Nutrition Science and Everyday Application - beta v 0.1

376 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
BMR depends not only on body composition but also on body size, sex, age, nutritional
status, genetics, body temperature, and hormones (Table 9.1). People with a larger frame
size have a higher BMR simply because they have more mass. On average, women have a
lower BMR than men, because they typically have a smaller frame size and less muscle mass.
As we get older, muscle mass declines, and therefore BMR declines as well.
Nutritional status also affects basal metabolism. If someone is fasting or starving, or even
just cutting their caloric intake for a diet, their BMR will decrease. This is because the body
attempts to maintain homeostasis and adapts by slowing down its basic functions (BMR)
to help preserve energy and balance the decrease in energy intake. This is a protective
mechanism during times of food shortages, but it also makes intentional weight loss more
difficult.
Factors That Increase BMR
Higher lean body mass
Larger frame size
Younger age
Male sex
Stress, fever, illness
Elevated levels of thyroid hormone
Factors That Decrease BMR
Lower lean body mass
Smaller frame size
Older age
Female sex
Starvation or fasting
Lower levels of thyroid hormone
Pregnancy or lactation
Stimulants such as caffeine and tobacco
Table 7.1. Factors that Impact BMR
2. Thermic Effect of Food (TEF)
This is the energy needed to digest, absorb, and store the nutrients in foods. It accounts for
5 to 10 percent of total energy expenditure and does not vary greatly amongst individuals.
3. Physical activity
Physical activity is another important way the body expends energy. Physical activity usually