To hear people talk, you'd think metabolism was the only factor that comes into play when determining body weight. A skinny person may shrug off their ability to polish off a burger and large fries and never gain an ounce with "I have quick metabolism," while an overweight person may say slow metabolism is to blame for their extra body weight.

But metabolism is simply the process by which your body converts the food you eat into the energy your body needs to fuel you. In truth, it's the number of calories you consume compared to the amount of energy you burn that determines whether your weight stays the same or whether you gain or lose pounds.

So how can you determine that magic number?

For starters, think about the things your body needs energy for. Even if you were to lie in bed all day and not move, your body would still need energy. Just the acts of breathing, circulating blood through the body, and growing and repairing cells require fuel. The number of calories your body uses to accomplish just these things is called your basal metabolic rate. For most people, these basic processes account for most of the calories you burn every day. Physical activity and food digestion burn calories above and beyond this basic amount.

Basal metabolic rates vary from person to person, and there are a number of factors that can influence how many calories your body needs to function.

  • Body size: Just as an SUV requires more fuel to run than a regular sedan, bigger bodies require more calories for fuel than smaller ones. As a result, your height and weight will affect the number of calories you burn.
  • Body composition: Muscle requires more energy than fat, so the more muscle you have in relation to fat, the more calories your body needs to function.
  • Age: Metabolism naturally slows as you age. As well, many people lose muscle and gain fat as they age, contributing to the slowdown.
  • Gender: Because men usually have more lean muscle mass than women, their metabolism tends to be slightly higher.

The number of calories you need per day depends on your basal metabolic rate, the number of calories you burn through physical activity, and the number of calories your body takes to break down food and absorb nutrients.

The calculation's simple. If you want to maintain your weight, you need to burn the same number of calories per day as you consume. To lose weight, you need to burn more calories than you consume, while to gain, you need to consume more calories than you burn.