We’ve all heard that years of yo-yo-dieting can damage your metabolism, but perhaps a more accurate statement would be, years of gaining and losing fat can change the way your brain regulates your body weight.
The first law of thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system. This is also true for biological systems and looks like this:
Changes in body stores = energy in – energy out (for simplicity in this article, energy and calorie will be synonymous.) In theory, if you eat less energy than you expend, you will lose weight and vice versa.
Then why do so many people have trouble losing weight? Simply put, there is a mismatch between expectation and what the Energy Balance Equation tells us. Not because the laws of thermodynamics are wrong, but because the equation is more complicated than as it's presented.
The above equation does not tell us about the influences on our body composition based on several factors including sex hormones, macronutrient intake, exercise, age, genetics and much more. Many factors affect the equation and they are not mutually exclusive. Let’s dissect the equation and uncover the most common misconceptions.
Energy in is the number of calories consumed. This number is affected by several different variables including the fact that nutrition labels can be up to 25% inaccurate. This is only confounded when we learn that the amount of energy a food contains in the form of calories is not the amount of energy we absorb, store, and/or use.
Our bodies are unique and individual variations in digestion, processing, absorption, storage, use, and physiological makeup change the Energy Balance Equation.
For example, we absorb less energy from minimally processed carbohydrates and fats because they are harder for us to digest. Conversely, we absorb more energy from highly processed foods because they are easier to digest. We also absorb more energy from foods that are cooked because breakdown processes have already started - making the nutrients more bioavailable.
There is also some evidence that our microbiome may have an effect on how much energy we absorb. For example, Bacteroidetes are better at extracting calories from tough plant cell walls than other species.
Therefore, eating a diet rich in whole foods can lead to significantly less calorie absorption than eating a diet of highly processed foods. Furthermore, processed foods are less filling, more calorie dense, more likely to cause overeating, and you will burn fewer calories in the digestive process.
Energy out is the energy burned through daily metabolism. It is highly variable and consists of four basic processes.
The first is your resting metabolic rate (RMR). RMR is the number of calories you burn each day at rest. Around 60% of your energy out depends on your RMR which is influenced by weight, body composition, biological sex, age, genetic predisposition, and more.
Thermic effect of food (TEF) is the energy it takes to digest food. TEF accounts for about 10% of your energy out. You burn more energy when digesting protein, about 20-30% of its calories, and the least from fats, about 3% of its calories.
Activity is the third and fourth part of the “energy out” equation. Physical activity (PA) can be thought of as calories burned from purposeful activity. Non-exercise activity thermogenesis (NEAT) are the calories burned through basic activities (staying upright, typing, fidgeting). Any type of activity varies from person to person and changes on a daily basis regardless of routine.
New Energy Balance Equation based on the above discussion:
Changes in body stores = [actual calories eaten – calories not absorbed] – [RMR + TEF + PA + NEAT]
As if this equation doesn’t now seem complicated enough, there’s more. Altering any of these variables causes adjustment in the other variables. This is because our metabolism evolved to keep us alive and functioning when food was sparse.
The most basic example is that when “energy in” goes down, “energy out” goes down to match it. Meaning, you burn fewer calories in response to eating less to maintain homeostasis. Examples include:
o Thermic effect of food goes down because you’re eating less
o RMR goes down because you weigh less
o Energy burned through PA go down since you weigh less
o NEAT goes down as you eat less
o Calories not absorbed goes down and you absorb more of what you eat
o Reducing calories causes hunger signals to increase
o Increased hunger signals cause cravings
o Rises in cortisol from the stress of reduced calorie consumption causes water retention
How metabolism reacts to changes in the Energy Balance Equation is unique to each individual and depends on the many factors previously mentioned. In summary, the net effect can lead to much lower weight loss than intended.
So does dieting damage metabolism? The answer is complex. Losing weight, and keeping it off, is always accompanied by adaptive metabolic, neuroendocrine, autonomic, and other changes. Nothing has really been “damaged”, instead, bodies become more sensitive to various hormones and neurotransmitters.
While weight loss gets more difficult the older we get, the more extreme we’ve been with our diets, and the more hormone imbalances we acquire along the way, it is not impossible. Similarly, getting your metabolism to do what you want it to do is achievable with the help of someone who knows your individual biochemistry.
The best strategies to achieve your health and weight goals always include the help of a qualified practitioner. If you don’t have someone you enjoy working with, I’d love to hear from you.