The optimal number of calories you need to maintain a healthy weight is based on how much energy you need when you are resting and how much energy you use when you are working. This amount varies for people of different sexes, ages, activity levels, and weights.

All foods consist of three main nutrient groups: carbohydrates, proteins, and fats. Each of these is converted to usable energy, which is measured in calories.

Calories basically are a unit of measuring heat energy. It takes 1 calorie to raise the temperature of 1 gram of water 1 degree Celsius (opposed to Fahrenheit). The calories that you count, however, are really called kilocalories. Kilo- means 1,000, so kilocalories are larger units than calories. It takes 1 kilocalorie to raise the temperature of 1 kilogram (1,000 g) of water 1 degree Celsius.

The energy of a kilocalorie is used to measure the amount of energy that can be provided by foods. Each gram of carbohydrate or protein contributes 4 calories. But each gram of fat contributes 9 calories. Therefore, if you eat fewer grams of fat, you take in fewer calories.

For overall health, you need carbohydrates, proteins, and fats in the right proportions. The guideline is 60 percent of your calories from carbohydrates, 30 percent of your calories from fats, and 10 percent of your calories from proteins.

## How to calculate your basal metabolic rate (BMR)

You can determine the amount of calories you need just to cover the basics of what your body does all day long (breathing, heart beating, digesting, cellular processes, etc.). This amount is called the basal metabolic rate (BMR).

Follow these steps to determine your BMR:

1. Divide your weight (in pounds) by 2.2 to convert it to kilograms.

2. Insert your weight (in kg) into the applicable equation in the following table to determine your BMR.

This number does not account for your activity level (such as if you workout two days per week or six days per week, take leisurely strolls around your neighborhood once in a while, run marathons, garden, play tennis, or golf regularly).

Equations to Determine Your Basal Metabolic Rate
Males Equation
10–18 years old (17.5 x weight in kg) + 651
18–30 years old (15.3 x weight in kg) + 679
30–60 years old (11.6 x weight in kg) + 879
>60 years old (13.5 x weight in kg) + 487
Females Equation
10–18 years old (12.2 x weight in kg) + 746
18–30 years old (14.7 x weight in kg) + 496
30–60 years old (8.7 x weight in kg) + 829
>60 years old (10.5 x weight in kg) + 596

Source: The National Research Council, Recommended Dietary Allowances (Washington, D.C.: National Academy Press, 1989).

So, if your BMR is 2,000, it means that those 2,000 calories provide enough energy for all the metabolic functions that occur to keep you alive and functioning well. If, however, you take in 3,000 calories, the extra 1,000 calories are stored as fat in your adipose tissue. But, if you take in less than 2,000 calories per day, you will lose 1 pound for every 3,500 calories you do not consume.

## How to calculate your body mass index (BMI)

Whether you need to lose weight is based more on your body mass index (BMI).

Follow these steps to determine your BMI:

1. Divide your weight (in pounds) by your height (in inches) squared.

2. Multiply the result from Step 1 by 705.

The value you arrive at is your BMI.

A value of approximately 21 is a great goal. If your value is higher than 28, you have a definite increased risk of diseases such as diabetes and heart disease and should consider lowering your weight (since you can’t increase your inches to lower the BMI).

Every 3,500 extra calories equals 1 pound of fat. Since most exercise uses 350 to 500 calories per hour, that works out to 7 to 10 hours of exercise needed to lose one pound. While you are exercising to use up those extra calories, you cannot be taking in more calories than you need, or your weight will stay the same. If you prefer to do less exercise, you can take in less food (fewer calories) than you need, so that your body uses up some of its stored energy.