8 The Body’s Fuel

When you eat carbohydrates, you raise insulin, you burn carbohydrates for energy, and you store fat.

To understand why human bodies accumulate excessive fat, it helps to understand what our bodies are working to accomplish when they are healthy. We are endowed (as are all living organisms) with an exceedingly sophisticated system for surviving and ideally thriving in any contingencies (or at least those we might have faced during the past few million years). This system does innumerable critical jobs simultaneously. The relevant one is that it aims to ensure that all its myriad cells and cell types are properly fueled now and will continue to be adequately fueled in the future, with all the future’s attendant unpredictability.

This system has to take the macronutrients (the fuels) available in the foods we eat and those stored in our bodies—protein, fat, and carbohydrates—and maximize their utility. It has to make sure that if the body has too much of one kind of fuel and not enough of others, it makes do and limits harm that might result. Specifically, it has to control our blood sugar after carbohydrate-rich meals because high blood sugar is toxic to cells. The most obvious complications of diabetes—blood vessel, nerve, and kidney damage—are primarily due to the toxic effects of high blood sugar, and they are the reason this disease has to be diagnosed early before irreversible damage is done.

As Yalow and Berson and others were working out the role of insulin and other hormones in fat storage, British biochemists were simultaneously illuminating how our bodies and specifically our cells do this fuel-partitioning job—making fuel available efficiently where and when it’s needed—without those hormones. The hormonal system, as I’ll discuss, is layered on top to modulate this biochemical system and be prepared for emergencies. As these British biochemists showed, our bodies burn carbohydrates for fuel (specifically glucose, the stuff of blood sugar) when carbohydrates are available, and they burn fat when the carbohydrates have been effectively used up or stored (as a compound called glycogen). This makes eminent sense since our bodies have limited space for storing carbohydrates, about two thousand calories’ worth, but they can store relatively huge amounts of fat. Or at least most of us can. The protein is necessary to rebuild and repair cells, and although we don’t tend to think of it this way, it too can be stored in large amounts as muscle.

Now imagine eating a typical mixed meal containing all three of the macronutrients—protein, carbohydrates, and fat (leaving aside alcohol for the moment). The carbohydrates break down into glucose and enter the circulation, and your blood sugar (glucose) rises. That glucose has to be used for fuel or stored quickly to minimize the toxicity of this quickly elevating blood sugar. The fat can be stored while that happens and then used for fuel later, and the protein, ideally, will be used for cell and tissue repair.

Insulin is the hormone primarily responsible for orchestrating all this. It prompts cells in your lean tissues and organs to take up carbohydrates and use them for fuel; it inhibits them from burning fat and lets that fat escape back into the circulation, where it can be returned to storage. Insulin simultaneously causes the fat tissue to hold on to fat and the muscle cells to do the same with protein. Protein consumption also stimulates secretion of two other hormones, glucagon and growth hormone, the former of which will work to limit fat storage, while the latter will help promote growth and repair.

As we finish burning off or storing (as glycogen) the carbohydrates we’ve consumed, as our blood sugar is under control and now coming down, so should insulin. With insulin decreasing, the fat tissue will eventually experience that negative stimulus of insulin deficiency, and the fat cells will release the fat from storage—they will mobilize it—and we will burn that fat for fuel. This is what happens or should happen between meals; it happens overnight while we’re sleeping, and it will happen for days, weeks, or even longer if we have to survive a lengthy famine or self-imposed period of fasting. This cycle in which tides of carbs and fat alternatively fuel our cells, moving into and out of storage in the process, became known as the Randle cycle after Sir Philip Randle, the British biochemist who led this work in the 1960s.

Nutritionists and dietitians of the conventional school of thinking have been instructed and will tell us that carbohydrates are the preferred fuel for our bodies and our brains, thus implying that they are indispensable. But these nutritionists and dietitians are thinking about it the wrong way. The observable fact is that when carbohydrates are available in our diet, we do use them for fuel and we use them first. Whether or not the body and the brain somehow prefer using carbohydrates for fuel, the fact is that we have little or no choice. Since we have such limited storage space, our bodies have three options: Use the carbohydrates for energy, which at least puts them to use; turn them into fat, which the liver will do if necessary; or dispose of them in our urine, which used to be the diagnostic symptom of diabetes prior to the invention of more sensitive tests that can measure glucose levels directly (or indirectly) in the blood.

Once again it will help to quantify what we’re talking about, to establish the actual size of the phenomenon, so we can understand it, specifically why controlling these carbohydrates is so critical and tends to take precedence over other jobs that insulin does, particularly in our modern eating environment. So if you’re healthy (i.e., not diabetic) and you have not just eaten a carbohydrate-rich meal, you have around a teaspoon’s worth of carbohydrate (glucose) circulating in your blood.* That’s what the body considers a benign amount of blood sugar. That’s about four or five grams’ worth of glucose in your blood or about twenty calories’ worth. You’ll be diagnosed as diabetic if your blood sugar levels while you’re fasting (i.e., in the morning, before breakfast) are even moderately above that level: maybe a teaspoon and a half of glucose, or the equivalent of about thirty total calories of glucose circulating throughout your entire body. That very small number is the elevated blood sugar that causes so much damage in diabetes and that so many drugs are deployed to control.

If we follow conventional ideas about a healthy diet, we will consume about half our daily calories from carbohydrates, perhaps 1,000 to 1,500 each day, or 50 to 150 times more carbohydrates than are circulating in our bloodstream at any one time. That represents a significant engineering problem for the human body. These carbohydrates will enter the body in waves at mealtimes and from snacks and whatever beverages we’re consuming, but they can’t be allowed to accumulate in the bloodstream or the consequences will be dire. Yet the storage capacity, as glycogen, is minimal and may be full already. It helps that carbohydrate-rich foods tend to contain significant fiber (or at least this used to be the case before the food industry perfected the art of processing carbohydrates and removing all the fiber, let alone making sugary and other carbohydrate-rich, fiber-free beverages like beer). The fiber will slow down digestion and absorption of the carbohydrates and the time it takes for them to enter the circulation. But once they’re in the circulation, they have to be dispensed with quickly.

Our bodies begin to handle this engineering problem by having the pancreas secrete insulin even before we eat. This is known as the cephalic phase insulin release, with cephalic meaning “pertaining to the head” or, in this case, what the head and brain are doing rather than the body. The insulin prompts our fat cells to hold on to fat and our lean tissue to take up glucose and burn that for fuel because the body is assuming that more is coming. Just by reading the words fresh, hot doughnuts, for instance, you very likely thought about eating, and this cephalic process was put in motion. You may also notice that you’re salivating a bit, which is the classic reaction that Pavlov described in dogs—another cephalic phase effect. All these effects prepare the body for the flood of carbohydrates and other macronutrients that it now expects.

The pancreas continues to secrete insulin and the levels in the circulation continue to rise as we start eating, even before the food hits our stomach and we begin to digest and absorb it into our circulation. Once that happens and the tide of blood sugar begins to rise, the glucose stimulates the pancreas to secrete still more insulin. All through this process, the insulin is inducing cells in lean tissue and organs to take up the glucose as quickly as possible, and to store or burn it up for fuel. It’s causing those cells to burn glucose rather than fat (fatty acids), and it’s stimulating fat cells to take up and continue to hold on to fat.

In essence, our bodies make a calculated decision with each meal we eat. They are maximizing health and utility in the short term with the expectation that the long-term consequences can be minimized. We deal with the immediate problem—this flood of carbohydrate and the damage done to our cells by pumping large amounts of glucose through the mitochondria and the Krebs cycle—in part by putting off until later any problems that might arise from the storage of the relatively benign fat that is consumed with the carbohydrates or made from the carbohydrates. Once the carbohydrate situation is under control, the tide of insulin drops (or it does if you’re healthy); the fat cells now see the negative stimulus of insulin deficiency and release fat into the circulation, where the cells of lean tissues and organs can and will take it up and use it for fuel. The same insulin deficiency signal causes cells of lean tissues and organs to also burn the fat for energy.

When this system is working well in lean, healthy individuals, it’s highly adaptive. Metabolism researchers refer to it as metabolic flexibility. We shift back and forth easily from burning fat to burning carbohydrates: As the carbohydrates come in, the fat is stored. As the carbohydrates are depleted, the fat is mobilized and takes its place as an energy source.

All that is fine, except that this wonderfully dynamic system is dependent on insulin and the negative stimulus of insulin deficiency to function correctly, and that signal can be disrupted with relative ease by what we eat and how we live in our modern world. Without that negative stimulus of insulin deficiency—if insulin remains elevated above some unknown baseline threshold—we will store fat. Our systems, as Hilde Bruch phrased it, will be shifted in the direction of fat storage and away from oxidation (i.e., burning that fat for energy).

This is a critical problem. Excess fat, specifically above the waist, is an exceedingly good sign of insulin resistance, in which case insulin is indeed elevated higher than it should be and elevated for longer than it should be. Those who are insulin resistant are in fat-storage mode (which is the kind of phrase used by diet book authors but one that is nonetheless biologically appropriate) for much longer in the day than ideal and will be predisposed to hold on to fat rather than mobilize it or burn it. They will fatten easily, at least until their fat cells also become insulin resistant, at which point their weight will plateau. As Yalow and Berson noted, it wouldn’t take much insulin resistance for a few extra calories every day to be stored as fat, eventually manifesting itself as obesity. This was clearly an implication. This elevation of insulin, alas, could easily be small enough that it would not be measurable by any assay known to man.