Nutrition Science and Everyday Application - beta v 0.1

BEST PRACTICES FOR WEIGHT MANAGEMENT 421
thousand participants published in the November 2011 issue of the International Journal of
Obesity estimated that more than one in six Americans (17 percent) who were overweight
or obese were successful at both achieving and maintaining a significant level of weight
loss. 4 While this estimate is more promising than other studies suggest, it still raises the
question: Why is achieving long-term weight loss so difficult? Much of the explanation lies in
understanding the biology of weight loss.
Weight loss has often been viewed as a simple formula: energy in versus energy out. If
you consume more calories than you expend, you gain weight. If you expend more calories
than you consume, you lose weight. This is the general principle of energy balance, as
discussed earlier in this unit, and this principle gives foundation to the basic premise of
weight management.
However, the body is more complex than a simple formula. And much like many functions
within the body, weight is tightly regulated. In order to prevent perpetual weight loss or
weight gain every time environmental or behavioral factors change, mechanisms within the
body adjust to help normalize weight at a steady point. 5 But our obesogenic environment
often promotes behaviors that encourage excessive caloric intake and lower energy
expenditure, leading to a higher steady weight over time. When an individual focuses on
losing weight, active weight loss efforts often yield initial weight loss. But those same
mechanisms that work to maintain a steady weight also kick in during periods of weight loss
to help the body defend the original weight. 5 The body recognizes weight loss as a threat to
survival, lowering basal metabolic rate to preserve calories and protect against starvation.
Additionally, as someone loses weight, there is less physical mass to the body that has to be
moved from place to place throughout the day, resulting in fewer calories burned through
physical movement and activity, and less metabolically active tissue using calories for fuel
throughout the day.
Biological differences in individual metabolism may also impact weight loss success.
Researchers have found that some individuals have a “thrifty” metabolism, meaning that
they have a lower metabolic rate and expend significantly fewer calories when in a fasting
(or calorie-restricted) state, common in weight loss efforts. This results in a lower level of
weight loss. In contrast, individuals with a “spendthrift” metabolism tend to have a higher
metabolic rate in a fasting state, burning more calories and thus yielding bigger weight loss
results. 6 According to researcher Martin Reinhardt, M.D., “The results corroborate the idea
that some people who are obese may have to work harder to lose weight due to metabolic
differences.” 7
Figure 7.24. Illustration of the concept of spendthrift and thrifty metabolisms, characterized by
their response to overfeeding and fasting.