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’re not losing weight (or not losing it as fast as you’d like or expect).
Or you were losing weight consistently… until recently. Now you’re stuck — even though you’re working as hard as ever.
Can months or years of dieting do some kind of long-term harm to the way the human body processes food?
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 (e.g. your fat tissue).
- If you eat less energy than you expend, you should lose weight.
- If you do the opposite (i.e. eat more energy than you expend), you should gain weight.
So, weight loss is as simple as eating/taking in less energy than you expend, right!?
That equation is based on constants. In order for that equation to work, you need to know how much energy you're using, and how much you're taking in. Without that information, knowing that equation is kind of pointless.
This is where weight loss is tricky, because the amount of energy we take in varies a lot, and the amount of energy we expend changes a lot.
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 can all change the energy balance game.
So, for instance:
- 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.)
For example, research has shown that we absorb more fat from peanut butter than from whole peanuts. The researchers found that almost 38 percent 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.
So, if the way we prepare our food dictates how much energy we get out of it, you can imagine how much variability that causes on the energy "in" side of the equation.
‘Energy out’ — again, energy burned through daily metabolism and moving you around — is 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 percent 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.
- A 150-pound man might have an RMR of 1583 calories a day.
- A 200-pound man might have an RMR of 1905 calories.
- A 250-pound man might have an RMR of 2164 calories.
Crucially, RMR varies up to 15 percent from person to person. If you’re that 200-pound guy with an RMR of 1905 calories, another guy just like you on the next treadmill might burn 286 more (or fewer) calories each day with no more (or less) effort.
2. Thermic effect of eating (TEE)
This may surprise you, but it takes energy to digest food. Digestion is an active metabolic process. (Ever had the “meat sweats” or felt hot after a big meal, especially one with lots of protein? That’s TEE.)
TEE is the number of calories you burn by eating, digesting, and processing your food. This represents roughly 5-10 percent of your ‘energy out’.
In general, you’ll burn more calories in your effort to digest and absorb protein (20-30 percent of its calories) and carbs (5-6 percent) than you do fats (3 percent).
And as noted before, you’ll burn more calories digesting minimally processed whole foods compared to highly processed foods.
3. Physical activity (PA)
PA is the calories you burn from purposeful exercise, 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.
4. 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.
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, does dieting damage the metabolism?
Despite what you may have heard:
No, losing weight doesn’t “damage” your metabolism.
But because of the adaptations your body undergoes in response to fat loss ‘energy out’ for those who have lost significant weight will always be lower than for people who were always lean.
These changes mean that we expend less energy — around 5-10 percent less (or up to 15 percent less at extreme levels) than what would be predicted based on just weighing less.
So, you can't damage your metabolism, but as you lose weight, you'll need to eat less than someone who was always lean in order to keep weight off, or lose weight.
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