Eat, Drink, and Be Healthy

by Walter Willett

  . . . AND EYE DISEASES . . .

  Eating plenty of fruits and vegetables helps keep those portals to your soul healthy, clear, and focused. This goes way beyond the common admonition to eat carrots for better vision (actually better night vision). People who regularly eat dark green leafy vegetables like spinach and collard greens are less likely to develop two common aging-related eye diseases, cataract and macular degeneration. Together, these two afflict millions of Americans over age sixty-five. A cataract is the gradual clouding of the eye’s lens, a disk of protein that focuses light on the light-sensitive retina. Like clear floor wax that turns dull and cloudy from the pounding and scuffling of feet, decades of “insults” damage and cloud the lens. Macular degeneration, the leading cause of blindness among older people, is caused by cumulative damage to the macula, the center of the retina. It starts as a blurred spot in the center of what you see. As the degeneration spreads, vision shrinks.

  In both diseases, free radicals are believed to be responsible for causing much of the damage. Free radicals are highly reactive and out-of-control substances generated inside the eye by bright sunlight, cigarette smoke, air pollution, and infection. Dark green leafy vegetables contain two pigments, lutein and zeaxanthin, that accumulate in the eye. These two, along with phytochemicals called carotenoids, can snuff out free radicals before they can harm the eye’s sensitive tissues.8 Getting lutein and zeaxanthin from fruits and vegetables is probably better than taking them as pills. Good sources include dark green lettuce, kale, turnip greens, collards, spinach, and broccoli.

  . . . AND BOWEL TROUBLE . . .

  What you can’t digest of fruits and vegetables is as healthful as what you can. As I describe later (see “Fiber: Praise for the Indigestible,” page 161), fiber, or what some call roughage, is essential for healthy bowel function. Without enough indigestible material in the diet, stools can become hard and difficult to pass. Fiber sops up water like a sponge and expands as it moves through the digestive system. This can calm the irritable bowel. By prompting regular bowel movements, fiber can relieve or prevent constipation. The bulking and softening actions of fiber also decrease pressure inside the intestinal tract and so may help prevent diverticulosis (the development of tiny, easily irritated pouches inside the colon) and diverticulitis (the often painful inflammation of these pouches). Almost twenty years ago, my colleagues and I found that men who ate more fiber were less likely to develop symptoms of diverticular disease.9 This was recently confirmed in a six-year study of nearly 700,000 women in the United Kingdom.10

  . . . AND CONTROL WEIGHT

  Adding more fruits and vegetables to your diet won’t necessarily help you lose weight, or even maintain it, unless you cut back on something else. As I described earlier, ideally that something else would be highly refined carbohydrates (like breads, crackers, and other foods made from white flour, or sugar-sweetened beverages) and red meat (see “A Low-Carb Diet May Help,” page 53). That said, data from the Nurses’ Health Studies and the Health Professionals Follow-Up Study show that women and men who increased their intakes of fruits and vegetables over a twenty-four-year period were more likely to have lost weight than those whose fruit and vegetable consumption remained steady or decreased.11 An interesting pattern appeared: increased intake of berries, apples, pears, soy, and cauliflower were all linked to weight loss or control, while increased intake of starchy vegetables such as potatoes, corn, and peas was linked to weight gain.

  BUT DO THEY PROTECT AGAINST CANCER?

  Three decades ago, two eminent epidemiologists estimated that “dietary factors”—not enough of something or too much of something else—accounted for 35 percent of cancer deaths in the United States, or roughly the same percentage that was chalked up to smoking at the time. Major reports from the U.S. National Academy of Sciences (Diet and Health: Implications for Reducing Chronic Disease Risk) and the World Cancer Research Fund and the American Institute for Cancer Research (Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective), among others, have echoed this conclusion. While 35 percent may be overly optimistic, the basic message that better diets—heavy on the plant foods, please—can help guard against a variety of cancers is perfectly sound.

  Fast Fact: A Shout Out for Blueberries

  * * *

  In our analyses of connections between specific fruits and vegetables and disease risk, one group of foods kept rising to the top: berries. Several studies have linked eating these tasty fruits, especially blueberries, to lower risks of heart disease, memory loss, diabetes, estrogen-receptor negative breast cancer, Parkinson’s disease, and more. Berries are no miracle food, mind you, and eating them can’t undo the harms caused by less-than-healthful food choices. But sprinkled on cereal, added to a fruit salad, munched as a snack, and turned into low-sugar desserts, berries can be a terrific addition to your diet.

  So far, several hundred studies have looked at the connection between diets high (or low) in fruits and vegetables and the development of cancer. Initially, they estimated a 50 percent reduction in most major cancers if everyone got at least five servings of fruits and vegetables a day. That was the basis of the National Cancer Institute’s ongoing 5 A Day program.

  Most of the early studies were case-control studies (see page 32). In a nutshell, these involve comparing differences in diet, habits, and other possible causes of cancer between a group of people with a particular cancer and a group without it. Such comparisons aren’t always fair or without bias. People with cancer, for example, tend to seek reasons why they were stricken and may be more apt to find fault with their diets than those without the disease. The consistency of results from case-control studies created a deceptively strong idea that eating plenty of fruits and vegetables helped ward off cancer.

  Cohort studies, in which information on diet and other lifestyle factors are collected before cancer, heart disease, and other conditions occur, tend to give more reliable and durable results. More than a decade ago, my team at the Harvard School of Public Health combined information on fruits and vegetables and cancer from our two large cohort studies, the Nurses’ Health Studies and Health Professionals Follow-Up Study, after the 110,000 participants had been followed for almost twenty years. During this time, 9,100 had developed some type of cancer. Those who averaged eight or more servings of fruits and vegetables a day developed cancer at about the same rate as those who ate fewer than one and a half servings a day.12 Also, in two randomized trials in which fiber supplements and a high-fiber, low-fat diet were compared with control groups, higher fiber intake didn’t reduce the recurrence of new polyps.13 The lack of a strong relation with overall cancer incidence was confirmed in a large prospective study in Europe.14

  Does this mean that eating fruits and vegetables has no impact whatsoever on cancer? No. Although they don’t have a blanket anticancer effect, some specific fruits and vegetables may work against specific cancers. Drill down a bit into the data and there’s some evidence that certain types of fruits or vegetables work against specific cancers. Examples include the following:

  • Bladder cancer. Eating cruciferous vegetables like broccoli has been linked with lower rates of bladder cancer.

  • Breast cancer. One problem with studying breast cancer is that it isn’t a single disease. It is several different diseases, each with its own risk factors. One type, estrogen-receptor-negative breast cancer, is particularly aggressive and more likely to be deadly. By combining data from cohort studies around the world, my team was able to examine breast cancers by their estrogen receptor status and found that consuming more vegetables was linked to a lower risk of developing estrogen-receptor-negative breast cancer.15 Eating broccoli and other cruciferous vegetables has been linked to lower risk of developing breast cancer.16

  • Colon and rectal cancer. There is strong evidence that the vitamin folate (also called folic acid) helps protect against colon and rectal cancer. Vegetables such as spinach and beets are good sources o
f folic acid and so can help fight these cancers. Today, though, with so many foods fortified with folic acid (see chapter eleven), the contribution of this vitamin from fruits and vegetables to protection against colon and rectal cancer may be dwindling.

  • Prostate cancer. Lycopene from tomatoes and cooked or processed tomato products, such as tomato sauce and ketchup, seems to be involved in the prevention of prostate cancer. In the Health Professionals Follow-Up Study, for example, men who consumed several servings of tomatoes, tomato sauce, or tomato juice a week were less likely to develop advanced prostate cancer than those who ate one to two servings a week.17 This finding has been supported by studies that look at blood levels of lycopene and other carotenoids.

  Although the anticancer effects of fruits and vegetables aren’t quite what they were thought to be a few years ago, every little bit helps. It is also possible that the benefits of fruits and vegetables may be underestimated, because almost all studies so far have examined intakes during midlife and later, while the critical time period for preventing cancer may be in childhood, adolescence, or young adulthood. For example, in one of the few studies to examine diet during adolescence, we have seen that consumption of fruits and vegetables was more strongly related to lower risk of breast cancer than was diet in midlife.18

  The genes you inherited from your parents play a role in determining whether or not you will get cancer. So do habits like smoking cigarettes, drinking too much alcohol, getting too much sun, and not exercising. Your occupation may also play a role. Still, a nutritious diet—and that includes plenty of fruits and vegetables—is an important part of any stay-healthy strategy.

  FIBER: PRAISE FOR THE INDIGESTIBLE

  From a health standpoint, one of the wonderful things about eating fruits and vegetables is that they contain much you can’t digest. Many of the substances that give plants their strength and flexibility aren’t broken down by the acids and enzymes in the human stomach or intestines. These substances, generically called fiber, include cellulose, pectin, and gums. There are two classes of fiber, soluble and insoluble. Both pass through the digestive system largely untouched. The big difference is that soluble fiber dissolves in the intestinal fluid, while insoluble fiber doesn’t.

  Soluble fiber is plentiful in peas, apples, and citrus fruits, as well as in oats and other grains and seeds. It forms a sticky, gooey, Jell-O–like mass as it passes through the intestines. This gummy substance traps cholesterol-rich bile acids and carries them out of the body in the stool. The more cholesterol you excrete, the less is available for transfer into the blood and the more your cholesterol will be lowered. The lower your cholesterol, the lower your risk of heart disease and other circulatory problems.

  Insoluble fiber comes from the cell walls of plants. The main component is cellulose, a long string of glucose molecules linked in a way the human digestive system can’t separate and that can’t dissolve in the intestines’ fluids. Several decades ago, research among the Bantu people of South Africa suggested that their high-fiber diet was responsible for their low rate of colon cancer. As insoluble fiber passes unchanged through the intestine, so the thinking went, it carries along partly digested food, and by speeding the passage of food through the digestive system, it may reduce the intestine’s exposure to toxic or cancer-causing substances found in food. After a few small studies showed much the same thing, the fiber craze was on. Media reports prompted many of us to start crunching through bran flakes or bran muffins for breakfast, and food manufacturers began adding fiber to cereals, breads, and pastries. In reality, though, most studies did not show lower colon cancer risks among persons who ate higher amounts of fiber from grain products. In a combined analysis of thirteen studies from around the world that included more than 725,000 men and women, fiber intake was not related to risk of colon cancer.19 Earlier analyses had failed to find a link between dietary fiber and colon polyps, the tiny growths from which most cancers arise. Also, in two randomized trials in which fiber supplements and a high-fiber, low-fat diet were compared with control groups, higher fiber intake didn’t reduce the recurrence of new polyps. Taking these findings together, high-fiber diets don’t appear to be an effective way to prevent colon cancer.

  Despite the disappointments for colon cancer, don’t throw out the All-Bran and stock up on Wonder Bread. By dragging partly digested food through the intestines, insoluble fiber delays the absorption of sugars and starch. This helps blunt the spikes in blood sugar and insulin that occur after eating foods that are easily converted into glucose and a similar spike in triglycerides, particles that ferry fat from the intestines to the tissues. Consistently high levels of insulin and triglycerides in the blood increase the chances of having a heart attack, and the repeated demand for large amounts of insulin can increase the risk of developing type 2 diabetes. Not surprisingly, many studies have shown that eating more fiber can lower the risks of having a heart attack or developing diabetes, providing more reason to consume an abundance of fruits, vegetables, and whole grains.

  The vast community of microbes that live in your digestive system, called your gut microbiome, helps digest food as it passes through your intestines. It also makes certain vitamins, breaks down toxins, and trains your immune system. Your microbiome plays an important role in keeping you healthy or making you sick, and may help control your weight or nudge you toward gaining weight. A powerful way to keep your microbiome in shape is to feed it plenty of fiber, a preferred food of gut microbes. This promotes what is believed to be the healthiest mix of microbes.

  Over the past decade or so, research in animals has shown that adding more fiber to the diet can change the community of microbes from one that promotes weight gain to one that is linked to a leaner physique. Fiber-starved microbes in the gut can start to feed on the protective mucus that lines the gut, which may trigger inflammation and disease. How well this applies to humans is a hot area of research.

  PHYTONUTRIENTS AT WORK

  How fruits and vegetables protect us from certain cancers, heart disease, gastrointestinal problems like diverticulitis, age-related eye diseases, and other ailments is still something of a mystery. Although we’ve been eating plants for eons and seriously studying them for decades, what we know today is the proverbial tip of the iceberg.

  Identifying the benefits of fruits and vegetables has been a challenging job, especially since plants have tremendous nutritional variability. A single type of plant—say, a Macoun apple—isn’t a stable, well-defined entity. Instead its chemical composition varies with the season, the soil in which it grew, the amount of water it got, what pests it had to withstand, how ripe it was when picked and eaten, and under what conditions it was stored. What’s more, the nutrients it delivers depend on how it is processed or cooked.

  It will be decades before we have identified all of the complex compounds in food and even longer before we truly understand how they interact with one another and what they do in our bodies. Even so, scientists have isolated a number of substances that plants make or store that may play critical roles in keeping us healthy. These include the following:

  Vitamins. The first set of phytochemicals discovered were what we today call vitamins. By definition, vitamins are carbon-containing compounds the body needs in small amounts to maintain tissue and keep metabolism humming. Vitamins have traditionally been defined by studying diseases of deficiency, things like rickets (too little vitamin D), pellagra (not enough niacin), and beriberi (not enough thiamine). More and more it looks as though cancer, heart disease, stroke, diabetes, osteoporosis, and other chronic diseases are, in part, diseases of deficiency. Exactly what the deficiencies are is the focus of intense research. Inadequate intake of folic acid is a likely risk factor for cardiovascular disease and some cancers. Low consumption of a special class of vitamins known as antioxidants, which capture and neutralize free radicals, appears to be involved in the early stages of heart disease, cancer, aging-related eye disease, dementia, and possibly aging itself (see
chapter eleven). Perhaps some of the known or yet-to-be-discovered phytochemicals will earn vitamin status for preventing these diseases. You could think of whole fruits and vegetables as vitamins, given their already proven ability to prevent these new diseases of deficiency, but they are much more.

  Essential elements. Plants are excellent sources of potassium, magnesium, and other elements the body needs for a host of critical tasks. Magnesium and potassium help control blood pressure and may reduce the risk of fatal rhythm disturbances of the heart.

  Plant hormones. The Food and Drug Administration has given food manufacturers the go-ahead to claim in ads and on packages that eating protein from soybeans lowers the risk of heart disease. One group of compounds found in soy, the isoflavones, can mimic or inhibit the hormone estrogen (see chapter seven). Another group, the phytosterols, can influence the absorption and metabolism of cholesterol.

  Carotenoids. These pigments give plants their orange or reddish colors. Some of them, such as beta-carotene and alpha-carotene, can be converted to vitamin A and so are considered to be vitamins. Others, like lutein and beta-cryptoxanthin, don’t contribute to vitamin A, but the evidence is strong that they play important roles in maintaining vision and memory, probably by acting as antioxidants.

  Because the chemical makeup of fruits and vegetables can vary so widely—what does a carrot have in common with a blueberry?—we should not expect that they would all have the same effects on health. Until recently, however, nutrition recommendations have just lumped them together as “fruits,” “vegetables,” or “fruits and vegetables.” There are now a few studies, including the Nurses’ Health Studies and Health Professionals Follow-Up Study, that are large enough to look at specific fruits and vegetables. For example, in an analysis combining these cohorts, with more than 185,000 men and women followed for up to twenty-four years, we examined consumption of specific fruits in relation to risk of diabetes. Total fruit consumption was related to lower risk of diabetes. Consumption of blueberries, grapes, raisins, and prunes seemed particularly beneficial for preventing diabetes, while oranges, strawberries, and cantaloupe were not.20 Drinking fruit juice was linked to a higher risk of diabetes, probably because of the large amount of rapidly absorbed sugar.