Human metabolism and the calorie conundrum

‘Energy in minus energy out’ is not as simple as it may seem, with a host of factors affecting both parts of the equation, writes dietitian and sports nutritionist Brian St. Pierre.


THE QUICK READ

  • The Energy Balance Equation of ‘Energy in - Energy out’ is commonly used for calculating weight loss, or gain, over time
  • People’s frustrations at failing to lose weight despite following the equation is due in part to inaccurate nutrition information labels, and in part to variations in how much energy we actually absorb and how much energy we use
  • The factors that affect absorption include how processed the food is; how the food has been prepared or cooked; and our unique gut bacteria
  • The amount of energy we use is affected by four main factors of resting metabolic rate; thermic effect of eating; physical activity; and non-exercise activity thermogenesis
  • Altering any one of the variables causes adjustments in other, seemingly unrelated variables.

Gaining and losing fat changes the way your brain regulates your body weight. To understand this, let’s have a refresher on how human metabolism actually works and explore what that means for your clients, most of whom are probably counting calories.

Energy balance

You need a certain amount of energy (in the form of calories) to stay alive, as well as to move around. You can get this energy from food, or you can retrieve it from stored energy, such as your fat tissue. If you consume less energy than you expend, you will lose weight – and if you do the opposite, i.e. consume more energy than you expend, you will gain weight.

In other words: Changes in bodily tissues = Energy in - Energy out.

This relationship between ‘energy in’ and ‘energy out’ is called the Energy Balance Equation, and it’s the most commonly accepted model for calculating how much weight one will lose or gain over time.

While the Energy Balance Equation determines body weight, it doesn’t tell us much about body composition, which is influenced by things like sex hormone levels; macronutrient intake (especially protein); exercise style, frequency and intensity; age; medication use; genetic predisposition, and more.

Understandably, people get very frustrated and confused with the Energy Balance Equation when the numbers don’t seem to add up, or their results don’t match their expectations. And it’s a fair frustration. Most of the time, the numbers don’t add up.

Expectations versus reality

This mismatch between expectations versus reality is not because the Energy Balance Equation is wrong, or a myth. Nobody’s body defies the laws of physics, even though it seems like that sometimes. It’s because the equation is more complicated than it sounds.

Many factors affect the Energy Balance Equation. What you do to ‘energy in’ affects what happens to ‘energy out’ and vice versa. The opposing sides aren’t mutually exclusive.

‘Eat less, move more’ is a good start – but that advice alone isn’t enough, because it doesn’t take all of the complex, intersecting factors and feedback loops into account.

Let’s take a look at some of these factors, starting with the ‘energy in’ part of the equation.

Energy in

‘Energy in’ is trickier than it first sounds, for two main reasons:

Reason 1: Inaccurate nutrition labelling

It might sound hard to believe, but the number of calories in a meal probably doesn’t match the number of calories/kilojoules on the labels or menu. The way companies, and even the government, come up with energy (kilojoule/calorie) and nutrient estimates is incredibly complex, rather imprecise, and centuries-old. As a result, the Nutrition Information label that’s mandatory on the packet of every foodstuff in your local supermarket can be off by as much as 20-25%, making it hard to accurately gauge what your ‘energy in’ actually is.

Nutrition Information labels can have a margin of error of up to 25%


Even if those food labels were correct, there’s another reason that ‘energy in’ is no picnic to calculate.

Reason 2: We don’t absorb all the energy we consume

The amount of energy a food contains in the form of calories is not necessarily the amount of energy we absorb, store, and/or use.

Remember that the food we eat has to be digested and processed by our unique bodies. The innumerable steps involved in digestion, processing, absorption, storage, and use — as well as our own individual physiological makeup — can all change the energy balance game.

How processed?
We absorb less energy from minimally processed carbohydrates and fats because they’re harder to digest, and we absorb more energy from highly processed carbohydrates and fats, because they’re easier to digest. The more processed a food is, the more digestion work is already done for you.

Research has shown, for example, that we absorb more fat from peanut butter than from the same volume of whole peanuts. The researchers found that almost 38% of the fat in peanuts was excreted in the stool, rather than absorbed by the body, whereas seemingly all of the fat in the peanut butter was absorbed.

How prepped? 
We often absorb more energy from foods that are cooked (and/or chopped, soaked, blended) because those processes break down plant and animal cells, increasing their bioavailability.

When eating raw starchy foods (like sweet potatoes), we absorb very few of the calories. After cooking, however, the starches are much more available to us, tripling the number of calories absorbed. Interestingly, allowing starchy foods to then cool before eating them decreases the amount of calories we can extract from them again – mostly due to the formation of resistant starches.

The way in which a food is prepared and whether it is cooked can affect its bioavailability


Gut bacteria
We may absorb more or less energy depending on the types of bacteria in our gut. Some people have larger populations of a Bacteroidetes (a species of bacteria), which are better at extracting calories from tough plant cell walls than other bacteria species.

Margins of error

By eating a diet rich in whole, minimally processed foods, the number of calories you absorb can be significantly less than you may expect. Plus, they require more calories to digest.

Conversely, you will absorb more calories by eating lots of highly processed foods, and burn fewer calories in the digestive process. In addition, highly processed foods are less filling, more energy dense, and more likely to cause overeating.

Since the number of calories someone thinks they’re consuming could be off by 25% or more, their carefully curated daily intake of 1,600 calories could really be 1,200… or 2,000.

Taking all of these factors into consideration, it becomes clear that this part of the equation should more accurately be: Energy In = Actual calories eaten - Calories not absorbed

Clearly, there’s a big margin of error with regards energy input, even if you’re a conscientious calorie counter. This doesn’t invalidate the Energy Balance Equation. It just means that if you want an accurate calculation, you probably have to live in a fancy metabolic lab. For most people, it’s not worth the effort.

Energy out

‘Energy out’ varies a lot from person to person: it’s a dynamic, always-changing variable.

There are four key parts to this complex system:

1. Resting metabolic rate (RMR)

RMR is the number of calories you burn each day at rest, just to breathe, think, and live. This represents roughly 60% of your ‘energy out’ and depends on weight, body composition, sex, age, genetic predisposition, and possibly (again) the bacterial population of your gut.

A bigger body, in general, has a higher RMR, but, crucially, RMR varies up to 15% between individuals. So, a 90kg guy with an RMR of 1905 calories might find himself running alongside an identically-sized guy on the next treadmill who burns 286 more, or fewer, calories each day with no more, or less, effort.


2. Thermic effect of eating (TEE)

Digestion is an active metabolic process. Thermic effect of eating (TEE) is the number of calories you burn by eating, digesting and processing your food. This represents roughly 5-10% of your ‘energy out’.

In general, you’ll burn more calories in your effort to digest and absorb protein (20-30% of its calories) and carbs (5-6%) than you do fats (3%).

And, as noted before, you’ll burn more calories digesting minimally processed whole foods than you will highly processed foods.

The less processed a food is, the greater the amount of energy you will burn digesting it


3. Physical activity (PA)

You’re a fitness professional, so you know this, but we’ll recap anyway. Physical activity is the calories you burn from purposeful movement, such as walking, running, working out at the gym, gardening and riding a bike. Obviously, how much energy you expend through physical activity will change depending on how much you intentionally move around.

4. Non-exercise activity thermogenesis (NEAT)

Non-exercise activity thermogenesis (NEAT) is the calories you burn through fidgeting, staying upright, and all other physical activities except purposeful exercise. This, too, varies from person to person and day to day.

Considering all of these factors, this part of the equation should more accurately be: Energy out = RMR + TEE + PA + NEAT

Each of these is highly variable. Which means the ‘energy out’ side of the equation may be just as hard to pin down as the ‘energy in’ side.

Revising the equation...

So, while the Energy Balance Equation sounds simple in principle, all these variables make it hard to know or control exactly how much energy you’re taking in, absorbing, burning, and storing.

So, revisiting that ‘simple’ Energy Balance Equation, we can see that, actually:
Changes in bodily tissues = [actual calories eaten - calories not absorbed] - [RMR + TEE + PA + NEAT]

Knock-on effects of variables

Even if all the variables in the final equation above were static, the Energy Balance Equation would be complicated enough. But things get crazy when you consider that altering any one of the variables causes adjustments in other, seemingly unrelated variables.

This is a good thing, of course. Our human metabolisms evolved to keep us alive and functioning when food was scarce. One consequence is that when ‘energy in’ goes down, ‘energy out’ goes down to match it, because we burn fewer calories in response to eating less.

Likewise, when ‘energy in’ goes up, ‘energy out’ tends to go up too, because we burn more calories in response to eating more.

This isn’t the case for everybody, and it doesn’t work ‘perfectly’, but generally, that’s how it goes and how our bodies avoid unwanted weight loss and starvation. It’s how humans have survived for two million years. The body fights to maintain homeostasis.

To illustrate this point, here’s how your body tries to keep your weight steady when you take in less energy and start to lose weight:

  • Thermic effect of eating goes down because you’re eating less.
  • Resting metabolic rate goes down because you weigh less.
  • Calories burned through physical activity go down since you weigh less.
  • Non-exercise activity thermogenesis goes down as you eat less.
  • Calories not absorbed goes down and you absorb more of what you eat.

It’s important to note that if you have lots of body fat to lose, many of these adaptations don’t happen right away or are very modest initially. As you become leaner, however, this adaptive thermogenesis really ramps up.

Calorie cutting counterproductive?

In addition to these tangible effects on the equation, reducing actual calories eaten also causes hunger signals to increase, causing us to crave (and maybe eat) more. The net effect leads to a much lower rate of weight loss than might be expected – and in some cases, could even lead to weight re-gain.

Reducing calorie consumption can lead to a much lower rate of weight loss than might be expected, and can even lead to weight re-gain


Setting better expectations

The calorie-cutting effect is just one example of the amazing and robust response to trying to manipulate one variable. There are similar responses when trying to manipulate each of the other variables in the equation. The point is, metabolism is much more complicated, and interdependent, than most people realise.

Therefore, trying ‘what used to work’ for you, or relying on calorie counting, often won’t get you the results you want. As your energy balance evolves, so must your strategies for losing fat or maintaining your weight.

Understanding energy balance means setting better expectations about body change. Losing weight, and keeping it off, is accompanied by adaptive metabolic, neuroendocrine, autonomic, and other changes.

It’s also important to remember that how your metabolism reacts to changes in energy balance will be unique to you. How much you can lose or gain will depend on your age, your genetic makeup, your biological sex, if you’ve had relatively more or less body fat and for how long, what medications you’re taking, the makeup of your microbiome… and probably a whole lot of factors we don’t even know about yet.


Brian St. Pierre, MS, RD, CSCS
Brian is a US-based registered dietitian (RN), certified sports nutritionist (CISSN) and certified strength and conditioning specialist (CSCS). He spent three years as the Head Sports Nutritionist and as a Strength and Conditioning Coach at Cressey Sports Performance and is a nutrition coach with Precision Nutrition. precisionnutrition.com