There’s a lot of discussion in the fitness industry about whether crash dieting can cause metabolic damage. In this article, we’ll take on this interesting topic and separate fact from fiction. We’ll also teach you exactly why crash diets might be linked to struggling to maintain your weight in the future. Despite working out consistently and intensely, plus eating carefully, you are not losing weight. To understand this answer let’s explore how human metabolism actually works. Then we will talk about whether the metabolism can actually be damaged. This post delves into the science of energy balance, thermodynamics, and metabolic regulation. If you love learning this stuff, feel free to dig in.
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.
We absorb less energy from minimally processed carbohydrates, and fats, because they’re harder to digest. We absorb more energy from highly processed carbohydrates and fats because they’re easier to digest. (Think of it this way: The more “processed” a food is the more digestion work is already done for you. We often absorb more energy from foods that are cooked (and/or chopped, soaked, blended) because those processes break down a 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 number of calories we can extract from them again. (This is mostly due to the formation of resistant starches).
There are three 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 percent of your ‘energy out’ and depends on weight, body composition, sex, age, genetic predisposition, and possibly the bacterial population of your gut.
2. Physical activity (PA)
PA is the calories you burn from purposeful exercises, such as walking, running, going to the gym, gardening, riding a bike, etc. Obviously, how much energy you expend through PA will change depending on how much you intentionally move around?
3. Non-exercise activity thermogenesis (NEAT)
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. Scientists at the National Institutes of Health have studied the data from people who have lost weight and created a mathematical model that represents how weight and fat loss actually happens in the real world.