But a growing number of top nutritional scientists blame excessive carbohydrates — not fat — for America's ills. They say cutting carbohydrates is the key to reversing obesity, heart disease, Type 2 diabetes and hypertension.
"Fat is not the problem," says Dr. Walter Willett, chairman of the department of nutrition at the Harvard School of Public Health. "If Americans could eliminate sugary beverages, potatoes, white bread, pasta, white rice and sugary snacks, we would wipe out almost all the problems we have with weight and diabetes and other metabolic diseases."
It's a confusing message. For years we've been fed the line that eating fat would make us fat and lead to chronic illnesses. "Dietary fat used to be public enemy No. 1," says Dr. Edward Saltzman, associate professor of nutrition and medicine at Tufts University. "Now a growing and convincing body of science is pointing the finger at carbs, especially those containing refined flour and sugar."
Americans, on average, eat 250 to 300 grams of carbs a day, accounting for about 55% of their caloric intake. The most conservative recommendations say they should eat half that amount. Consumption of carbohydrates has increased over the years with the help of a 30-year-old, government-mandated message to cut fat.
And the nation's levels of obesity, Type 2 diabetes and heart disease have risen. "The country's big low-fat message backfired," says Dr. Frank Hu, professor of nutrition and epidemiology at the Harvard School of Public Health. "The overemphasis on reducing fat caused the consumption of carbohydrates and sugar in our diets to soar. That shift may be linked to the biggest health problems in America today."
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To understand what's behind the upheaval takes some basic understanding of food and metabolism.
All carbohydrates (a category including sugars) convert to sugar in the blood, and the more refined the carbs are, the quicker the conversion goes. When you eat a glazed doughnut or a serving of mashed potatoes, it turns into blood sugar very quickly. To manage the blood sugar, the pancreas produces insulin, which moves sugar into cells, where it's stored as fuel in the form of glycogen.
If you have a perfectly healthy metabolism, the system works beautifully, says Dr. Stephen Phinney, a nutritional biochemist and an emeritus professor of UC Davis who has studied carbohydrates for 30 years. "However, over time, as our bodies get tired of processing high loads of carbs, which evolution didn't prepare us for … how the body responds to insulin can change," he says.
When cells become more resistant to those insulin instructions, the pancreas needs to make more insulin to push the same amount of glucose into cells. As people become insulin resistant, carbs become a bigger challenge for the body. When the pancreas gets exhausted and can't produce enough insulin to keep up with the glucose in the blood, diabetes develops.
The first sign of insulin resistance is a condition called metabolic syndrome — a red flag that diabetes, and possibly heart disease, is just around the corner. People are said to have the syndrome when they have three or more of the following: high blood triglycerides (more than 150 mg); high blood pressure (over 135/85); central obesity (a waist circumference in men of more than 40 inches and in women, more than 35 inches); low HDL cholesterol (under 40 in men, under 50 in women); or elevated fasting glucose.
About one-fourth of adults has three or more of these symptoms.
"Put these people on a low-carb diet and they'll not only lose weight, which always helps these conditions, but their blood levels will improve," Phinney says. In a 12-week study published in 2008, Phinney and his colleagues put 40 overweight or obese men and women with metabolic syndrome on a 1,500-calorie diet. Half went on a low-fat, high-carb diet. The others went on a low-carb, high-fat diet. The low-fat group consumed 12 grams of saturated fat a day out of a total of 40 grams of fat, while the low-carb group ate 36 grams of saturated fat a day — three times more — out of a total of 100 grams of fat.
Despite all the extra saturated fat the low-carb group was getting, at the end of the 12 weeks, levels of triglycerides (which are risk factors for heart disease) had dropped by 50% in this group. Levels of good HDL cholesterol increased by 15%.
In the low-fat, high-carb group, triglycerides dropped only 20% and there was no change in HDL.
The take-home message from this study and others like it is that — contrary to what many expect — dietary fat intake is not directly related to blood fat. Rather, the amount of carbohydrates in the diet appears to be a potent contributor.
"The good news," adds Willett, "is that based on what we know, almost everyone can avoid Type 2 diabetes. Avoiding unhealthy carbohydrates is an important part of that solution." For those who are newly diagnosed, he adds, a low-carb diet can take the load off the pancreas before it gets too damaged and improve the condition — reducing or averting the need for insulin or other diabetes meds.
Americans can also blame high-carb diets for why the population has gotten fatter over the last 30 years, says Phinney, who is co-author of "The New Atkins for a New You" (Simon & Schuster, 2010).