11.9: Starvation and Low-Carbohydrate Diets

The body constantly needs a certain amount of glucose for energy production. Brain cells, for example, need a lot of energy as ATP, but under normal circumstances can use only glucose as fuel. Where does the glucose come from during starvation or when on a low-carbohydrate diet?

When a person isn’t consuming carbohydrate, for whatever reason, glycogen stores are soon depleted. Our cells must then make the glucose it needs from certain amino acids. We can’t make glucose from fatty acids.

Glucose can be made from pyruvate, but not from acetyl CoA. The conversion of pyruvate to acetyl CoA (see Fig. 11-3) is a crucial reaction in that it’s irreversible. Glucose can’t be made from fatty acids because fatty acids are broken down to acetyl CoA rather than pyruvate (see Fig. 11-3). However, the amino acids that can be converted to pyruvate can be made into glucose.

Figure 11-5: Excess energy-providing nutrients are stored as body fat.

The amino acids come from protein. During starvation, the body first breaks down the proteins least essential for survival (e.g., some proteins in the liver and skeletal muscles) to provide the necessary amino acids. As a last resort, the body starts breaking down such proteins as those in the heart muscle.

Starvation would be rapidly fatal if the body had to incessantly break down its protein to provide the brain with glucose. The body in its wisdom produces an alternative fuel (ketones) for the brain during starvation. It does this by using the lack of carbohydrate as a signal of starvation.

When carbohydrate is lacking, acetyl CoA does not readily proceed to aerobic metabolism. As a result, acetyl CoA accumulates. This unusual accumulation of acetyl CoA causes acetic acid (from acetyl CoA) to combine with each other to form ketones (see Fig. 11-6).

The brain uses the ketones as fuel in increasing amounts during starvation. This reduces the need for glucose, thereby slowing the breakdown of body protein. Of course, once the body’s fat stores are used up, body protein is used unremittingly as fuel, which is eventually fatal.

Ketones are acidic, and excessive amounts can cause ketosis or acidosis, a condition that can become dangerous. About 50 grams (200 calories worth) of carbohydrate (“carb”) a day will prevent ketosis.

Figure 11-6: Glucose and ketones are made when carbohydrate is scarce.

A low-carb diet in one form or another shows up regularly in the steady stream of popular weight-loss diets. One reason why this diet is so popular is that it promotes rapid weight loss because body protein (and glycogen) is broken down to maintain glucose in the blood.

The early weight loss is mainly from water loss. Recall that protein and carbohydrate (which includes glycogen) hold about three times their weight in water, whereas fat doesn’t hold any (see Chap. 8).

Another reason why low-carb diets are popular is because the production of ketones causes a loss of appetite (sometimes in the form of nausea). Some of the ketones are excreted in the urine and breath. Ketones have caloric value, and this “easy loss of calories” is touted in popular books promoting these diets. But, typically, this loss is less than 60 calories a day.

Long-term weight loss from low-carb diets is typically from simply eating much fewer calories. Cutting out such carbs as candy, desserts, sugared drinks, bread, pasta, rice, not only cuts out a lot of calories, but steers us to a healthier diet. Lowcarb, high-fat diets that are also low in saturated fat are being studied for possible health benefits.¹