Carbohydrates are NOT easily converted into body fat

Here is an interview I did a number of years ago with Jean-Pierre Flatt, Ph.D. about the role of carbohydrates and their conversion to body fat.

Here is why I found it so interesting.

My impression is that most people, including most doctors and most obesity researchers, believe that carbohydrates are easily converted into fat and that is why they make you fat. This is not true.

I believe this is probably the biggest myth in obesity research.

As J.P. Flatt explains, carbohydrates are NOT easily converted into fat.

I assume this is because the body does not want to waste energy doing this conversion as long as we get enough fat in our diet. One book on biochemistry notes if we get at least 10% of our calories from fat, there is very little conversion of carbohydrates to fat.

When the book “Sugar Busters” came out, the doctors who wrote the book were interviewed on television and said that “most of the fat on your body comes from carbohydrates in your diet.” This is complete nonsense. I don’t think the authors were lying. I imagine they were simply unaware of this research and assume, like everyone else, that what they were saying must be true.

There are a lot of these myths in science and medicine like this which are believed simply because people have heard it repeated so many times that they assume that they must be true when, in fact, they are not. This includes beliefs about blood pressure and blood pressure drugs, about cholesterol and cholesterol drugs, about blood sugar and diabetes drugs, about depression and antidepressants. My guess is that, unfortunately, a lot of these other myths are perpetrated by the drug companies in order to sell more drugs. This would not apply to the belief that carbs are easily converted into fat.

And by the way, the reason that carbohydrates can make you fat is not because they are converted into fat, but because the body can only store a very small amount of carbohydrates—a pound or two—but can store an unlimited amount of fat, therefore, when you eat carbohydrates, the body burns them first and tells your fat cells, “Hey guys, don’t release any fat right now, I have to burn these carbohydrates first.”

So if you’re snacking on carbs all day, it reduces the amount of fat that you burn. That is why low-carb diets work. They allow your body to burn more fat.

Jean-Pierre Flatt, Ph.D. is a leading researcher in field of energy metabolism and body weight regulation. His group was one of the first to discover than despite being a common belief, carbohydrates are not easily converted to fat in humans. In 1995 he received the McCollum Award from the American Society of Clinical Nutrition for his work on weight maintenance.

Dr. Flatt earned his Ph.D. in chemistry in Switzerland in 1959, followed by post-doctorate work at the Harvard Medical School. He was then on the faculty at the Institute of Biochemistry in Lausanne, Switzerland after which he spent six years in the Department of Nutrition Food Science at the Massachusetts Institute of Technology. Since 1973 Dr. Flatt has been Professor of Biochemistry at the University of Massachusetts Medical School in Worcester, Massachusetts.

Larry Hobbs interviewed Dr. Flatt by phone.

Hobbs: I heard one of the physician-authors of the book The Sugar Busters say that virtually all of the fat on our bodies came from sugar in our diet. Is this true?

Flatt: No, absolutely not. That is complete nonsense.

Hobbs: Are carbohydrates a major source of body fat?

Flatt: No.

Hobbs: Are carbohydrates easily converted to fat?

Flatt: No. We were the first group—well, actually the second but we didn’t know this at the time—to show excess carbohydrates are not easily converted to fat. We had young men consume a very large quantity of carbohydrates—about 500 grams or 2,000 calories in a single meal—to determined how much was converted to fat. We were surprised to find that virtually none of the excess carbohydrates were converted to fat, but were simply stored as glycogen—the storage form of sugar in the body.

Acheson KJ; Flatt JP; Jequier E. Glycogen synthesis versus lipogenesis after a 500 gram carbohydrate meal in man. Metabolism: Clinical and Experimental, 1982 Dec, 31(12):1234-40.

Hobbs: Do obese people eat more carbohydrates than they burn?

Flatt: No. The body is very efficient at balancing the amount of carbohydrates that are burned with the amount that is eaten. The same is true of protein. Protein and carbohydrate balance is maintained in all individuals, whether they are fat or thin, sedentary or athletic.

Hobbs: How much fat is made from carbohydrates?

Flatt: On a very-high carbohydrate diet you might make from 1 to 10 grams per day, but that is of no consequence. In the study I mentioned above the men still burned more fat than they made. They burned 17 grams of fat over a ten hour period. (454 grams = 1 pound.) Even under conditions of unrestricted access to food, the amount of fat made from carbohydrates is negligible. The common belief that sugar is easily converted to fat is not true.

Hobbs: Has this been verified?

Flatt: Yes. Numerous studies have shown this to be true. We also know that the amount of saturated fat and unsaturated fat found in adipose tissue reflects that amount found in the diet. This also shows that body fat comes from dietary fat and not carbohydrates.

Hobbs: Do obese people make more fat from carbohydrates than lean people?

Flatt: No. We studied this and found that after consuming an unusually large amount of carbohydrates obese people make the same amount of fat as lean people. After consuming 2000 calories of carbohydrates obese people made 5 grams of fat more than they burned compared to 4 grams more in lean people. Obese people do not make more fat from carbohydrates than lean people.

Acheson KJ; Schutz Y; Bessard T; Flatt JP; Jequier E. Carbohydrate metabolism and de novo lipogenesis in human obesity. American Journal of Clinical Nutrition, 1987 Jan, 45(1):78-85.

Hobbs: Is it possible that fructose is a major source of body fat, being that it bypasses a control point in the lipogenic process?

Flatt: No. Fructose is slightly more likely to be converted to fat than other carbohydrates, but still is not a major source of body fat.

Hobbs: Is it possible that high-fructose corn syrup—which not only contains fructose, but also raises insulin levels considerably more than fructose alone—is a major source of body fat?

Flatt: No. Carbohydrates of any kind do have the effect of reducing the amount of fat that is burned, and elevated insulin levels increase the amount of dietary fat that is stored. But high-fructose corn syrup is not a major source of body fat either.

Hobbs: Is there ever an increase in the conversion of carbohydrates to fat?

Flatt: Yes. When fat intake falls below 10 percent of calories in laboratory mice and rats there is an increase in the conversion of carbohydrates to fat, but the amount of fat made is still somewhat limited. These animals are leanest consuming a diet containing 10 percent fat. Below this level of fat intake there is a modest increase in body fat due to an increase in the conversion of carbohydrates to fat.

Hobbs: Does the amount of fat that you eat determine the amount of fat that you burn?

Flatt: No. Fat intake has little to do with the amount of fat that you burn. In 1989 we published a study showing that adding 106 grams of fat—nearly 1,000 calories of fat—to young men’s diet had no effect on the amount of fat that they burned. Before adding the fat they burned 1,032 fat calories per day compared to 1,042 fat calories after adding the fat. Fat oxidation decreases following a meal, even after a high-fat meal.

Schutz Y; Flatt JP; Jequier E. Failure of dietary fat intake to promote fat oxidation: a factor favoring the development of obesity [see comments]. American Journal of Clinical Nutrition, 1989 Aug, 50(2):307-14.

Hobbs: Does dietary fat increase the number of calories burned?

Flatt: No. In the same study we found no difference in the amount of calories they burned with or without the added fat. They burned 2,783 calories per day without the fat, and 2,820 calories with the added fat. Increasing fat intake does not increase the amount of calories that are burned.

Hobbs: Are carbohydrates important in weight control?

Flatt: Yes. Carbohydrates determine the amount of fat that is burned. The more carbohydrates you eat, the less fat you burn. The less carbohydrates you eat, the more fat you burn.

Hobbs: How much fat is burned on a low-carbohydrate diet?

Flatt: A person can burn 150 to 250 grams of fat per day if carbohydrate intake is restricted to 50 grams or less. That is roughly one-third to one-half a pound of fat per day.

Hobbs: Is dietary fat burned immediately after it is consumed, if needed?

Flatt: No. Dietary fat is not absorbed in a form that utilized immediately for energy. First it must be deposited in adipose tissue. Then later, between meals or during exercise, fatty acids are released to be burned.

Hobbs: Do differences in metabolic rate play much of a role in obesity?

Flatt: No, not as much as people would like to think. A five percent difference in resting metabolic rate could only explain an 11- to 18-pound difference in body weight in a sedentary person if food intake were held constant, and less than this in an active person. Differences in resting metabolic rate are of limited importance in explaining the differences in body weight between people. Differences in food intake are overwhelmingly the most important factor in explaining differences in body weight.

Hobbs: What is the effect of alcohol?

Flatt: Alcohol slightly increases calories burned, but not nearly enough to account for the extra calories consumed as alcohol. Alcohol does not affect the amount of carbohydrates or proteins that are burned, but it markedly decreases the amount of fat that is burned. Because of this alcohol does not reduce food intake. However, because alcohol reduces fat oxidation, it should be counted along with fat rather than carbohydrates when determining the relative portions of fat and carbohydrates.

Hobbs: Do stress hormones affect weight?

Flatt: Yes, probably. In a study done in Phoenix, Arizona young men ate nearly 1,700 calories per day more when they were given methylprednisolone, a glucocorticoid or stress hormone, than when they were given a placebo (4,554 versus 2,867 calories per day, respectively).

Tataranni PA; Larson DE; Snitker S; Young JB; Flatt JP; Ravussin E. Effects of glucocorticoids on energy metabolism and food intake in humans. Am J Physiol, 1996 Aug, 271(2 Pt 1):E317-25.

Jean-Pierre Flatt Ph.D. can be reached at:
Dept. of Biochemistry and Molecular Biology
University of Massachusetts Medical School
Worcester, MA 01655-0103
(508) 856-2353 phone
(508) 856-6231 fax
.(JavaScript must be enabled to view this email address)

Articles on the same subject can be found here:

• Carbohydrates