Why Do People Become Obese On High-carb, Low-fat Diets?

While a low-fat, high-carbohydrate diet can be a healthy lifestyle choice, it's no guarantee you won't gain weight. A low-fat, high carb diet that stresses fruits, vegetables and whole grains is healthy, but even too many "good" carbohydrates will cause weight gain if you consume more calories than you expend. Certain medical disorders can also cause obesity on a high-carbohydrate, low-fat diet.

Definition

The American Heart Association supports a diet composed of 20 to 35 percent of calories from fat, 10 to 35 percent from protein and 45 to 65 percent of calories from carbohydrate. Sticking to the lower levels for fat intake and higher levels for carbohydrates would comprise a high-carb, low-fat diet; a diet containing 20 percent of calories from fat is considered low fat. Most low-fat diets are high-carbohydrate diets because it's hard to eat low-fat and high protein, because proteins, particularly animal proteins, often contain large amounts of fat.

Excess Calories

If you take in more calories from carbohydrates that you expend, you will gain weight, regardless of the type of carbohydrate. Carbohydrates all have the same number of calories, 4 calories per 1 g and all carbohydrates break down into glucose before they're absorbed. While it's certainly possible to eat too much bread, rice, pasta or potatoes and gain weight, it's even easier to consume too many refined carbohydrates because they're denser in calories. The increase in American portion sizes makes it easier to overeat. Studies show that people who are offered a larger portion will eat more of it than they would normally eat from a smaller plate. One study, published in the December 2002 issue of "The American Journal of Clinical Nutrition," offered four different portion sizes ranging from 500 to 1,000 g to diners. When offered the larger portions, subjects ate 30 percent more than they did when offered a smaller portion.

Insulin Resistance

People who have insulin resistance don't utilize insulin well. Insulin is essential for the uptake of glucose into cells. Abnormal insulin levels can increase the risk of metabolic syndrome, a condition caused by excess insulin production, high blood glucose levels and accumulation of extra fat around the waist, called central obesity. High carbohydrate intake is associated with the metabolic syndrome, according to a collaborative study reported in the February 2003 issue of "Archives of Internal Medicine" which examined data from the Third National Health and Nutrition Examination Survey, 1988 to 1994.

Considerations

While weight gain can occur when you take in too many carbohydrates of any type, choosing healthy whole grains is still better for your overall health. Refined sugars have less nutritional value than complex carbohydrates, such as whole grains, fruits and vegetables, which help protect you against chronic disease.