How the Body Uses Carbs, Fat, and Protein for Energy

The body needs a continuous supply of glucose to fuel energy metabolism. To keep blood glucose stable, the body works to maintain tight glucose homeostasis within a narrow range, roughly 70 to 90 mg/dl.

It does this in more than one way. The body can convert digested carbohydrates into cellular energy, or it can synthesize glucose in the liver from fatty acids and amino acids through a process called gluconeogenesis. These systems complement one another and provide backup in case one raw nutrient, such as carbohydrates, fats, or protein, is temporarily unavailable.

While fasting and at relative rest, a 155-pound, or 70-kilogram, person requires approximately 200 grams, or about 7 ounces, of glucose over a 24-hour period. The formula used to calculate this demand is 2 mg of glucose per kilogram of body weight per minute.

That 200-gram number is approximate. The actual amount changes depending on the person, body temperature, outside temperature, physical activity, intellectual activity, and other factors. “Additional” activity means anything above and beyond the body’s regular baseline functions, such as heart function, breathing, walking, vision, hearing, and thought.

Any additional activity increases energy needs. This is why both physical and intellectual exertion can increase the body’s energy demand and contribute to weight loss when energy intake is controlled.

Beyond the glucose needed for energy metabolism, the body also needs a continuous supply of fatty acids and amino acids. These are used to build new cells and synthesize hormones, enzymes, vitamins, and other critical substances. These are sometimes called plastic, organic, or replacement needs because they help rebuild or replace dead cells and substances lost through feces, urine, perspiration, and exhaled air.

In simple terms, the body does not need calories only for energy. It also needs raw materials for repair, replacement, and maintenance.

If you consume more than the approximate 200 grams of glucose needed daily, the body can convert the excess into body fat. That is one way fat gain happens.

The rate of conversion is approximately 1 gram of fat for every 3 grams of glucose. This comes from the difference between 9 calories per gram of fat and 4 calories per gram of carbohydrate, with additional allowance for the energy required for consumption, digestion, and conversion.

If you consume less than 200 grams of glucose, the body compensates for the shortage by using fat at a rate of about 1 gram of fat for every 2 grams of glucose. That is one way fat loss happens.

Dr. Atkins incorrectly referred to this process as ketosis because ketones are intermediary products of the biochemical reactions involved in converting fatty acids into cellular energy. The more accurate name for the breakdown of fat is lipolysis.

Before the body converts stored body fat into usable energy, it will use fatty acids derived from food. This means that if dietary fat intake is too high, the body will use fat from the diet before turning to its own fat stores.

According to this framework, consuming above 75 grams of dietary fat can stop the loss of body fat because the body must first dispose of the fat coming from food.

If you consume less than 75 grams of fat, the body will draw from its own fat stores to produce enzymes, hormones, vitamins, cell membranes, and other essential substances. That is another way fat loss occurs.

If you consume more than 75 grams of fat, the excess can be stored under the skin as body fat. That is another way fat gain occurs.

Protein works differently.

If you consume less than 53 grams of protein, the body will break down muscle tissue into amino acids needed for building cells, neurotransmitters, hormones, digestive enzymes, and other essential structures and substances. This process is called muscle wasting.

You can lose weight this way, but it is not desirable weight loss because it is not primarily a loss of body fat. Losing muscle tissue weakens the body, lowers functional capacity, and can negatively affect metabolism.

If you consume more than 53 grams of protein, the body can use the excess amino acids to support muscle tissue. The stronger the muscles, the more protein they can use. In this case, weight gain can occur, but that weight is not from fat. It is desirable weight because it reflects the building or maintenance of lean tissue.

However, if someone does not have strong muscles or does not provide the body with a reason to build muscle, excess protein may not be used for muscle tissue. Instead, some of that excess can be converted into glucose. If the glucose exceeds the body’s energy needs, it can then be converted into body fat.

That is how body fat can be gained from overeating protein.

The larger point is that the body is always trying to solve three problems at once. It needs energy, it needs stable blood glucose, and it needs raw materials for repair and replacement.

Carbohydrates, fats, and proteins all contribute to these needs in different ways. Glucose provides immediate energy. Fat provides stored energy and structural material. Protein provides amino acids for tissue repair, hormones, enzymes, neurotransmitters, and muscle maintenance.

Fat gain and fat loss are not random. They are the result of how the body handles incoming nutrients relative to its current energy needs, replacement needs, and storage demands.

When glucose intake exceeds demand, excess can be stored as fat. When glucose intake is below demand, the body can use fat to compensate. When dietary fat intake is too high, the body may use incoming fat before stored fat. When protein is too low, muscle tissue may be broken down. When protein is adequate and muscles have a reason to use it, protein supports lean tissue. When protein is excessive and not used for muscle, some of it may be converted into glucose and eventually stored as fat.

This is why body composition is not only about calories. It is about how the body uses each nutrient, what it needs at the time, and whether the diet supports energy, repair, and the maintenance of lean tissue.