Blog #2 in Metabolism Series
The concept of energy balance, or weight management, is based on the fundamental thermodynamic principle that energy cannot be destroyed, it can only be gained, lost, or stored by an organism. Many people are familiar with the static energy balance equations to explain weight gain or loss:
· Energy in = energy out = weight maintenance
· Energy in > energy out = weight gain
· Energy in < energy out = weight loss
Energy in is all the food and drink that we intake during the day, while energy out is a combination of our resting metabolic rate (RMR), thermal effect of feeding (TEF), and physical activity and exercise. Energy in vs energy out, is it really that simple for weight management?
Static vs Dynamic Energy Balance Models
There are two views on the relationship of the sides of the energy balance equation. Static or linear energy balance simply assumes that a change in energy intake does not change or influence energy expenditure. A typical misconception associated with static energy balance is the so-called 3,500-kcal rule. One pound of fat corresponds to energy of about 3,500 kcal, and the “3500-kcal rule” claims that, if a negative energy balance of 3,500 kcal is created by eating less and exercising more for one week, weight is slowly and steadily reduced by one pound of body fat. To create a deficit of 3500 kcal in a week, you need to have a negative balance of 500 kcal/day. You hear this 3500 calorie rule over and over, but does it always work?
When data from real-world weight loss studies were applied, the 3,500-kcal rule overestimated weight loss in most cases. The actual weight loss pattern is not linear as the 3,500-kcal rule indicates and has different configurations. On the other hand, the dynamic energy balance equation models result in a curvilinear pattern of weight loss over time and depends on age, sex, height, baseline weight, and degree of caloric restriction, according to the first law of thermodynamics. In other words, differences in the amount of weight gained or lost are dependent upon age, sex, baseline weight, caloric restriction, etc. and cannot be determined strictly by the 3500 calorie rule.
It is commonly assumed that energy intake and energy expenditure can be independently modified, through changes in food intake and physical activity, to achieve energy balance. So, I either can reduce my food intake or I can increase my energy expenditure and I will be able to lose weight. However, energy input and expenditure are interdependent and regulated at several levels. For example, both neural and hormonal signals to the hypothalamus of the brain work to keep us at a constant body weight. Let’s look at how this works:
When calorie intake is reduced, the body (brain) responds by both stimulating hunger and reducing the RMR so that less energy is expended
When energy expenditure or activity increases, this results in increased hunger or decreased activity during some other part of the day.
Both of these scenarios work to keep our weight constant…. BUT despite this internal control system, the majority of adults gain weight over time. Why? Well..It seems that in energy balance terms, the compensation in response to positive energy balance seems to be weaker that the compensation in response to negative energy balance….meaning the body is really good at maintaining or gaining weight. So, when you say I am reducing my calories and/or increasing my energy expenditure and I am not losing weight this is most likely due to additional influences from the brain, hormones, etc. While this can be frustrating, it doesn’t mean that weight loss is impossible.
Next month we will take a look at hormonal influences on metabolic rate.