3.1: How Many Calories Do We Need?

As we’ve seen, the energy need for most people is mainly governed by their basal metabolic rate (BMR). We also saw in the last chapter how genetics and gender affect basal energy needs, through differences in body type, shape, and composition.

A number of other factors affect our use of energy. For example, the older we get, the less fuel we need. Much of this is due to changes in body composition and chemistry, but a decline in physical activity can also play a big part.

Exposure to extremes of climate also has an effect. If the body is exposed to either high or low temperatures, it must spend energy to compensate, for the body must stay at a fairly even temperature to survive. One has only to remember how bad one feels with a fever—an extra couple of degrees of body temperature. The body becomes seriously threatened when its internal temperature is changed by more than a few degrees.

In cold environments, the body must spend more energy to generate heat, often by shivering— a way to use vigorous muscle activity to generate warmth. If the weather is hot, the cooling processes take energy, in pumping more blood to the skin surface, so that heat can escape. Even sweating takes energy.

And there are endless subtleties. Energy needs are conserved by wearing several layers of clothing in cold weather because this reduces loss of body heat. But then, energy needs are increased by the added weight of the clothing and the drag that the clothing exerts upon movement. Then again, energy needs are reduced when bulky clothing—or hot weather—restricts our physical activity.

Thus, energy needs depend on a complex interrelationship of many variables, such as physical activity, body size and composition, age, and climate. In general, while it’s common to speak of energy balance as a rather tidy and precise matter, it’s neither tidy nor precise.

What this means is that even scientists can’t predict energy use, storage, etc., as accurately as they’d like. But this doesn’t mean that we can’t predict and plan energy use in approximate ways which have great practical value.

Making a Personalized Energy Budget

There are quick ways to estimate your energy budget, e.g., the online Body Weight Planner niddk.nih.gov/bwp approximates calories/day to maintain your current weight, in addition to calories/day to lose weight. For instructional purposes, let’s look at the components in more detail.

Calculating Your Basal Energy Needs

There are many ways to approximate basal metabolic rate (BMR), some of which are quite complex. One can begin by measuring body size and using BMR tables which show averages. But however carefully these averages are determined, they encompass a rather broad range, with variations of 20% and more. This is a huge variation when we consider the fact that small variations in energy balance can make a dramatic difference in long-term weight gain or loss.

Without a basal metabolism test, you might still get a fairly good idea of your daily energy need for basal metabolism from a simple rule of thumb:

• For women: Add a zero to your weight in pounds. Add to the result, your weight in pounds.
• For men: Add a zero to your weight in pounds. Add to the result, twice your weight in pounds.

By this rule, a 128-pound woman adds a zero, making 1,280 calories, and then adds her weight (128), for a total of 1,408 calories a day.

Correcting for Age: The rule-of-thumb calculation makes no allowance for age, and BMRs change with age. The changes reflect of a decline in muscle, and changes in various hormones.

To correct your BMR for age, simply reduce it by 2% for each decade above age 20. Thus, a man of 30 with a basal need of 2,000 calories would subtract 40 calories (.02 x 2,000 = 40) from his basal need.

Basal metabolic rates of children are a special matter, especially those of infants, and the energy calculating methods shown here don’t work for them. Among other things, their body proportions are different from adults, and the factor of growth enters in. The small, growing body uses an impressive part of its fuel as building material. This is one reason why children can take in so many more calories for their weight than adults can.

Correcting for Body Shape and Composition: Going back to the rule-of-thumb calculation of basal need, there’s a simple way to make this reflect our individual body shape and our fat-lean composition. It goes like this:

• If you’re thinner than average, add 5% to what you’d otherwise calculate to be your total basal need. You need more calories for your weight.
• If you’re pudgier than average, so that you might describe yourself as “plump,” then you are probably some 5% higher in fatness. So subtract 5% from your basal calorie need.
• If you’d have to describe yourself as “fat,” or about 10% higher in fatness than the average, subtract 10% from your basal calorie need. (Note that % of greater fatness is not the same as % above desired weight.)

Adding Calories for Physical Activity

To our basal calorie need, add calories for physical activity. In the last chapter we were given some idea of the calories needed for various activities. But even if we could measure to the minute how we spend our days, it’s hard to use activity tables to estimate the number of calories we need for physical activity on a day-to-day basis. Also, we tend to overestimate how active we are.

Keep in mind that when an activity is strenuous, it’s hard to sustain that activity for long —we have to stop to “catch our breath.” One might spend an hour “working out,” but most of that time may be spent resting between lifts and strolling from one piece of exercise equipment to the next. Weightlifting, for example, is hard work, but a lot of time is spent resting between lifts.

In reality, the calories we need in a day is the amount of calories we take in when we are neither losing nor gaining weight. Of course, what this amount is, is hard to figure. Practically speaking, all we can do is use another rule-of-thumb to estimate the calories expended per day above basal needs, based on our general level of activity:

• Sedentary (mostly activities like reading, keyboarding): Add 50% to basal calorie needs (multiply your corrected basal need by 0.5).
• Lightly active (mostly activities like lab work, cooking): Add 60% to basal calorie needs (multiply your corrected basal need by 0.6).
• Moderately active (mostly activities like carpentry, housework): Add 70% to basal calorie needs (multiply your corrected basal need by 0.7).
• Very active (mostly activities like unskilled labor, running, dancing): Add 80% to basal calorie needs (multiply your corrected basal need by 0.8).
• Strenuously active (like professional athletes during training): Add 90+% to basal calorie needs (multiply your corrected basal need by 0.9-1.0).

For a sedentary person with a basal calorie need of 1,400 calories, the calories for physical activity would be 700 (0.50 x 1400 = 700), for a total of 2100 calories. An additional 10% (210) is added for the digestion and assimilation of food, making a total calorie need of about 2,310 (1,400 + 700 + 210) calories per day.