13.2: Physical Activity

While we can’t make drastic changes in basal metabolism, it goes without saying that we can make big changes in physical activity. In fact, during adulthood (except for pregnancy and lactation), being more active is virtually the only way we can use substantially more calories under normal circumstances. It’s important to keep several points in mind when discussing the calories used in various activities:

• Calories used in activities are often given as specific numbers (e.g., 100 calories for walking a mile). This can be misleading. Calories used for a weight- bearing exercise like walking, depend not only on how far we walk, but on how much we weigh (i.e., how much weight is moved).

Calories used when walking are also affected by other factors: speed, walking surface (e.g., sand or pavement, uphill, downhill, or flat), distribution of body weight, physical fitness, etc. In non-weight-bearing exercise, other factors (e.g., buoyancy in water when swimming) make a difference.

A 120-lb person uses about 70 calories walking a mile in 20 minutes, but someone 200 lbs uses about 105 calories walking that same mile. (Good reason to carry an infant in a baby carrier— or books in a backpack—if you’re looking to use more calories.) This also means that if you lose weight, you use fewer calories for that same walk—unless you balance your weight loss with an infant of the same weight, or add a counterbalancing number of books to your backpack.

• It’s important to distinguish between calories used during an activity and calories that the activity itself uses. Energy used during exercise includes calories used for the activity and the calories used for basal metabolism. For example, Sue (140 lb) walks 1½ miles in a half hour, and uses about 150 calories during that time—about 120 for walking and about 30 for basal metabolism. In business terms, it is 150 gross, 120 net. Someone “selling exercise” is more likely to use the 150-calorie number.
• People vary in how many calories they use in an activity (e.g., playing tennis). Even in sleeping, some people lie still the entire night, whereas others toss and turn. In listening to a lecture, some people sit on the edge of their seats, using more calories than others who are almost asleep—or actually are!

Table 13-1 gives a rough estimate of the calories used for various activities by someone weighing 140 pounds. Keep in mind that when an activity is strenuous, it’s hard to sustain—we stop to catch our breath.

Table 13-1: Calories Used per Minute for Activities of a 140-lb Person

Weightlifting, for example, is very hard work. In an hour spent working out at a gym, a lot of it may be spent resting between lifts and strolling from one piece of equipment to the next. We tend to overestimate our physical activity (and underestimate how much we eat).

Generally, you can estimate the calories spent per day on physical activity as a percent of basal metabolism:

• Sedentary (mostly sitting, typing, etc.; no routine, strenuous activity) = 50% (If 1400 cal for basal metabolism, energy for physical activity = 0.50 X 1400 = 700 cal)
• Lightly active (laboratory work, cooking, etc.; no routine, strenuous activity) = 60%
• Moderately active (carpentry, housework, etc.; very little sitting) = 70%
• Very active (unskilled labor, dancing, running; very little sitting) = 80%
• Strenuously active (professional football players/strenuous training, etc.) = >80%