The weight loss industry in the U.S., including weight-loss surgeries, medications and diet books, generates $20 billion per year. At any time, about 108 million Americans are on a diet, and most of them make four or five attempts at dieting per year. Despite all of the information that we have and despite all of our attempts at dieting, 34 percent of Americans qualify as obese.

One of the biggest challenges that American dieters face is the ocean of conflicting nutrition messages. After years of scientific research into nutrition, experts can’t seem to agree on what we should eat. These conflicting messages present real challenges for people studying for nutrition careers and jobs. How do nutrition educators know when they’re giving the right advice?

Fat, Our Frenemy

For years, nutrition scientists have demonized fat, especially saturated fat, as the reason Americans are overweight. However, scientists now concede that fat isn’t the demon that they thought it was. Walter Willett, chairman of the department of nutrition at Harvard’s School of Public Health, told NPR that research shows that replacing saturated fat with carbohydrates hasn’t cut down on obesity or obesity-related diseases, like heart attack and stroke.

When Willett first submitted his data to medical journals in 1996, no journal would publish it. Almost 20 years later, scientists know that although saturated fat does raise LDL cholesterol to some degree, it also raises HDL cholesterol and lowers triglycerides. A wider consensus has emerged linking the obesity epidemic to another more widespread enemy.

Why We Cling to Our Carbs

When nutritionists preached the lowfat diet, they recommended replacing fat with unlimited carbohydrates. Unfortunately, they’ve learned that the refined carbohydrates contained in pasta, cookies, cereal and bread have a far more negative effect on the body than saturated fats. A 1997 study of 65,000 American women found that the women who ate the most refined carbs were 79 percent more likely to develop coronary vascular disease. High-glycemic carbs have a yo-yo effect on insulin in the body, promoting fat production, lowering insulin sensitivity, increasing appetite and producing inflammation.

Attempts to commercialize the low-carb diet, such as Atkins, have made many people skeptical of its validity. Also, shopping for and preparing a low-carb menu means fighting our entrenched food system, a system that subsidizes the mass production of corn and grains. Meir Stampfer, professor of nutrition and epidemiology at the Harvard School of Public Health, told Scientific American that the sugared beverage industry has lobbied politicians hard, persuading them to dismiss scientific evidence against refined carbs. Because of inertia and resistance to change, America continues to get fatter.

Why Scientists Look So Confused

Much of nutrition science is based on observational studies, which monitor the nutrition habits of people over time and try to identify trends. For example, an observational study, like the famous Nurses’ Health Study, may identify an association between certain nutrition habits and more health problems, but an observational study can’t prove that an observed habit results in a predictable outcome.

The best that an observational study can do is to generate a hypothesis, which then should be tested to determine causality. However, the news media often seizes observational studies and reports them as fact long before scientists can prove their assertions. Although the headline may state that a study “suggests” something instead of stating that it “proves” something, many Americans fail to understand the distinction. Therefore, nutrition science appears to constantly issue conflicting advice on diet.

Sound Advice

Ultimately, there is no single nutrition prescription for the entire human race. However, some general principles can help Americans to improve their diets and lose weight:

• Eat fruits and vegetables. It’s hard to go wrong with fruits and vegetables, so enjoy them.
• Avoid refined carbohydrates. Cut back on pasta, bread, cereal, cookies, cakes and sweetened beverages.
• Worry less about fat. David Ludwig, director of Children’s Hospital Boston’s obesity program, suggests that the next time people put butter on a piece of toast, they should assume that the butter is healthier than the piece of toast.
• Eat whole foods. Instead of eating dinner from a package or meals made from convenience foods, put together healthful meals from whole foods.

A final principle: Put down that fork and get moving. People that have lost 30 pounds and kept it off for at least five years tend to exercise about an hour each day.