We all know people who seem to be able to eat anything they wish and never gain weight. Likewise, there are those who “merely have to look at food” to put on pounds. It turns out there is scientific support for these subjective observations. Studies that have looked at weight gain in response to overfeeding have shown that people vary greatly in how much fat they accumulate. The biological mechanism that allows some individuals to resist weight gain more than others, however, has not been identified.

In 1999, Levine set out to solve the mystery. He and a team of researchers at the Mayo Clinic in Rochester, Minnesota, recruited 16 nonobese adults (12 men and 4 women from 25 to 36 years of age). These individuals underwent measures of body composition and energy expenditure before and after eight weeks of supervised overfeeding by 1,000 kcal per day.

The researchers found that fat gain varied tenfold among individuals in the study, ranging from a gain of only 0.36 kg to a gain of 4.23 kg (1 kg = 2.2 lb). Why was the weight gain so disparate? One obvious possibility is each person expended a different amount of energy.

A person’s total energy expenditure (TEE) can be divided into three main components: (1) basal metabolism, (2) the thermic effect of food, and (3) activity energy expenditure.

Basal metabolism is the energy expenditure required to maintain the essential functions that sustain life. This energy is required for the chemical reactions in our cells, the maintenance of muscle tone, and the work done by our heart, lungs, brain, liver, and kidneys—with much of this work depending on the active transport of electrolytes and other nutrients in our cells. In research studies, basal metabolism is measured while the person is lying completely still but awake, at a comfortable temperature, following an overnight fast, and without any physical activity for the preceding eight hours. For most individuals, basal metabolism is the largest component of total daily energy expenditure, accounting for about 60% of TEE in a typical sedentary individual.

Thermic effect of food (TEF) is the energy needed to digest, absorb, and metabolize nutrients in our food. TEF is generally equivalent to 10% of the energy content of the food ingested and does not vary greatly between people. Activity energy expenditure (AEE) is the amount of energy individuals expend in physical activity per day, both planned and spontaneous. It includes all the energy expended in the contraction of skeletal muscles to move our body and to maintain posture (sitting or standing versus lying down). AEE is the most variable component of TEE. (INFOGRAPHIC 15.5)

Question 15.4

Though people can and do vary in the amount of energy expended by their basal metabolism, having a “low metabolism” is not a widespread cause of obesity. Nearly all of the variation in basal metabolism from person to person is accounted for by individual differences in fat-free mass (FFM), which is total body mass minus the fat mass (adipose tissue). The greater a person’s FFM, the higher his or her basal metabolic rate (BMR) will be. Although increasing fat mass will also increase BMR to some degree, adipose tissue is far less metabolically active than other tissues. Our organs, such as the brain, kidney, heart, and liver, have the highest metabolic activity, but even at rest, skeletal muscle is about three times more metabolically active than adipose tissue. What this means is that a lean individual expends more energy at rest than someone of the same weight who has more body fat. Proportionally higher fat mass and lower FFM in women nearly completely accounts for the lower BMR in women than in men. People also tend to have higher fat mass as they get older, which accounts for the decline in BMR that generally occurs with age. Some external factors, such as how much caffeine we consume and whether we smoke cigarettes can also affect BMR. (INFOGRAPHIC 15.6)

Question 15.5

In Levine’s study, he found only very minor increases in BMR and TEF in his study participants, which was not enough to account for the tenfold variance in fat gain among them. By contrast, levels of physical activity varied markedly between study participants. Interestingly, intentional exercise was not the crucial difference in activity level.

Levine designed his study in such a way that exercise was kept at a constant and minimum level across all study participants. Therefore, the differences in physical activity were principally non–exercise-related. Levine has a name for this type of activity: he calls it NEAT, short for nonexercise activity thermogenesis.

NEAT includes all the activities of daily living, such as household chores, yard work, shopping, occupational activity, walking the dog, and playing a musical instrument. It also includes the energy expended to maintain posture and spontaneous movements such as fidgeting, pacing, and even chewing gum. In other words, NEAT encompasses all the activities we do as part of daily living, separate from planned, intentional exercise like going for a run, taking an aerobics class, or hopping on an exercise bike.

In Levine’s study, NEAT varied greatly among individuals (fewer than 100 kcal to more than 700 kcal per day). More important, changes in NEAT were inversely correlated with changes in weight gain. In other words, NEAT proved to be the principal mediator of resistance to fat gain in these individuals. (INFOGRAPHIC 15.7)

Question 15.6

Question 15.7

Next, Levine wanted to know if NEAT plays a role in obesity. Do lean and obese people differ in their levels of NEAT, for example? To get at this question, Levine and his colleagues recruited 20 healthy volunteers who were self-proclaimed “couch potatoes.” Ten participants (five women and five men) were lean and 10 participants (also five women and five men) were mildly obese. The volunteers agreed to have all of their movements measured for 10 days. They were instructed to continue their normal daily activities and not to adopt new exercise regimens.

To measure NEAT, Levine and colleagues devised a novel way to track activity levels in test volunteers. They built a special kind of underwear outfitted with electronic sensors that detect movement. The undergarments were built to allow people to wear them essentially all the time—even while going to the bathroom and having sex. “We literally have snapshots as to how real people live their lives every half second of every day for days and days and days on end,” says Levine.

From those measurements, they can then calculate how many calories a person expends per day. Over the 10-day period, Levine’s team collected 25 million data points on NEAT for each individual.

The bottom line? Obese people sit on average 2.25 hours longer than their leaner counterparts. Moreover, this tendency seems to be innate: Lean individuals sit the same amount of time even after they are forced to gain weight, and obese individuals sit the same amount of time even after they are forced to lose it.

What does all this have to do with obesity? Levine thinks that it’s not only that people are eating more than they did a few decades ago, but also that we have been “seduced” (his word) by our environment into expending less energy by sitting more.