Metabolic Rate and Weight Loss

Karen Frazier
doctors’ scale
Good Healthy Food Diets

If you're on a diet, it's important to know "how many calories should I eat to lose weight?" The answer actually depends on quite a few factors, including your metabolic rate.

What Is a Calorie?

A calorie is a unit of energy. Calories are most commonly associated with food, but calories can refer to anything that contains energy. Calories in food refer to kilocalories (1000 calories). In the case of food, calories are units of energy used by your body.

Calories and Weight Gain or Loss

It is commonly accepted that if you eat more calories than you burn, you will gain weight. Conversely, if you eat fewer calories than you burn, you lose weight. In most cases, this simple formula works. Sometimes, however, it's hard to determine what your individual caloric needs are, and much depends on an individual's metabolism.

Metabolic Rate

Do you know someone who barely exercises, eats like a pig and never gains an ounce? Have you ever gone on a diet with your husband and watched him lose huge amounts of weight while your weight creeps off in spite of the fact that he is eating a lot more and exercising a lot less than you? Do you know someone who barely eats and still gains weight? All of these can be explained by variations in metabolic rate.

There are a number of factors that affect metabolic rate. These include:

  • Body size: In general, the larger your body is, the more calories you need to eat to maintain your weight. This explains in part why, as you get closer to your ideal weight, you need to eat fewer calories to continue weight loss.
  • Body composition: Your body composition is how much lean body mass you have versus how much body fat you have. Lean body mass, especially muscle, burns more calories than fat, so having more muscle mass can help to boost your metabolism and allow your body to burn more calories.
  • Gender: It's not good news for women, but women naturally burn fewer calories than men do. There are a number of reasons for this, including hormonal factors and lean body mass.
  • Activity Level: The more active you are, the more calories you burn. This includes not only formal exercise, but also factors like how much movement you engage in at work, fidgeting and other types of movement. Those who work at a desk job and then come home and then come home and sit in front of the television burn fewer calories than those with active jobs or people who engage in sports in addition to an active lifestyle.
  • Age: Starting at about 30 years of age, metabolic rate generally declines. This is because of changes in hormones and body composition.
  • Hormones: Your hormones control the rate at which your body burns calories too. Hormonal disorders such as thyroid disorders or excess cortisol can cause the body to burn fewer calories.
  • Dietary Habits: Eating smaller, more frequent meals and eating foods that are nutritious for the body are associated with a higher caloric burn rate.
  • Heredity: Some aspects of metabolism are hereditary. If your parents had quick metabolisms, it is more likely that you, too, will have a quicker metabolic burn rate.
  • Stress: Stress and anxiety cause hormonal changes, which can lead to a slower metabolic rate.
  • Medications: Many medications can influence your metabolism. Caffeine, for instance, has been shown to slightly elevate a metabolic burn rate, while certain anti-depressants and anti-anxiety drugs have been shown to slow metabolism.

How Many Calories Should I Eat to Lose Weight?

Now that you have an idea of factors that affect your metabolic rate, it is time to answer the question "how many calories should I eat to lose weight?" A basic formula can be used to calculate your body's minimal caloric needs, also known as your basal metabolic rate (BMR). This is the minimum number of calories that your body needs to sustain your day to day life functions. You should never eat fewer than your basal metabolic rate, or your body could go into starvation mode and start burning fewer calories in order to conserve.

Calculate Your Basal Metabolic Rate

  • To calculate BMR for women: BMR (calories your body needs each day) = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)
  • To calculate BMR for men: BMR (calories your body needs each day) = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) - (6.8 x age in years)

Factor in Your Activity Level

After calculating your BMR, you must factor in your activity level. One way to do this is to use the Harris Benedict Equation, which is as follows:

  • If you get little or no exercise: Daily Caloric Requirement (DCR) = BMR x 1.2
  • If you do light exercise 1-3 days per week: DCR = BMR x 1.375
  • If you engage in moderate exercise 3-5 days per week: DCR = BMR x 1.55
  • If you engage in intense exercise 6-7 days per week: DCR = BMR x 1.72
  • If you exercise intensely or have a very physical job: DCR = BMR x 1.9

Calculate Your Caloric Needs to Lose Weight

To determine how many calories you need to eat to lose weight, take your daily caloric requirement from the Harris Benedict Equation above and subtract 300 to 500 calories. Eat between your basal metabolic rate calculation and your DCR.If you eat this number of calories and still have difficulty losing weight, you may want to check with your physician to see if any of the above metabolic factors come into play.

Metabolic Rate and Weight Loss