Good Calories, Bad Calories

In 1965, the American Physiological Society published an eight-hundred-page Handbook of Physiology dedicated to the latest research on adipose-tissue metabolism. As this volume documented, several fundamental facts about the relationship between fat and carbohydrate metabolism had become clear. First, the body will burn carbohydrates for fuel, as long as blood sugar is elevated and the reserve supply of carbohydrates stored as glycogen in the liver and muscles is not being depleted. As these carbohydrate reserves begin to be tapped, however, or if there’s a sudden demand for more energy, then the flow of fatty acids from the fat tissue into the circulation accelerates to take up the slack. Meanwhile, a significant portion of the carbohydrates we consume and all of the fat will be stored as fat in our fat cells before being used for fuel. It’s this stored fat, in the form of fatty acids, that will then provide from 50 to 70 percent of all the energy we expend over the course of a day. “Adipose tissue is no longer considered a static tissue,” wrote the Swiss physiologist Albert Renold, who coedited the Handbook of Physiology; “it is recognized as what it is: the major site of active regulation of energy storage and mobilization, one of the primary control mechanisms responsible for the survival of any given organism.”

Since the excessive accumulation of fat in the fat tissue is the problem in obesity, we need to understand this primary control mechanism. This means, first of all, that we have to appreciate the difference between triglycerides and free fatty acids. They’re both forms fat takes in the human body, but they play very different roles, and these are tied directly to the way the oxidation and storage of fats and carbohydrates are regulated.

When we talk about the fat stored in the adipose tissue or the fats in our food, we’re talking about triglycerides. Oleic acid, the monounsaturated fat of olive oil, is a fatty acid, but it is present in oils and meats in the form of a triglyceride. Each triglyceride molecule is composed of three fatty acids (the “tri”), linked together on a backbone of glycerol (the “glyceride”). Some of the triglycerides in our fat tissue come from fat in our diet. The rest come from carbohydrates, from a process known as de novo lipogenesis, which is Latin for “the new creation of fat,” a process that takes place both in the liver and, to a lesser extent, in the fat tissue itself. The more carbohydrates flooding the circulation after a meal, the more will be converted to triglycerides and stored as fat for future use (perhaps 30 percent of the carbohydrates in any one meal). “This lipogenesis is regulated by the state of nutrition,” explained Wertheimer in an introductory chapter to the Handbook of Physiology: “it is decreased to a minimum in carbohydrate deficiency and accelerated considerably during carbohydrate availability.”^*114

A second critical point is that while the fat is stored as triglycerides it enters and exits the fat cells in the form of fatty acids—actually, free fatty acids, to distinguish them from the fatty acids bound up in triglycerides—and it’s these fatty acids that are burned as fuel in the cells. As triglycerides, the fat is locked into the fat cells, because triglycerides are too big to slip through the cell membranes. They have to be broken down into fatty acids—the process technically known as lipolysis—before the fat can escape into the circulation. The triglycerides in the bloodstream must also be broken down into fatty acids before the fat can diffuse into the fat cells. It’s only reconstituted into triglycerides, a process called esterification, once the fatty acids have passed through the walls of the blood vessels and the fat-cell membranes and are safely inside. This is true for all triglycerides, whether they originated as fat in the diet or were converted from carbohydrates in the liver.

Inside the fat cells, triglycerides are continuously broken down into their component fatty acids and glycerol (i.e., in lipolysis), and fatty acids and glycerol are continuously reassembled into triglycerides (i.e., esterified)—a process known as the triglyceride/fatty-acid cycle. Any fatty acids that are not immediately repackaged back into triglycerides will slip out of the fat cell and back into the circulation—“a ceaseless stream of [free fatty acids], a readily transportable source of energy, into the bloodstream,” as it was described in the Handbook of Physiology by one team of NIH researchers.