‘Fat burns in the flame of carbohydrate.’ That dieting adage refers to the idea that you must have a minimal level of carbohydrate for the body to efficiently burn, or oxidize, fat. Although the reason is rarely explained, it has to do with the synthesis of substances that activate a metabolic energy-producing process known as the Krebs, or citric-acid, cycle. Some carbohydrate may be required for the synthesis of the substances, but muscle tissue lacks the enzymes needed to convert by-products of glucose metabolism into citric-acid intermediates. Instead, amino acids in muscle, especially branched-chain aminos (leucine, isoleucine and valine), degrade, and their carbon skeletons make the required substrates.
Even the most stringent low-carb plans rarely suggest that a dieter eat no carbs at all. The Atkins diet originally had a zero-carb phase, but that was later amended to a minimal 25 grams per day. The lowest-carb phase is supposed to accustom the body to becoming a fat-burning machine rather than a sugar burner.
The late Dan Duchaine, an IRON MAN columnist and a self-taught ergogenics expert, wrote a book called Underground Body Opus, in which he suggested a five-day zero-carb diet for those interested in obtaining the ‘ultimate level of low bodyfat and extreme muscularity.’ But Duchaine’s plan called for a relatively large intake of carbs during the weekend because carbs fuel bodybuilding workouts. It’s difficult to run a high-performance body on low-octane fuel.
On the other hand, it surprises many to learn that there’s no actual physiological requirement for carbohydrate. The same isn’t true for protein or fat. The body needs a certain amount of essential amino acids (coming from protein) to maintain health, as well as two essential fatty acids. The lack of a need for carbs seems curious when you consider that they’re the body’s preferred fuel, and certain organs, such as the brain, run best on carbohydrate.
In fact, the body can synthesize carbs from amino acids or from the 10 percent glycerol portion of triglyceride, which is fat. The process, called gluconeogenesis, occurs in the liver. In addition, the body is fully capable of using alternative fuel sources, such as lactate and ketones from fat metabolism.
Those physiological facts would seem to indicate that zero-carb diets are safe and effective, but other factors come into play that make them not only bad but actually a hindrance to losing bodyfat. Take thyroid metabolism, for example. The thyroid gland is the master controller of metabolism. Studies show that for the thyroid to function properly, it needs about 40 grams of carbs a day. If it doesn’t get enough carbs or calories, it produces an inert thyroid hormone called reverse T3, which brings on the dreaded diet plateau. That’s the body protecting itself. The lack of calories and carbs is interpreted as acute starvation, and the body halts thyroid production to prevent the breakdown of lean mass.
Careful readers may be asking, ‘What about the gluconeogenesis backup’won’t that supply enough carbs?’ Possibly. The key is to ensure that you’re getting enough protein, since much of the protein you take in converts to glucose in the liver in the absence of carbohydrate.
Scientists suggest still another reason to avoid a zero-carb diet, however, even if you’re getting an abundance of protein. A recent study involving rats that had adapted to a high-protein, zero-carb diet points up some metabolic issues.1 Fat mobilization is triggered when adrenergic hormones, such as epinephrine and norepinephrine, interact with adrenergic receptors in fat cells. When the hormones connect to the receptors, a biochemical cascade begins that culminates in the release of stored fat.
What the scientists found in rats that had adapted to a high-protein, zero-carb diet were aberrations in cellular systems that promote the stored-fat release. The rats weren’t releasing fat from their fat cells. At rest they showed an average 17 percent decrease in the activity of hormone-sensitive lipase, an enzyme activated by adrenergic hormones that releases fat from fat cells. The content of the enzyme didn’t change, only its activity.
The lack of carbs didn’t affect the activity of adrenergic cell receptors; only the process that activated fat release was disturbed. When the rats’ fat cells were exposed to a drug that mimics the effects of adrenergic hormones, normal fat release appeared blunted. Researchers noted that hormone-sensitive lipase, which is normally activated and relocates to a portion of fat cells where it’s most active, didn’t move under zero-carb conditions.
The study shows that even a high-protein diet won’t prevent a significant drop in fat mobilization when you’re eating zero carbs. The question that still needs to be answered is whether the same thing happens in humans. Rats don’t burn fat the way humans do. In rats the primary adrenergic cell receptors involved in fat mobilization are beta-3 receptors. In humans the beta-2 receptors serve that purpose.
It would be prudent not to undertake a long-term zero-carb diet. A cyclical zero-carb plan, such as the one suggested by Duchaine, with a heavy carb intake every few days, would probably resolve metabolic problems, but eventually, a long-term zero-carb diet may actually inhibit fat burning. IM
1 Martins-Afferri, M.P., et al. (2004). Response to intra- and extracellular lipolytic agents and hormone-sensitive lipase translocation are impaired in adipocytes from rats adapted to a high-protein, carbohydrate-free diet. J Nutr. 134:2919-2923.