Dr. McDougall, in his March 2009 newsletter, discusses “de novo lipogenesis,” a term that describes the pathway for converting dietary carbohydrate into body fat. He says the pathway is not utilized much in humans, that “excess starch does not turn to body fat.” He provides several references to support his claim. They do indeed back the notion that excess carbohydrate or replacement of dietary fat by carbohydrate does not induce de novo lipogenesis to any substantial degree.
“A widely held belief is that the sugars in starches are readily converted into fat and then stored unattractively in the abdomen, hips, and buttock. Incorrect! And there is no disagreement about the truth among scientists or their published scientific research.5-13 After eating, the complex carbohydrates found in starches, such as rice, are digested into simple sugars in the intestine and then absorbed into the bloodstream where they are transported to trillions of cells in the body in order to provide for energy. Carbohydrates (sugars) consumed in excess of the body’s daily needs can be stored (invisibly) as glycogen in the muscles and liver. The total storage capacity for glycogen is about two pounds. Carbohydrates consumed in excess of our need and beyond our limited storage capacity are not readily stored as body fat. Instead, these excess carbohydrate calories are burned off as heat (a process known as facultative dietary thermogenesis) or used in physical movements not associated with exercise.9,13
The process of turning sugars into fats is known as de novo lipogenesis. Some animals, such as pigs and cows, can efficiently convert the low-energy, inexpensive carbohydrates found in grains and grasses into calorie-dense fats.5 This metabolic efficiency makes pigs and cows ideal “food animals.” Bees also perform de novo lipogenesis; converting honey (simple carbohydrates) into wax (fats). However, human beings are very inefficient at this process and as a result de novo lipogenesis does not occur under usual living conditions in people.5-13 When, during extreme conditions, de novo lipogenesis does occur the metabolic cost is about 30% of the calories consumed—a very wasteful process.11
Under experimental laboratory conditions overfeeding of large amounts of simple sugars to subjects will result in a little bit of de novo lipogenesis. For example, trim and obese women were overfed 50% more total calories than they usually ate in a day, along with an extra 3.5 ounces (135 grams) of refined sugar. From this overfeeding the women produced less than 4 grams (36 calories) of fat daily, which means a person would have to be overfed by this amount of extra calories and sugar every day for nearly 4 months in order to gain one extra pound of body fat.10 Obviously, even overeating substantial quantities of refined and processed carbohydrates is a relatively unimportant source of body fat. So where does all that belly fat come from? The fat you eat is the fat you wear.”
That study where women were overfed by 50% is something else. That’s like having them eat 3000 calories/day instead of 2000, in sugar no less. And they put on hardly any body fat. That reminds me of the Cubans during their Special Period, where they lost weight and lowered their rates of diabetes, heart disease, and cancer, by eating a diet of primarily rice and sugar.
Americans (and other rich populations) defeat this by eating a lot of fat. When you add fat to the mix, it’s a whole different ball game. See my posts:
“Meat And Potatoes” Dietary Pattern And Risk For Colon Cancer
High-Fat High-Sugar Dietary Pattern (“Meat & Potatoes”) Linked To Colon Cancer, Diabetes
“For example, the influence of metabolism seems particularly relevant for the de novo synthesis of even-numbered saturated fatty acids in the body, compositions of which are largely determined by dietary factors, including carbohydrate and alcohol consumption (33–35), and other metabolic pathways (36, 37) rather than direct dietary intake.”
If carbohydrate intake doesn’t induce de novo lipogenesis to any substantial degree, why would these authors (whose study gave saturated fat consumption the green light) say it does?