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Can eating too little actually cause your body to hold onto weight or even damage your metabolism?

Updated: Apr 16, 2022

This one is complicated because the metabolism is very complicated. No stress, I'll tell you what you need to know. Feel free to skip to THE SUMMARY at the end and #10 is the best advice!

Are you one of these?

  • 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.

  • Or when you were younger, you were lean & fit. Maybe you did some crash diets and always saw results in a week or two at least. But now, even when you put in the same effort, you just can’t seem to get as lean.

The thought has probably crossed your mind, “Is my metabolism damaged?”

This post delves into the science of energy balance, thermodynamics, and metabolic regulation just a little bit. If you love learning this stuff, feel free to keep reading. If, on the other hand, you’re simply looking for solid, research-backed advice on how to lose fat and break weight-loss plateaus, go to THE SUMMARY at the end.

Energy balance: The laws of physics still apply.

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).

In theory:

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.

In other words:


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 /intensity, age, medication use, genetic predisposition, and more.

Understandably, people get really 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. This is a good lesson, by the way, about the importance of adjusting your expectations to match observable reality. And it’s a fair frustration. Most of the time, the numbers don’t add up.

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.

Here's the thing, most of us could probably benefit from eating a little less and getting a little more daily activity.

But that advice alone isn’t enough.

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

‘Energy in’ is trickier than you think.

Reason 1: The number of calories in a meal likely doesn’t match the number of calories on the labels or menu.

This might sound hard to believe, but it’s true… the way companies (and even the government) come up with calorie and nutrient estimates is incredibly complex, rather imprecise, and centuries-old. As a result, food labels can be off by as much as 20-25 percent.

And even if those food labels were correct:

Reason 2: 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, processed, absorbed, stored, and used by our unique bodies.

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. Aha, moment!

In addition:

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.

Also, 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.

Here’s another interesting example of this whole process at work. Recently, USDA researchers asked test subjects to consume 45 grams (about 1 ½ servings) of walnuts daily for three weeks. What they found was that, on average, people only absorbed 146 of the 185 calories in the nuts. That’s 79 percent of the calorie content on the label.

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

Conversely, you will absorb more calories by eating lots of highly processed foods, plus burn fewer calories in the digestive process. Since the number of calories someone thinks they’re consuming could be off by 25 percent (or more), their carefully curated daily intake of 1,600 calories could really be 1,200… or 2,000.

As you can see, there’s a big margin of error for 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. LOL!

‘Energy out’ varies a lot from person to person.

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.

For instance:

A 150-pound person might have an RMR of 1583 calories a day.

A 200-pound person might have an RMR of 1905 calories.

A 250-pound person might have an RMR of 2164 calories.

Sorry, my female friends, males differ from females. Men have a higher RMR naturally.

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 food (TEF)

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 TEF.

TEF is the number of calories you burn by eating, digesting, and processing your food. This represents roughly 5-10 percent of your ‘energy out’.

'TAKE HOME': 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.

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.

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.

It’s important to note that if you have lots of body fat to lose, many of these adaptations (i.e. lowered RMR, PA, NEAT, etc) don’t happen right away. But, as you become leaner, this “adaptive thermogenesis” kicks in. It’s also important to know that how your metabolism reacts to changes in energy balance will be unique to you.

THE SUMMARY: The physiology of weight loss is complicated, but the best strategies for losing fat and keeping it off don’t have to be. Here are my suggested TEN things you can start working on, and step by step you will get the results you're looking for:

1. Eat plenty of protein.

Protein is essential when trying to lose weight/fat for a few reasons. Protein helps you keep that all-important lean body mass (which includes connective tissues, organs, and bone as well as muscle). Protein significantly increases satiety, which means you feel fuller despite eating less. And eating more protein often causes people to eat less overall.

Just by eating more protein, you burn more calories, because of the increased thermic effect of food.

For most active men: 6-8 palm-sized servings of protein per day.

For most active women: 4-6 palm-sized servings per day.

2. Eat a wide variety of fruits, vegetables, quality carbs, and healthy fats.

Vegetables are loaded with vitamins, minerals, phytonutrients, water, and fiber to help you fill up during meals, stay full between meals, keep you healthy, and recover from your workouts.

Recommendation is 6-8 fist-sized servings per day for most active men.

And 4-6 fist-sized servings per day for most active women.

3. Eat a moderate amount of quality carbs—whole grains (when tolerated), fruit, potatoes, sweet potatoes, beans, and legumes, etc. (We emphasize moderate, of course). They will fuel training, boost leptin (a super important hormone), keep up sex hormones, and prevent feelings of deprivation. And the fats also keep up sex hormones, boost the immune system, suppress excess inflammation, and make food taste really good.

For most active men, this would be 6-8 handfuls of quality carbs and 6-8 thumbs of healthy fats per day.

For most active women, 4-6 handfuls of quality carbs and 4-6 thumbs of healthy fats per day.

4. Adjust your intake as you plateau, or to prevent plateaus.

IMPORTANT: As your weight loss progresses, you will need to lower your calorie intake further to continue to progress, as your smaller body will burn fewer calories, and your body is adapting to your diet.

Be ready, willing, and able to adjust portion amounts by removing 1-2 handfuls of carbs and/or 1-2 thumbs of fats from your daily intake. Then reassess and continue to adjust as needed.

However, one study found that weight loss plateaus have less to do with metabolic adaptations and more to do with “an intermittent lack of diet adherence”. In other words, not actually sticking to a nutrition plan consistently.

Research shows that we usually think we’re eating less and exercising more than we truly are. So do an objective review of your actual energy in and out before assuming your body is blocking your efforts.

5. Create an environment that encourages good food choices and discourages poor ones. This can mean making changes to your daily routine, who you spend time with, where you spend time, and what food is readily available to you. But remember that weight loss can and should be relatively slow, so aim to lose about 0.5-1 percent of your body weight per week. This helps to maintain muscle mass and minimize the adaptive metabolic responses to a lower calorie intake and resulting weight loss. Faster weight loss tends to result in a larger adaptive response.

6. Cycle calories and carbs.

For folks who are trying to get really lean, at some point you can’t just rely on linear dieting to get you there. By strategically cycling calories and carbs, you can help to limit how much the metabolism-regulating hormone leptin drops or temporarily boost it back up.

7. Do a mixture of resistance, cardiovascular, and recovery activity.

Resistance training helps you maintain vital muscle mass, burn calories, and improve glucose tolerance. Cardiovascular exercise improves the health of your cardiovascular system, helps you expend energy, and can improve recovery.

Recovery work (e.g. foam rolling, walking, yoga) helps you maintain consistency and intensity with resistance and cardio training, making them more effective. And it helps to decrease stress (lowering cortisol), which also helps you lose body fat and keep it off.

Aim for 3-5 hours per week of purposeful activity.

8. Find ways to increase NEAT.

Even small increases in activity can account for hundreds of daily calories, and therefore make a big difference in fat loss efforts. Some ideas: Get a stand-up desk or a treadmill desk; fidget; pace while on the phone; take the stairs; park your car farther away from where you’re going.

9. Develop a solid nightly sleep routine and manage your stress.

Sleep is just as important to your success as nutrition and activity levels. Don’t pretend that you can get by with less.

10. Have some self-compassion.

There are going to be meals or days where you don’t eat as you “should”. It’s OK. It happens to everyone. Recognize it, accept it, forgive yourself, and then get back on track. Research actually shows that self-compassion and flexible eating is associated with lower BMI and a healthier body weight, lower self-reported calorie intake, less anxiety and stress, and a better relationship with food.

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