The reasons you gain weight are varied, complex, and to some extent a mystery: Even experts don't know everything yet.
But how your body puts on pounds is really quite simple. Food provides calories, and calories — whether from a chocolate bar or a carrot stick — provide energy.
If you eat more calories than you use, you'll gain weight, and if you use more calories than you eat, you'll lose weight. Let's take a closer look at the process.
Taking the mystery out of calories
The word "calorie" seems to have taken on an importance nowadays out of proportion to its simple meaning. A calorie is nothing more than a unit of measurement, like an inch or an ounce. An inch is a small amount of distance, and a calorie is a small amount of energy.
Technically speaking, one calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. Calories are a way to measure the amount of energy contained in foods. A peach, for instance, has about 68 calories. That means when you eat a peach, your body gains about 68 units of energy it can use to fuel physical activity.
Calories can come from three nutrients in food: carbohydrates, fat, and protein. They can also come from alcohol, which isn't a nutrient. As your body digests what you've eaten, it converts carbohydrates, fat, protein, and alcohol into glucose. Your body's cells use glucose to fuel everything you do, from breathing and digesting your food to running, talking, and even reading this article.
Carbohydrates, fat, protein, and alcohol all provide calories, but they supply them in different amounts. Take a look at where your energy is coming from:
Technically speaking, one calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. Calories are a way to measure the amount of energy contained in foods. A peach, for instance, has about 68 calories. That means when you eat a peach, your body gains about 68 units of energy it can use to fuel physical activity.
Calories can come from three nutrients in food: carbohydrates, fat, and protein. They can also come from alcohol, which isn't a nutrient. As your body digests what you've eaten, it converts carbohydrates, fat, protein, and alcohol into glucose. Your body's cells use glucose to fuel everything you do, from breathing and digesting your food to running, talking, and even reading this article.
Carbohydrates, fat, protein, and alcohol all provide calories, but they supply them in different amounts. Take a look at where your energy is coming from:
| Energy Source | Calories per Gram |
Fat |
9 |
Alcohol |
7 |
Carbohydrates |
4 |
Protein |
4 |
It's important to understand that even though fat has more calories per gram than, say, protein, this doesn't mean eating fat will always cause you to gain more weight than eating protein will. It depends on how much fat or protein you eat. For instance, four grams of fat and nine grams of protein both contain the same 36 calories.
So you'll get the same number of calories — about 36 — eating one teaspoon of butter or one cup of asparagus. You could also get those 36 calories from one-third cup of unsweetened applesauce, one-half cup of fresh blackberries, one cup of cooked carrots, one-half cup of tomato juice, or one rice cake. So it isn't just what you eat, but also how large a portion of it you take, that determines how many calories you're getting.
When you take in more calories than your body needs, that extra energy is stored as body fat. This excess energy is what makes the numbers on your scale go up and your size 12 jeans feel as snug as a size 2. The only way to get rid of that stored energy is to burn it — by using up more energy in physical activity than you take in through what you eat.
So you'll get the same number of calories — about 36 — eating one teaspoon of butter or one cup of asparagus. You could also get those 36 calories from one-third cup of unsweetened applesauce, one-half cup of fresh blackberries, one cup of cooked carrots, one-half cup of tomato juice, or one rice cake. So it isn't just what you eat, but also how large a portion of it you take, that determines how many calories you're getting.
When you take in more calories than your body needs, that extra energy is stored as body fat. This excess energy is what makes the numbers on your scale go up and your size 12 jeans feel as snug as a size 2. The only way to get rid of that stored energy is to burn it — by using up more energy in physical activity than you take in through what you eat.
Metabolism: the key to how calories are used
How many calories does your body need? That depends on a number of factors, including your body composition, how active you are, your age, your gender, and how much food you eat.
The rate at which your body uses up energy when it's at rest — maintaining all the involuntary processes that usually go unnoticed, such as perspiration, breathing, and the beating of your heart — is called your basal metabolic rate. These basic processes account for about 60% of the calories your body needs. Another 30% goes into fueling physical activity, such as walking, picking up your children's toys, going out dancing with your spouse, and so on. Digestion uses up the remaining 10%.
Your metabolism — how your body burns the energy it takes in — is very personal, but it isn't static. For example, metabolism tends to go down as you age, so you're likely to need fewer calories as the years go by, even if your level of physical activity stays about the same. Your metabolism can also rise and fall in response to lifestyle changes, such as changes in how many calories you eat or how physically active you are.
In fact, metabolism is more sensitive than you might think. The following factors each play a part in whether you will lose, gain, or maintain your weight. Your body composition Maintaining muscle requires more energy — more calories — than maintaining body fat. That means the more muscle mass you have, the higher your metabolism and the more calories you can eat without gaining weight. Resistance training, such as lifting weights, boosts your metabolism by building more calorie-hungry muscle. How active you are Exercise speeds up your metabolism in two ways. As mentioned above, resistance training increases your metabolism by building muscle, which burns more fuel than fat. Plus, if you exercise long and hard enough, exercise of any kind can also temporarily speed up your metabolism for several hours post-workout. Your age Growing bodies need extra energy to develop, so children burn more calories per pound than full-grown adults do. As we age, we also lose muscle mass. When our energy needs decrease, but our energy consumption (the number of calories we eat) doesn't also go down, the result is weight gain. Your gender Men, who are usually bigger and more muscular than women, tend to need more energy and so have higher metabolisms in general. Significant calorie restriction This is why strict dieting can actually backfire. When you drastically cut how many calories you eat, your body instinctively reacts by slowing down its metabolism. This is your body's natural way of conserving energy when food is in short supply.
The rate at which your body uses up energy when it's at rest — maintaining all the involuntary processes that usually go unnoticed, such as perspiration, breathing, and the beating of your heart — is called your basal metabolic rate. These basic processes account for about 60% of the calories your body needs. Another 30% goes into fueling physical activity, such as walking, picking up your children's toys, going out dancing with your spouse, and so on. Digestion uses up the remaining 10%.
Your metabolism — how your body burns the energy it takes in — is very personal, but it isn't static. For example, metabolism tends to go down as you age, so you're likely to need fewer calories as the years go by, even if your level of physical activity stays about the same. Your metabolism can also rise and fall in response to lifestyle changes, such as changes in how many calories you eat or how physically active you are.
In fact, metabolism is more sensitive than you might think. The following factors each play a part in whether you will lose, gain, or maintain your weight. Your body composition Maintaining muscle requires more energy — more calories — than maintaining body fat. That means the more muscle mass you have, the higher your metabolism and the more calories you can eat without gaining weight. Resistance training, such as lifting weights, boosts your metabolism by building more calorie-hungry muscle. How active you are Exercise speeds up your metabolism in two ways. As mentioned above, resistance training increases your metabolism by building muscle, which burns more fuel than fat. Plus, if you exercise long and hard enough, exercise of any kind can also temporarily speed up your metabolism for several hours post-workout. Your age Growing bodies need extra energy to develop, so children burn more calories per pound than full-grown adults do. As we age, we also lose muscle mass. When our energy needs decrease, but our energy consumption (the number of calories we eat) doesn't also go down, the result is weight gain. Your gender Men, who are usually bigger and more muscular than women, tend to need more energy and so have higher metabolisms in general. Significant calorie restriction This is why strict dieting can actually backfire. When you drastically cut how many calories you eat, your body instinctively reacts by slowing down its metabolism. This is your body's natural way of conserving energy when food is in short supply.
Making peace with calories
Calories aren't bad in themselves — they are merely the "plus" side of your energy equation. They only cause trouble when there are too many of them.
Physical activity is also an important part of the formula — the "minus" side that can help balance the calories you take in. Sheah Rarback, R.D. — spokeswoman for the American Dietetic Association and director of nutrition at the Mailman Center at the University of Miami School of Medicine — likes to say that the problem isn't just overeating, but also "under-exercising."
Your goal should be to watch your energy intake (the number of calories you eat) as well as your energy output (how much physical activity you get). According to the experts — and proven by the simple math — this is the best way to reach and maintain a healthy weight.
Physical activity is also an important part of the formula — the "minus" side that can help balance the calories you take in. Sheah Rarback, R.D. — spokeswoman for the American Dietetic Association and director of nutrition at the Mailman Center at the University of Miami School of Medicine — likes to say that the problem isn't just overeating, but also "under-exercising."
Your goal should be to watch your energy intake (the number of calories you eat) as well as your energy output (how much physical activity you get). According to the experts — and proven by the simple math — this is the best way to reach and maintain a healthy weight.