Almost half of Americans are on a diet — not surprising, since two-thirds are overweight or obese, a frightening statistic that inspired Mayor Michael R. Bloomberg to push through a ban on large soft drinks in New York City. The country is preoccupied with calories. McDonald’s, for instance, is now posting them. But our widespread hope for weight loss makes us vulnerable to all kinds of promises, even ones that aren’t true, when it comes to food.

Perhaps the biggest misconception is that as long as you lose weight, it doesn’t matter what you eat. But it does. Yet being thin and being healthy are not at all the same thing. Being overweight is not necessarily linked with disease or premature death. What you eat affects which diseases you may develop, regardless of whether you’re thin or fat. Some diets that may help you lose weight may be harmful to your health over time.

A widely publicized study earlier this year showed that a low-carb Atkins-type diet might be a faster way to lose weight. That may have given many people the idea that eating meat and butter is the route to thinness and thus health.

In 35 years of medical research, conducted at the nonprofit Preventive Medicine Research Institute, which I founded, we have seen that patients who ate mostly plant-based meals, with dishes like black bean vegetarian chili and whole wheat penne pasta with roasted vegetables, achieved reversal of even severe coronary artery disease.

They also engaged in moderate exercise and stress-management techniques, and participated in a support group.

The program also led to improved blood flow and significantly less inflammation which matters because chronic inflammation is an underlying cause of heart disease and many forms of cancer. We found that this program may also slow, stop or reverse the progression of early stage prostate cancer, as well as reverse the progression of Type 2 diabetes.

Also, we found that it changed gene expression in over 500 genes in just three months, “turning on” genes that protect against disease and “turning off” genes that promote breast cancer, prostate cancer, inflammation and oxidative stress.

The program, too, has been associated with increased telomerase, which increases telomere length, the ends of our chromosomes that are thought to control how long we live (studies done in collaboration with Dr. Elizabeth Blackburn, who shared the Nobel Prize in 2009 with Carol Greider and Jack Szostak for discovering telomerase). As our telomeres get longer, our lives may get longer.

In a randomized controlled trial, patients on this lifestyle program lost an average of 24 pounds after one year and maintained a 12-pound weight loss after five years. The more closely the patients followed this program, the more improvement we measured in each category — at any age.

It’s not low carb or low fat. An optimal diet is low in unhealthful carbs (both sugar and other refined carbohydrates) and low in fat (especially saturated fats and trans fats) as well as in red meat and processed foods.

WHAT you eat is as important as what you exclude — your diet needs to be high in healthful carbs like fruits, vegetables, whole grains, legumes, soy products in natural, unrefined forms and some fish, like salmon. There are hundreds of thousands of health-enhancing substances in these foods. And what’s good for you is good for the planet.

Calories do count — fat is much denser in calories, so when you eat less fat, you consume fewer calories, without consuming less food. Also, it’s easy to eat too many calories from sugar and other refined carbs because they are so low in fiber that you can consume large amounts without getting full. Sugar is absorbed so quickly that you get repeated insulin surges, which promote Type 2 diabetes and accelerate the conversion of calories into body fat.

But never underestimate the power of telling people what they want to hear — like cheeseburgers and bacon are good for you. People are drawn to Atkins-type diets in part because, as the study showed, they produce a higher metabolic rate. But a low-carb diet increases metabolic rate because it’s stressful to your body. Just because something increases your metabolic rate doesn’t mean it’s good for you.

Amphetamines will also increase your metabolism and burn calories faster, which is why they are used to help people lose weight, at least temporarily. But they stress your body and may mortgage your health in the progress.

Patients on an Atkins diet in this study showed more than double the level of CRP (C-reactive protein), which is a measure of chronic inflammation and also significantly higher levels of cortisol, a key stress hormone.

Both of these increase the risk of heart disease and other chronic diseases.

A major research article published recently in the British Medical Journal studied 43,396 Swedish women over 16 years. It concluded that “low carbohydrate-high protein diets … are associated with increased risk of cardiovascular diseases.”

An important article in The New England Journal of Medicine examined data from a study showing that high-protein, low-carb diets promote coronary artery disease even if they don’t increase traditional cardiac risk factors like blood pressure or cholesterol levels. A diet low in fat and high in unrefined carbohydrates caused the least amount of coronary artery blockages, whereas an Atkins-type diet caused the most.

Outcomes from more than 37,000 men from the Harvard-sponsored Health Professionals Follow-Up Study and more than 83,000 women from the Nurses’ Health Study who were followed for many years showed that consumption of both processed and unprocessed red meat, a mainstay of an Atkins diet, is associated with an increased risk of premature death as well as greater incidence of cardiovascular disease, cancer and Type 2 diabetes.

About 75 percent of the $2.8 trillion in annual health care costs in the United States is from chronic diseases that can often be reversed or prevented altogether by a healthy lifestyle. If we put money and effort into helping people make better food and exercise choices, we could improve our health and reduce the cost of health care. For example, Medicare is now covering this program for reversing heart disease. In an increasingly polarized political landscape, this approach provides an alternative to some Republicans who want to privatize or dismantle Medicare and some Democrats who want to simply raise taxes or increase the deficit without addressing the diet and lifestyle choices that account for so much health spending.

This way of living helps you lose weight and keep it off while enhancing rather than harming your health.

By DEAN ORNISH/The New York Times