there’s no connection between dietary fat and cancer. If fat does influen
ce the development of cancer, though, evidence from large cohort studies with many years of follow-up shows that the effect is small. Given the strong and consistent association that has been observed between type of fat and heart disease, I think it makes sense to focus on dietary fats for their proven impact on heart disease, not for their hypothetical connections with cancers that so far have not been supported by extensive evidence.
SELECTING HEALTHY FATS
The phrase “heart-healthy diet” often conjures up images of steamed rice and vegetables, a platter of baked chicken breasts, pasta—easy on the sauce, please—and only dreams of fried onion rings.
If you believe, as I do, that a low-fat diet isn’t the best way to a healthier heart, there’s another option. This one requires a bit of cutting back, just as traditional low-fat diets do, but it also means consciously adding some fats to your diet. This takes some practice at first, but the effort will be well worth it, both in taste and health.
Cutting back. Stay away from trans fats and limit your intake of saturated fats. It’s gotten easier and easier to avoid trans fats, and it will soon be hard to find them even if you wanted to. When an FDA rule went into effect in 2006 that the amount of trans fat in a food had to be listed on the food label, many companies took that as an “opportunity” to find trans-free replacements (see “Trans Fats—A Special Concern” on page 83).
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