Everyone's Health

Dynamics of a Calorie

Is a calorie a calorie, or is there a difference between the calories in fat and the calories in carbohydrates? I am on a weight-loss program that has me counting calories, and I remember reading that the calories in carbohydrates should count for less because they require energy to be used.

A calorie is a unit of energy. In food, it represents the ability of the food to fuel work. Fats are a more concentrated sources of energy; they contain more than twice the calories per unit of weight as carbohydrates or protein.

Everything the body does requires energy, and the calories in the food we eat go to satisfy our energy needs. If we happen to take in more than is required, the excess gets changed into fat and put away in storage. There is even a caloric cost involved in doing this.

Changing dietary fat into stored fat has a small price tag – only about 3 percent of the ingested calories are expended. Transforming carbohydrates into stored fat is a multistep process that can use about 23 percent of the calories.

From this it might seem as though carbohydrates are a better dietary choice for those interested in losing weight. Indeed, this concept played a role in the popularization of low-fat, high-carbohydrate diets in the mid-1980s.

However, our metabolisms – and the relationship between food and what happens after it is consumed – are a bit more complicated. First and foremost, the body has a prime directive not to waste energy.

We also have to consider that the difference in caloric expense only comes into play when there are excess carbohydrate calories waiting to be put away into storage.

Glucose has many jobs to perform in the body; it can provide energy to just about any cell, and it's the primary fuel of the brain and the only one that can be used by red blood cells.

Prior to meal time, our blood glucose is sustained by a glucose storage compound named glycogen.

The liver contains the highest concentration of glycogen, but it's present in other cells as well. Once we begin to eat a meal that contains carbohydrates, they soon find their way into the blood stream as glucose. Incoming glucose will also go to replenish the glycogen stores in the liver and in cells throughout the body.

Prior to mealtime, the bulk of our energy needs are handled by fats coming out of the body's storage depots. As soon as dietary carbohydrates are on the scene, the enzyme that pulls the fat energy out of storage shuts down, and the glucose from the dietary carbohydrates takes over a larger share of the energy cost of running the body.

It is only after the intake of carbohydrates exceeds the amount needed for all bodily requirements that any excess will begin to be transformed into stored fat. This is signaled when a rise in the blood glucose level begins to stimulate the pancreas to release insulin.

It is only then that the 23 percent cost of turning carbohydrates into fat will be extracted; it is only for those carbohydrate calories that don't get used for normal metabolism or for any of the other glucose uses in the body.

Bottom line: On a diet that counts calories, it makes sense to consider carbohydrate and fat calories as equal. One study actually examined this issue, shifting people between equal-calorie diets based on high-fat or high-carbohydrate foods. The study found no difference in how the diets affected the body composition of the volunteers.

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