A couple of days ago a friend of mine sent me the draft of a medical paper he is submitting for publication. He wanted me to review it and get back to him with any criticisms before he sent the paper off, which I did. While going through the references at the end of the paper I came across one that I dimly remembered reading in the past and went and retrieved it from my files. The paper was stunning. As I read it I realized that I had seen the author present the data at a meeting somewhere, but, although I had the paper in my files, I had never really read it critically.
The paper addressed a question that all carbohydrate-restricted dieters run into sooner or later.
Since low-carb diets have been making such positive waves over the past couple of years, what with all the studies appearing demonstrating the superiority of these diets for weight loss as well as for blood pressure, blood sugar, and even lipid control, the horde of pro-low-fat naysayers has resorted to low-carb-bashing by sniffing that those diets work simply because they reduce calories. And we all know, after all, they say, that low-calorie diets cause us to lose weight, reduce our blood sugars and blood pressures; therefore, it’s simply a low-calorie phenomenon. And—they always add with a sinister tone to their voices—who knows what the long term effects will be?
This study published in the venerable American Journal of Physiology way back in 1992 demonstrates just the opposite.
If you want to lose weight one of the quickest—but not necessarily the most comfortable—way to do it is to fast. Fasting is the ultimate low-calorie diet. When a person fasts for longer than a day or so a host of metabolic changes kick in that serve to provide nourishment for all the critical tissues and organs of the body. During the fast, the body looks to the fat being released from adipose tissue (stored fat) as its primary energy source, while conserving as much glucose as possible to provide for those organs—primarily the brain and the red blood cells—that can use only glucose. As the fast goes on, greater and greater amounts of fat are consumed as even the brain weans itself from a diet of almost entirely glucose to one of ketone bodies, which are breakdown products of fatty acid metabolism. In short, the body is going to burn the maximal amount of fat it can possibly burn while on a fast.
If we were trying to lose weight wouldn’t it be nice to be able to burn fat at the fastest rate possible while at the same time getting to eat?
Well, you can. And that’s what the above referenced paper is all about.
The researchers used five subjects in a crossover study, which means that each subject served as his own control, eliminating genetics or individual biological variation from the equation.
The subjects were either fasted for 84 hours (31/2 days) or fasted for 84 hours while at the same time being given a fat solution intravenously. So, two groups, neither of them ate anything, but one group got about 5% more calories in the form of fat than their resting energy requirements. The fatty emulsion that was given intravenously contained less than 20 grams of carbohydrate as glycerol, the remainder was pure fat. The study took place in a hospital setting so that the subjects were under observation at all times so that no food could be eaten. This was not one of those notoriously error-prone dietary recall studies.
What did the researchers discover?

The mechanism by which carbohydrate restriction in the presence of adequate energy supply acts to stimulate lipolysis [fat breakdown] is presumably the same as that proposed to be responsible for increased lipolysis during total fasting. During fasting, there is a decrease in hepatic [liver] glycogen, hepatic glucose production, blood glucose concentration, and glucose availability for oxidation. The alterations in glucose metabolism ultimately cause changes in both the level of circulating hormones and the tissue sensitivity to hormonal interactions. Plasma insulin, a potent inhibitor of lipolysis, declines; plasma epinephrine, a potent lipolytic agent, increases. In addition, adipose tissue sensitivity to insulin is decreased, whereas sensitivity of epinephrine is enhanced, thereby maximizing the lipolytic effect of the changes in hormone concentrations.

The first sentence above is the crux of the entire paper. The researchers found that all the changes the body normally undergoes during a fast, the changes that maximize fat breakdown, take place in the face of energy consumption as long as that energy is restricted in carbohydrates, i.e., a low-carbohydrate diet. Even if the energy is in the form of virtually all fat.
So, it’s NOT the reduced calories that make the difference—it’s the reduced carbohydrates.
The study authors concluded:

Our results demonstrate that carbohydrate restriction, not the presence of a negative energy balance [reduced-calorie diet], is responsible for initiating the metabolic response to fasting.

This paper reinforces what we tell our patients and readers all the time—if you want a low-carb diet to work for you, it will work like a charm, but you’ve got to cut the carbs. And not just a little bit here and there. If you’re going to go on a low-carb diet, go on a low-carb diet.

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