How a Healthy Diet Can Prevent Disease

How a Eating a Healthy Diet Can Prevent Disease

“With the right food choices, physical activity, and not smoking, we could prevent about 80 percent of heart disease, about 90 percent of diabetes, and 70 percent of stroke,” says Walter Willett, chair of the nutrition department at the Harvard School of Public Health in Boston. “Those are the three pillars. They really do make a difference.”

The right food choices are simple: Eat less red meat, sweets, refined grains, and salt, and drink fewer sugary beverages. Replace unhealthy foods with vegetables, fruit, beans, and whole grains, and with smaller amounts of fish, poultry, and low-fat dairy. Those foods aren’t just good for our health. They can also help protect the Earth.

Here’s why—and how—to eat real.

Walter Willett, Nutrition Department Chair, Harvard UniversityWalter Willett is chair of the Department of Nutrition at the Harvard School of Public Health and professor of medicine at the Harvard Medical School. He has published over 1,400 scientific articles on diet and disease.

Willett spoke to Nutrition Action’s Bonnie Liebman from Boston.

Q: Can food keep us healthy?

A: The foods we choose have a huge impact on our long-term health and well-being. We’ve learned that in the last few decades. We’ve seen that, say, rates of heart disease in northern Europe are ten times higher than in southern Europe and that rates of cancer vary tenfold or more around the world. The foods we choose— along with physical activity and not smoking—are a major factor in those huge differences in rates of almost every disease that we look at, including heart disease, stroke, diabetes, and many cancers.


Q: What are the right foods?

A: Within that healthy market basket will be mostly plant foods. That means plenty of vegetables, but not potatoes, and plenty of fruit, but not fruit juices. It’s aiming more for the more-intact, less-processed fruits and vegetables and for whole grains as opposed to refined grains. Those factors have an enormous influence. And I should add healthy protein sources, which means a combination of plant-based foods like legumes and nuts and a modest amount of poultry, fish, and dairy. It’s not necessary to be a vegan, but to move in the direction of plant-based choices.

Q: Is any plant-based diet good?

A: No. Just aiming for a plant-based diet alone doesn’t cut it. If your plant-based diet is high in refined starch and sugar, that could be the worst possible diet.

Q: How much cancer can diet prevent?

A: Cancer is more complicated than heart disease, stroke, and diabetes. It’s pretty clear now that diet’s biggest impact on cancer is avoiding overweight and obesity, and they are almost entirely avoidable. Obviously, quantity of food is important. And choosing the right foods can help us control our calories and weight. It’s the same market basket that’s high in intact whole grains, fruits, and vegetables and low in sugar, sugary beverages, refined grains, potatoes, and red meat.

Q: And low in salt?

A: Keeping salt on the low side is definitely important for preventing heart attacks and strokes, but that tends to happen automatically if you eat fruits, vegetables, and whole grains that have been minimally processed. You still need to pay attention because you can find whole grains even at places like Whole Foods that are extremely high in salt even though they are marketed as healthy. If you go the processed, prepackaged route, you run the risk of a high salt intake. But if you prepare your own intact foods, most of the time your salt intake will be low.

Q: Why limit red meat?

A: The evidence has now become very strong that keeping red meat low is one of the most important steps in creating a healthy diet. Replacing meat with almost anything is better, but replacing it with poultry will move you in the right direction and replacing it with fi sh or nuts— something with positive health benefits— is even better. Red meat is high in unhealthy fats. And even lean cuts have unhealthy fats. So replacing them with unsaturated fats will move the risk of heart disease and diabetes in the right direction.

Q: How does red meat affect diabetes?

A: We’re not sure. There may be multiple factors in meat. Some evidence suggests that the heme iron increases risk. The link with diabetes hasn’t been appreciated until recently, but now it’s been seen in many studies.

Q: Does meat promote colon cancer?

A: Yes, particularly if it’s processed red meat. So much happens in the processing that we’re not sure what matters, but the evidence is quite strong. Breast cancer does not seem to be related to red meat consumption during midlife and later, but we have seen a relationship with red meat consumed in early adult life and high school. So far we have the only prospective data on high school diet and breast cancer, so that needs confirmation.

Q: How much red meat is okay?

A: Like almost everything, it’s frequency and amount that influence our risk. There’s no sharp cutoff. It’s like radiation. We can’t say that there’s any safe level. But the large majority of the risk would be removed if everyone would cut back from the current average intake, which is about once a day, to once or twice a week in a moderate amount. A 20-ounce steak once a week is still a lot of meat.

Q: A serving should be three ounces?

A: Yes, three or four ounces cooked. That would be a huge step in the right direction for the vast majority of Americans who are eating red meat on a daily basis.

Q: What’s wrong with refined grains?

A: They’re problematic in two ways. Much of their vitamins and minerals and fiber have been removed in the refining process. For example, there’s pretty good evidence that the magnesium that’s removed has some protective effect for diabetes and heart disease. Labels usually call the fl our “enriched,” and in parentheses you’ll see a list of vitamins that are added back. It should be called “depleted” flour. Or “depleted and partially restored.” So there’s a problem with what’s lost, but also with what’s created—which is a rapidly absorbed, high-glycemic form of carbohydrate that has clear adverse effects on the risk of diabetes, weight gain, and heart disease.

Q: A high-glycemic carbohydrate raises blood sugar rapidly?

A: Yes. The strongest evidence that glycemic index matters comes from the acarbose trial. Acarbose is a drug that inhibits an enzyme secreted by the intestine to split longchain starches into their glucose building blocks. This drug inhibits the enzyme, so it’s turning a high-glycemic-index carb into a low-glycemic-index carb. The drug reduces the risk of heart disease and diabetes. From a physiological standpoint, it’s the definitive study on glycemic index.

Q: Don’t most foods—like hamburgers, pizza, and cookies—mix refined carbs with fat, which blunts the glycemic index?

A: If you add fat to refined carbs or potatoes, that will lower the glucose response. But you’ll also lower blood sugar if you replace the high-glycemic carb with a lowglycemic carb like barley. No matter what your diet, if you switch from a higher- to a lower-glycemic-index carb, you’ll get the expected drop in blood glucose. That’s why glycemic load matters. It takes into account the amount of carbohydrate as well as the glycemic index.

Q: Can you overdo whole grains?

A: You can always overdo foods. A key principle that’s been documented in the last few decades is that if you are lean and active, you can tolerate more carbohydrate. If you’re not, a high-carbohydrate diet can exacerbate insulin resistance. What’s interesting is that if you’re really eating whole grains, you won’t overeat them. I think of my Kashi pilaf as a paradigm because it’s a blend of intact grains. We’re not talking about whole-grain bread or Kashi breakfast cereals. I can tolerate only 30 to 40 percent of calories from intact grains because more causes gastrointestinal disturbances. The healthiest whole grains are self-limiting.

Kashi Pilaf Whole Grain Healthy Breakfast Cereal
A Willet favorite. Whole intact grains like Kashi Pilaf beat breads, breakfast flakes, and other whole foods made from whole-grain flour.

Q: What’s wrong with sugars?

A: At this point, sugary beverages are the number-one problem in the American diet. They’re the number-one source of calories on average. Sugary beverages are also the number-one sales item in terms of dollars in grocery stores, and a lot are consumed at McDonald’s and other fast food places. They’re like cigarette smoking. They’re only bad and they have no redeeming virtue. And the adverse effects are weight gain, diabetes, heart disease, gout, cavities, and maybe some cancers as well.

Infographic: How Much Sugar is in Drinks


How Much Sugar is in Drinks Chart
Number-one problem. To lower the risk of obesity, diabetes, heart disease, gout, and cavities, minimize sugary beverages including juices.

Q: Even fruit juice is a problem?

A: Yes. It’s associated with a higher risk of diabetes. And you’d expect that because fruit juice has about the same number of calories per serving as Coke or Pepsi, and it will have the same metabolic effect. Some nutrients come along with fruit juice, but there’s a big metabolic price to pay for getting nutrients that you could get from foods that have fewer calories. And most people are getting those nutrients anyway, so two glasses of orange juice a day doesn’t have many health benefits and has many harms.

Q: What benefits do fruits and vegetables provide?

A: The benefit for lowering the risk of cardiovascular disease is clear. Some of that is due to their potassium, which lowers blood pressure. That’s proven. But some is also likely due to their folic acid, antioxidants, and other micronutrients. Fruits and vegetables are a primary source of vitamins and many minerals. We don’t understand all of their benefits. There may be some modest benefit for preventing some cancers, but a large benefit isn’t there.

Q: Why do we need oils and nuts?

A: Replacing saturated fat or carbohydrate with unsaturated fat definitively lowers LDL cholesterol and triglycerides. And there’s a huge amount of evidence that that will reduce the risk of heart attacks. It does seem that there’s some added benefit of polyunsaturated fats above monounsaturated fats, but virtually all oils and nuts have some of both.

Q: With 120 calories in a tablespoon of oil and about 175 calories in just a quarter cup of nuts, isn’t it easy to go overboard?

A: Of course. It’s too much of a good thing. Eating a horrible diet and pouring oil on it is not a good idea, just like sprinkling bran on a horrible diet is not a good idea. If you consume nuts and oils as part of an overall healthy diet, it’s easier to control calories, but that means replacing bad stuff with good stuff.

Q: The food industry keeps repeating that there are no good or bad foods.

A: I think we should be using the words good food and bad food liberally. That’s what it’s all about…replacing bad foods with good foods.

Q: And good food tastes good?

A: Putting good foods together and preparing them well can make a way of eating that’s a pleasure, not a sacrifice. In fact, it’s more interesting, enjoyable, and varied than the mainstream American diet.

This article is based on content from the October 2011 Nutrition Action Healthletter article “EAT LIKE IT MATTERS: How diet can prevent disease.” Click here to subscribe.

2 Replies to “How a Healthy Diet Can Prevent Disease”

  1. Recently I stopped drinking my morning orange juice ( fresh squeezed) because of the sugar. I substituted half a fresh squeezed lime mixed in a pot of a hot water as a vitamin C source. Does the hot water degrade the quality of the Vitamin C ????

    1. Robert: Only a trivial amount of vitamin C would be lost over a short period of time in water that’s not too hot to drink.

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