Nutrition Science and Everyday Application - beta v 0.1

ENERGY BALANCE: ENERGY IN, ENERGY OUT—YET NOT AS SIMPLE AS IT SEEMS 379
brain does not respond as well to it. Therefore, when you administer leptin to an overweight
or obese person, there is generally no sustained effect on food intake. 3
Figure 7.7. The structure of the hormone leptin (left), which is primarily produced by adipose
tissue. The obese mouse in the photo has a gene mutation that makes it unable to produce leptin,
resulting in constant hunger, lethargy, and severe obesity. For comparison, a mouse with normal
leptin production is also shown. Such gene mutations are rare, but they serve as a dramatic
illustration of the importance of the hormone in signaling energy balance.
THE COMPLEXITY OF ENERGY BALANCE
Energy balance seems like it should be a simple math problem, and in fact, it is based on a
fundamental truth in physics—the first law of thermodynamics. This law states that energy
can’t be created or destroyed; it can only change form. That is, calories that are consumed
must go somewhere, and if they aren’t metabolized (which converts caloric energy to heat
and work energy), they’ll have to be stored, usually in the form of adipose tissue. What makes
energy balance challenging is the reality that both energy intake and energy expenditure are
dynamic variables that are constantly changing, including in response to each other. 4,5
Let’s first look at the energy intake side. As we’ve already discussed, how much food we
eat each day is not just a matter of willpower or self-control. It’s the result of powerful
physiological and psychological forces that tell us if we need to eat, or if we’ve had enough.
Our brains are hard-wired to seek food if we’re in negative energy balance, an instinct
required for survival. This means that if you start to exercise more—increasing your energy
expenditure—you will also feel hungrier, because your body needs more fuel to support the
increase in physical activity. If you eat fewer calories, perhaps in an effort to lose weight, your
stomach will produce more ghrelin, and your adipose tissue will produce less leptin. These
shifting hormone levels work together to increase hunger and make you focus on obtaining
more calories. People who try to gain weight run into the opposite problem. Their leptin
levels increase, suppressing hunger. It’s also uncomfortable to eat beyond satiety, and food
doesn’t taste as good.
Even measuring how much energy is consumed is not as simple as you might think. We can
measure the caloric content of food from a chemical standpoint, but we can only estimate
how much energy a person will absorb from that food. This will depend on how well the food