Calories-In and Calories-Out: Energy Expenditure Models

CALORIES-IN AND CALORIES-OUT: ENERGY EXPENDITURE MODELS

Whether you’re a frequent flyer at the gym, a weekend warrior, or thinking about starting to workout, you’ve no doubt been faced with an energy expenditure scenario. Energy expenditure is the rate at which our bodies burn calories, and is connected to the rate at which we consume calories.

So, most of us try to make equations for ourselves:

“If I burn 200 calories at the gym, I get an extra 200 calories today.”

“If I burn an extra 500 calories working out each day, I’ll lose a pound this week.”

And some of us may even ponder things like:

“The more I train, the more ice cream I can get away with tonight…”

All of these calculations are well-intentioned, and guided by common attitudes in the fitness industry. But accurately calculating what’s really happening in our metabolism is a bit like trying to shoot a moving target. As you can see in the image at the top of the page, the human metabolism is probably far more complex than we’d like it to be. Ah, the beauty of science.

The human body regulates itself at incredibly high levels, so tricking it into making us shed body fat and get six-pack abs is probably going to be a bit harder than the general accounting equations that we’re told to follow. We need to assume (to a degree) that the body wants homeostasis.

Ho·me·o·sta·sis /hōmēəˈstāsəs/ [noun]: the tendency toward a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes.

In other words, the body doesn’t seem to jump for joy when we try to get it to change–physiologically or psychologically. In fact, it might be downright stubborn, because change means extra work. And from an evolutionary standpoint, why work extra hard (and burn extra calories) if you don’t have to? After all, you never know when you might need those calories. And with a preference for homeostasis comes an ability to be quite flexible.

Human beings are primates that evolved to hunt, acquire, and metabolize energy from food and then use the available energy sources (calories + stored fat) to successfully reproduce. Those that could do this best were likelier to survive. Flexibility is an important feature of this biological performance because it would allow our ancestors to adjust as energy availability waxed and waned with varying food resources and metabolic demands. Confounding variables such as pregnancy, lactation, growth, injury, and periods of inactivity or intense physical activity all had to be accounted for. If one couldn’t be flexible enough to maintain homeostasis, they could die quickly.

So how does the body seem to burn energy? There are two main models:

The Additive Energy Expenditure Model

and

The Constrained Energy Expenditure Model

These two models have been used as tools to investigate how the human body adapts to metabolic stimulus. Does more exercise burn more calories? Or do our bodies regulate themselves so that energy expenditure stays the same?

Let’s look at what the research says…

The Additive Energy Expenditure Model

The additive energy expenditure model is the model that assumes that exercise burns more calories. This model is the more well-known of the two models, and is heavily promoted in the fitness industry.

According to the additive energy expenditure model, we exercise to burn more calories because that will either:

1. Make us lose stored body fat

2. Allow us to eat more calories without gaining weight

Additive models of energy expenditure view total energy expenditure (TEE) simply as a product of body size and physical activity. It’s often calculated like a math equation and assumed to be accurate–which is why people get frustrated when they’re exercising more and not losing weight.

The Constrained Energy Expenditure Model

The constrained energy expenditure model is one that accounts for potential changes in energy allocation in response to variation in activity levels. This model argues that the body regulates itself with such flexibility and complexity, that more exercise does not necessarily equal more calories burned or weight lost, and thus also does not mean that we can eat far more because of activity levels.

“In an additive energy expenditure model, the energy spent each day on non-physical activity physiological activity (i.e., on organ systems other than the musculoskeletal) is fixed and does not change, regardless of variation in physical activity. Contrarily, in a constrained model non-physical activity energy expenditure adapts dynamically to variation in activity in order to maintain total energy expenditure within some narrow physiological range. In both cases, during the long-term, mean total energy expenditure must equal mean food energy intake (accounting for digestive efficiency) for organisms to maintain weight stability and viability.” -Pontzer 2015

The Constrained Energy Expenditure Model is More Accurate

Recent studies have shown that energy expenditure does increase when there are increases in small amounts of exercise; but that intense, regular exercise does not correlate with drastically increased energy expenditure. In other words, a body going from no exercise to some exercise will increase energy burned, but eventually energy expenditure will plateau.

However, this does not in any way suggest that there are not benefits to physical activity. The profound effects of exercise on health markers, happiness, and longevity are undisputed.

But the research on energy models does tell us two important things:

1. We can’t out-train bad diets. Consuming an appropriate, nourishing, balanced diet is an essential element to weight regulation.

2. Energy turnover is critical to physiological regulation. Physical activity helps our bodies regulate our energy intake and expenditure properly.

The constrained energy model research demonstrates that physical activity is essential for appetite regulation. Having high energy turnover from physical activity is a key to appetite control, long-term weight loss, and vitality. A lack of movement can result in appetite dysregulation and an uncoupling between energy intake and expenditure. These events can lead to issues like leptin and insulin resistance, weight gain, and metabolic disorders. Metabolic dysregulation undoubtedly can lead to obesity–which under the assumed circumstances of inflammation, reward addiction, and lack of movement–is difficult to change because the body and mind both balk at the new work and behaviors.

Copious amounts of movement–whether scheduled exercise or long walks, hikes, sports, kayaking, and other favorite activities–combined with whole foods are the stimulus and nourishment our bodies need in order to properly regulate its energy intake and expenditure. While sweating for an extra long time at the gym in order to “earn” an ice cream would be a little easier to follow, we have to appreciate and respect the body’s incredible ability to regulate itself at higher levels. Because of this, we have to avoid the inciting events of metabolic disease in the first place–namely spending time on screens instead of out in the sun and trusting corporations to make us meals instead of ourselves.

CALORIES-IN AND CALORIES-OUT? ENERGY EXPENDITURE MODELS