When you read that a serving of food — say, one banana — has 105 calories, that means metabolizing the banana produces 105 calories of heat that your body can use for work. You may wonder which kinds of food have the most calories. Here’s how the calories measure up in 1 gram of the following foods:

• Protein: 4 calories
• Carbohydrates: 4 calories
• Alcohol: 7 calories
• Fat: 9 calories

In other words, ounce for ounce, proteins and carbohydrates give you fewer than half as many calories as fat. That’s why — again, ounce for ounce — high-fat foods, such as cream cheese, are high in calories, while low-fat foods, such as bagels (minus the cream cheese, of course), are not.

Sometimes foods that seem to be equally low-calorie really aren’t. You have to watch all the angles, paying attention to fat in addition to protein and carbohydrates. Here’s a good example: A chicken breast and a hamburger are both high-protein foods. Both should have the same number of calories per ounce.

But if you serve the chicken without its skin, it contains very little fat, while the hamburger is (sorry about this) full of it. A 3-ounce serving of skinless chicken provides 140 calories, while a 3-ounce burger yields 230 to 245 calories, depending on the cut of the meat.

All food provides calories. All calories provide energy. But not all calories come with a full complement of extra benefits such as amino acids, fatty acids, fiber, vitamins, and minerals. Some foods are said to give you empty calories. This term has nothing to do with the calorie’s energy potential or with calories having a hole in the middle.

It describes a calorie with no extra benefits. The best-known empty-calorie foods are table sugar and ethanol (the kind of alcohol found in beer, wine, and spirits). On their own, sugar and ethanol give you energy — but no nutrients.

People who abuse alcohol aren’t always thin, but the fact that they often substitute alcohol for food can lead to nutritional deficiencies, most commonly a deficiency of thiamin (vitamin B1), resulting in loss of appetite, an upset stomach, depression, and an inability to concentrate.

Of course, it’s only fair to point out that sugar and alcohol are ingredients often found in foods that do provide other nutrients. For example, sugar is found in bread, and alcohol is found in beer — two very different foods that both have calcium, phosphorus, iron, potassium, sodium, and B vitamins.

In the United States, some people are malnourished because they can’t afford enough food to get the nutrients they need. The school lunch program started by President Franklin Delano Roosevelt in 1935 and expanded by almost every president, Republican and Democrat, since then has been a largely successful attempt to prevent malnutrition among poor schoolchildren.

But many Americans who can afford enough food nevertheless are malnourished because they simply don’t know how to choose a diet that gives them nutrients as well as calories.

For these people, eating too many foods with empty calories can cause significant health problems, such as having weak bones; being underweight (yes, being too thin can be a problem); getting bleeding gums, skin rashes, and other nasties; and developing mental disorders, including depression and preventable retardation.

People who say that “calories don’t count” or that “some calories count less than others” are usually trying to convince you to follow a diet that concentrates on one kind of food to the exclusion of most others. One common example that seems to arise like a phoenix in every generation of dieters is the high-protein diet.

The high-protein diet says to cut back or even entirely eliminate carbohydrate foods on the assumption that because your muscle tissue is mostly protein, the protein foods you eat will go straight from your stomach to your muscles, while everything else turns to fat.

In other words, this diet says that you can stuff yourself with protein foods until your eyes bug out, because no matter how many calories you get, they’ll all be protein calories and they’ll all end up in your muscles, not on your hips. Boy, wouldn’t it be nice if that were true? The problem is, it isn’t.

Here’s the absolute truth: All calories, regardless of where they come from, give you energy. If you take in more energy (calories) than you spend each day, you’ll gain weight. If you take in less than you use up, you’ll lose weight. This nutrition rule is an equal opportunity, one-size-fits-all proposition that applies to everyone.