by Jack Norris, RD
USDA study looks at the body weight of people eating low-carbohydrate, vegetarian, and other diets.
During the 1980s and 90s, low-fat diets were promoted by some as the key to weight-loss. A plethora of low-fat foods were put on the market. Many of these low-fat foods replaced the fat with carbohydrates. These low-fat foods were consumed, yet people continued to get fatter.
As Americans became more overweight, diabetes rates increased. Diabetes is a disease defined by high levels of sugar (a carbohydrate) in the blood, further giving carbohydrates a bad name.
The perception that low-fat, high-carbohydrate diets and foods have caused the rise in obesity and diabetes has opened the floodgates for nutrition and diet gurus to sell their books and programs touting the benefits of low-carbohydrate diets. These people blame carbohydrates for the rise in obesity and diabetes and base this claim on simplistic notions of the interaction between insulin and dietary carbohydrates.
In April of 2001, some results of a USDA study of popular diets were published (1). The results consisted of two parts.
As part of the USDA's Continuing Survey of Food Intake by Individuals, 10,014 adults had 2 days of diets recorded and analyzed from 1994-96. The diets were rated according to the Healthy Eating Index (HEI) and the average Body Mass Index (BMI) was calculated for different diet groups.
HEI is a measure of whether a diet meets both the Food Guide Pyramid and some of the components of the U.S. Dietary Guidelines (limiting total fat, saturated fat, cholesterol, and sodium while eating a variety of foods).
BMI is a measure of healthy body weight. A BMI of 20-24.9 is considered healthy; 25 or greater is considered overweight.
642 people ate no meat, poultry, or fish on either day, and were labeled Vegetarian. These people were not necessarily vegetarian all the time, but were called "vegetarian" for the purpose of this study. Most of the vegetarians ate dairy or eggs. Results:
*Scale of 0 - 100
The Vegetarian group had a lower average calorie intake and BMI. They also had a lower HEI, but not by much.
The Nonvegetarian group was further divided into categories based on the percentage of carbohydrate in their diet:
*Scale of 0 - 100
The Low-Carbohydrate group had a higher BMI, but not a higher calorie intake, than the High-Carbohydrate group. They also had a much lower HEI. It is interesting to note that the High-Carbohydrate group had a much higher intake of added sugars (21 vs. 4% of their calories), yet they did not weigh as much. This does not support the theory that high intakes of added sugars cause obesity.
The High-Carbohydrate group was then broken down into the categories of eating a diet 1) meeting Food Guide Pyramid recommendations or 2) not meeting Food Guide Pyramid recommendations. While the Food Guide Pyramid group ate more calories (2184 vs. 1847), the men had a lower BMI (25.2 vs. 26.2). This was partly due to a higher activity level in the Food Guide Pyramid group.
And finally, the Non-Food Guide Pyramid Diet was divided further into Very Low-Fat (< 15%) and Moderate-Fat (>= 15%). The only noteworthy difference between these two groups was that the number of calories eaten was less in the Low-Fat group (1360 vs. 1887).
The researchers concluded that energy intakes are the lowest for those eating low-fat diets including a Vegetarian diet. BMIs are also the lowest for women on a Very Low-Fat (BMI = 24.4) diet, and for those in the vegetarian group. The highest BMIs were for those in the Low-Carbohydrate group.
There are some problems with looking at this data in isolation. One is that we do not know the what these people's BMIs were prior to their adopting these diets. It could be that people with higher BMIs are more likely to choose a low-carbohydrate diet in an effort to lose weight. However, Part II of this study sheds some light on this issue.
The researchers conducted a review of all the studies of the effects of popular diets, which date back to the 1950s. These studies tend to include a small number of people and to be conducted for a short period of time.
The conclusions the researchers drew about each of the diet groups are as follows.
Very Low-Fat (< 10% of calories as fat) diets:
High Carbohydrate / Moderately Low-Fat diets:
In looking at both Part I and Part II, the researchers concluded that:
The authors point out that data from the National Weight Registry reinforce the principles that limiting fat intake to 20-30% of calories is key for sustaining weight-loss over the long term. These data also indicate that regular physical activity is an important component.
The long-term effect of a low-carbohydrate diet with limited nutritional quality (as shown by the poor HEI scores) in the context of reduced energy intake is unknown. The USDA plans to follow up this study with further research to determine the long-term health effects of popular diets.