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hen Mom told you to eat your vegetables or that an apple a day would keep the doctor away, you may have shrugged it off as just one more piece of advice based on motherly love. Now, however, Mom has hard science on her side.
Ever since the Food and Drug Administration first released its U.S. Recommended Daily Allowances in 1968, the American public has come to rely on researchers to figure out which foods are better, what amounts should be consumed and even what food combinations produce desirable effects. As a result, over the past 30 years almost everyone connected with health – from the federal government on down – has felt compelled to offer their own dietary guidelines.
Indeed, this research has changed the way we eat. “High-fiber,” “low-fat” and “cholesterol-free” weren’t household concepts, much less advertising buzzwords, just 25 years ago. Bacon-and-egg breakfasts have been replaced by cereal and skim milk, three-martini lunches by salad bars and steak-and-potato dinners by broiled fish and pasta. Consumption of fresh fruits and vegetables has skyrocketed, up 39 percent since the 1970s. There has been, as Howard Hodis, director of the USC Atherosclerosis Research Unit puts it, “almost a cultural change in some of the ways we view food.”
IF ANYTHING, THE SCIENTIFIC PERSUIT of the optimum diet continues to grow. By studying the menus of different cultures, the interactions of different foods, the chemistry of foods and food supplements and the relation of disease to specific diets, scientists are hoping to clarify what constitutes a good diet.
In addition, they’re looking at food as a way to prevent illnesses like diabetes, cancer and heart disease. Eventually, they say, the goal is to provide specific dietary guidelines tailored to individual environmental and genetic situations that could reduce disease and and lead to increased vitality.
Yet before they can get that specific, scientists still must help the public sort through a mass of conflicting information about what it means to “eat right.”
For example, although early studies suggested a link between the consumption of fat and breast cancer, later studies seemed to indicate that fat in the diet was only a small variable in overall risk profiles. Similarly, there is confusion about whether some alcohol consumption is beneficial in that it lowers risk for heart attack and Alzheimer’s disease, or whether it increases risk for congestive heart disease, breast cancer and cancers of the esophagus and larynx.
Lately, new findings suggest too much of a good thing can be harmful as well. A diet too high in fiber can reduce the absorption of certain minerals, like calcium, iron, magnesium and zinc. And too little fat may lead to increased risk for certain diseases.
Then there are the “reversals” that have taken place in the past few years. For example, while margarine was once touted as a preferable alternative to the saturated fats in butter, it turns out that the partially hydrogenated oils in margarine have a similarly negative effect in that they increase risk of colon, rectal, prostate and endometrial cancers.
These “myths,” as Brian Hender-son, holder of the Kenneth T. Norris Jr. Chair in Cancer Prevention, calls them, plague researchers as well as the public.
For example, several early studies suggested that beta carotene, the form of vitamin A found in many orange fruits and vegetables, was helpful in preventing lung cancer. Then came more recent studies that suggest using a beta carotene supplement doesn’t help, and may in some cases hurt.
The reason for the confusion, Hen-derson says, is that much of the initial research on diet came from “case control” studies, where a group of people with a particular disease – such as heart disease or cancer – are compared with a random group. “The difficult thing is that you have to ask people to recall what they ate over the years,” he says. “People preferentially recall what they ate, remembering only the extraordinary and forgetting or miscalculating many of the day-to-day items.”
To correct this problem, scientists began putting together more expensive and time-consuming “prospective” studies, like Harvard University’s Nurses’ Health Study, in which a diet is surveyed first and then researchers observe over the years if and how that diet changes and what the health outcomes are. Many of these prospective studies began in the late 1980s, and the preliminary results are only now coming out. “It’s still too early for any consensus,” Henderson says.
In 1993, Henderson collaborated with colleagues in Hawaii to start their own prospective study, one of the largest and most ambitious ever assembled. By interviewing and following 215,000 African Americans, Latinos, Asians and Caucasians in Los Angeles and Hawaii, “we look for common links between a specific type of food and a specific outcome across four different cultural groups,” he says. “We can also examine whether the fat that comes from milk products and cheese in a Latino diet has the same effect as fat from the meat in the Asian diet.”
Still, there are a number of cases where scientists agree that definite links (between a food’s consumption and an outcome) have been firmly established. Drinking alcohol while pregnant, for example, can lead to birth defects and fetal-alcohol syndrome, while taking supplements of the vitamin B folic acid can help prevent the birth defect spina bifida. And oat bran’s ability to lower cholesterol has been so firmly demonstrated, notes Hodis, that the Food and Drug Administration allows products containing oat bran to advertise the beneficial health effects.
In fact, scientists have found it may not just be what you eat, but when you eat it, that determines health outcomes. Mimi Yu, professor of preventive medicine, has studied the incidence of nasal pharynx cancer in certain Chinese populations with extraordinarily high rates of the disease. Not only were the rates elevated, but the relative age at onset of the disease was 10 to 15 years before most other cancers appear on average. A series of longitudinal studies suggested the cancer was attributable to a salted fish popular among certain populations in southern China. “It was one of the main weaning foods given to babies,” she says. Today, with more widespread refrigeration available, salted fish is being replaced by fresh fish, and the rates of nasal pharynx cancer are declining.
CLOUDING THE ISSUE, FOR THE PUBLIC, is how and where they get information about nutrition. At least once a month, the media reports on the latest dietary finding; but deciphering what is “science” and what is “marketing” can be extremely difficult, says Carol Kop-rowski, coordinator of USC’s graduate program in nutrition sciences. “What I find troublesome about the information that comes across,” she says, “is that there’s not always a lot of evidence to support the conclusions.”
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For example, vitamin manufacturers stress the need to “make up” any nutritional deficiencies with their product. “We only know about some nutrients,” she says. “We don’t know if it is that nutrient in the context of a food that’s important, or that specific nutrient. A pill could have the same effect as food, but maybe not.”
For the elderly, the problem is particularly acute, says Pamela Wendt, research assistant professor at the Andrus Gerontology Center. “There’s a big ad campaign out now for liquid diet supplements that supposedly keep older people healthy and strong,” she says. “It plays on people’s fears of not having enough nutrition, particularly as they get older.” But supplements, like vitamin pills, “don’t have the fiber or the phytochemicals or any of the components we’re discovering are important in disease prevention.”
Researchers are also finding that what you eat as a young person helps determine how you feel when you’re in your golden years. “Most problems older adults have are problems of the last 45 years of living, not the last five years,” says Wendt.
CURRENTLY, SCIENTISTS SEEM TO BE less enthralled with researching “supplements” – vitamins or minerals or any single nutrient – than with the whole question of balanced diet and the inter-actions of whole foods. Noting that complex foods contain more than 100 beneficial vitamins, minerals and other substances and that little is known about their interactions or even which substances in the foods protect against cancer, the American Cancer Society’s Guidelines on Diet, Nutrition and Cancer Prevention suggest there “is no evidence at this time that nutritional supplements can reduce cancer risk.”
Robert Haile, professor of preventive medicine and a researcher at the USC/ Norris Cancer Center, co-authored the paper in the New England Journal of Medicine that concluded beta carotene and vitamins C and E do not protect against colorectal polyps. “This large national trial didn’t look too encouraging for those who advocate supplements,” he says. “In the future we may find some micronutrients are beneficial. But the overall emphasis of research seems to be shifting in favor of studying food groups and whole foods.”
Similarly, Koprowski thinks that more attention will be paid to “junk diets” rather than individual junk foods.
“It’s not going to kill you to have a chocolate bar,” she says. “But you should look at your diet over a whole week, not just a meal or even a whole day, and see if overall it contains enough of the foods you should be eating.”
AS FOR THE FUTURE, HAILE OBSERVES, that genetics and diet are an area of intense scrutiny. In fact, he classifies his research as “genetic epidemiology,” or the interface of environment and genetics. But a future in which diets are designed according to an individual’s specific genetic makeup, he says, is at least 10 to 20 years away. Part of the reason: scientists are only now finding out that there are numerous “metabolic” genes – genes that direct metabolism – that seem to have an effect on certain nutrients.
“People thought naively that if you have a certain gene, it is ‘bad’ for you, so you should have a certain diet,” he says. “But what we’re finding is that it can also have valuable effects, so it is hard to come up with a genetic ‘net’ equation.”
That’s not to say science won’t provide more specific advice about how we should eat, he adds. “Until we know more about specific food components, the most universally accepted conclusions emphasize consuming a broad range of fruits and vegetables.”
Sounds like something Mom would recommend.
This article originally appeared in USC Health, a quarterly magazine from the USC Health Sciences Campus.
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