Body Mass Index and calories

by Jonny Bowden, M.A.

I'm 5 foot 8 inches, nine stone and four pounds and 28 years old. My body mass index (BMI) is supposedly 20. How many calories should I limit myself to in order to lose five pounds?

answer

First of all let me say that, based on what you told me, you don't need to lose weight. But, you probably don't want to hear that. Remember that what you may not like about your body probably has a lot more to do with body composition than body weight. With a lower percentage of body fat and a higher percentage of muscle, you’d look very different than you would with more body fat and less muscle, even at the same weight. You'll probably like the way you look more if you work on shaping your body with resistance exercises (using weights or machines), which will not only make you look leaner but will also help increase your metabolism and burn calories more efficiently, even when you’re at rest.

So, just to summarise, calories are important, but they are not the whole picture. What you eat, the mixture, the timing and your hormonal response to food are all important factors in the weight-gain equation. Your metabolic rate is one of those factors, and it is mainly determined by how much muscle you have. Muscle is metabolically active and burns calories while fat is basically inert.

Two main factors contribute to the number of calories a person burns a day:

The number-one, most important contributor is what's called Basal Metabolic Rate (BMR): this is the number of calories it takes to keep you alive. The BMR of a woman at your weight is about 1,300 calories. The health calculator has more information on this.
The second-biggest contributor to calorie expenditure is activity level. Activity calories could be anywhere from 400 a day for a sedentary lifestyle, right up to more than twice the BMR, depending on how much exercise you do.

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Body Mass Index and calories