The reason that low-carb diets work

One of the reasons that people on low-carb diets lose weight faster is that they eat fewer calories.

"Take the carbohydrates away, and I expected the participants would just eat more of the other stuff," says researcher Dr. Guenther Boden, a professor of biochemistry at Temple University, in Philadelphia. But they didn't. In fact, it turned out they ate 1,000 fewer calories every day.

His team recruited a group of 10 obese patients, all diagnosed with type II diabetes, in a controlled, clinical environment where diets were strictly monitored for three weeks [1].

For the first week, participants ate their usual mixed diet. But during the last two weeks the researchers restricted their intake of carbohydrates from an average of 300 grams per day to just 20 grams a day.

At the same time, a tempting array of fatty, sugary and other foods was readily available to all. Subjects were told they could eat as much of anything else as they wanted, whenever they wanted.

By the end of the two-week low-carb regimen, patients lost an average of 1.65 kilograms (3.6 pounds) and reduced their daily caloric intake by nearly 1,000 calories - from an average of 3,111 calories before they began the diet, to just 2,164 calories while on the low-carb diet.

As happens naturally with weight loss, insulin and blood sugar levels began to improve. So did levels of unhealthy blood fats called triglycerides -- a finding noted in previous studies that looked at the effects of low-carb diets on cardiovascular health.

It appears that the extra protein often associated with low carbohydrate diets, rather than their low carbohydrate content per se, is responsible for their effect on appetite control.

Recently, Dr. David Weigle and co-workers at the Oregon Health and Science University tested the hypothesis that increasing the protein content while maintaining the carbohydrate content of the diet lowers bodyweight by decreasing appetite and calorie intake [2].

Appetite, calorie intake, weight and fat mass were measured in 19 subjects placed on the following diets: 1) a weight-maintaining diet (15% protein, 35% fat, 50% carbohydrate) for two weeks; 2) an isocaloric diet (30% protein, 20% fat, 50% carbohydrate) for two weeks; and 3) an ad libitum (self-directed) diet (30% protein, 20% fat, 50% carbohydrate) for 12 weeks.

Subjects reported a clear decrease in hunger and an increase in fullness during weeks three and four of the study when following the high-protein diet. This increase in satiety was confirmed by a significant decrease in calorie intake.

"We found that an increase in dietary protein content comparable with that observed in popular low-carbohydrate diets," conclude the authors, "but no reduction in dietary carbohydrate content, resulted in rapid losses of weight and body fat."

These findings confirm the results of other studies showing that diets higher in protein and lower in carbohydrate produce greater weight loss. They work because protein increases your metabolic rate (so you burn more calories) and keeps you feeling fuller for longer (so you eat less).

Reference
1. Boden G, Sargrad K, Homko C, Mozzoli M, Stein TP. (2005). Effect of a low-carbohydrate diet on appetite, blood glucose levels, and insulin resistance in obese patients with type 2 diabetes. Ann Intern Med. Mar 15;142(6):403-1
2. Weigle DS, Breen PA, Matthys CC, Callahan HS, Meeuws KE, Burden VR, Purnell JQ. (2005). A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations. American Journal of Clinical Nutrition, 82, 41-48