Can You Gain Weight by Eating Too Little?

No, but here’s why it’s so easy to think you can. And three different solutions to help stop the problem.

You’re meticulously tracking your food and your workouts, but you’re still not seeing results. Has your metabolism slowed down to the speed of a snail? Have your hormones switched off and gone to the Maldives to sip Malibu and pineapple juice? Is it actually possible to gain weight while eating too little?

Here’s what’s really happening and how to fix it.

“How can I be eating so little and still gaining weight?”

Have you ever felt this way? Have you ever asked yourself this question, or had one of your clients come to you with this exact problem?

Over the years I’ve worked as a coach, this is a question that comes up again and again, both from clients and from fellow coaches.

They’re confused. Frustrated. Maybe even angry.

Despite everything they’re doing, including eating less, sometimes much less, they still aren’t losing weight. In fact, they might even be gaining weight.

Do a quick search online and you’ll inevitably find plenty of explanations.

Some people say that the laws of energy balance still apply and that people simply aren’t tracking their calories accurately. Others call it “starvation mode” because you’re supposedly not eating enough, or blame some strange metabolic or hormonal issue.

So what’s actually going on? Is something wrong with them? Have their bodies become obsolete? Is it all in their head?

Or can you really gain weight from eating too little?

Let’s find out together.

Truth: thermodynamics doesn’t lie.

You’ve probably heard the phrase “the laws of thermodynamics” before. Or maybe you’ve heard it described as energy balance. Or simply “calories in, calories out.”

Let’s break down what that actually means.

Thermodynamics is a way of describing how energy is used and transformed. In simple terms, we take in energy through food and we use energy through things such as:

• Basic metabolic functions (breathing, blood circulation, etc.),

• Movement (daily activity, intentional exercise, etc.),

• Heat production (also known as thermogenesis),

• Digestion and excretion.

And the truth is:

Energy balance, calories in versus calories out, determines body weight.

Quite simply, if we take in more energy than we expend, we gain weight.

If we take in less energy than we expend, we lose weight.

This has been tested again and again by researchers in many different settings.

Of course, many factors influence both sides of this seemingly simple equation, which can make things a little confusing. However, humans do not defy the laws of thermodynamics.

Factors that influence energy intake

• Appetite: the hormones that regulate hunger,

• Environment: your financial situation and, by extension, your food choices and your family,

• Psychology: your mindset, stress levels, and self-esteem.

Factors that influence your energy expenditure

• Basal metabolism: the calories you burn at rest,

• Exercise: the energy you expend depending on how much sport or exercise you do,

• Non-exercise activity: the energy you expend through your everyday activities.

The factors that influence energy intake can also influence energy expenditure, depending on how they affect your lifestyle. Here’s an example to make things clearer:

Take a family. It might be a sporty family that prioritises eating well and exercising regularly. In that case, appetite may be better regulated, the environment supports a healthier lifestyle, and the family’s mindset reflects those habits as well.

On the other hand, if it’s a family that doesn’t value physical activity and doesn’t pay much attention to what they eat, there’s a greater chance that appetite regulation won’t be as good, the environment won’t encourage a healthy lifestyle, and their mindset may reflect that lifestyle too.

But what about unexplained changes in weight? This time, you ate an entire wild boar for dinner, enough to make Obelix jealous, and somehow woke up lighter.

And what about when you feel like you’re “doing everything right” but still aren’t losing weight?

No, even when we think we’re defying the calories-in-versus-calories-out equation, we aren’t.

What about the so-called expert at your gym or your neighbourhood “apprentice Getafix” who suggests that insulin resistance, or some other hormone, breaks the equation?

Although hormones can influence the proportions of lean mass and fat mass you gain or lose, they still do not invalidate the energy balance equation.

However, as the title of this article suggests, it’s easy to understand why people, including online gurus and coaches, get confused about this.

One reason for this is:

Measuring metabolism is very difficult

Why? Because you’re human, not a machine. Calculating how much energy you expend during the day is much more complex than calculating the fuel consumption of a car.

It’s possible to estimate your basal metabolic rate, meaning the amount of energy your body needs just to keep you alive. But the result is only as accurate as the tools we use.

When it comes to measuring metabolism, the best tools are tightly controlled metabolic chambers. Unfortunately, very few of us spend much time inside one of those.

That means that even if our “metabolism” is estimated at the gym, through a fitness tracker, or using the calorie information on food labels, those estimates can be off by 20 to 30% in normal, young, healthy people. They may be even less accurate in other populations.

Of course, if we could accurately measure exactly how much energy you expend every day, and then accurately measure how much energy you consume and absorb, we could check whether you are truly eating too little for your body’s needs.

But even if we could measure that outside a laboratory, which for now is not really possible with perfect accuracy, it still wouldn’t solve everything because energy expenditure is dynamic. In other words, every variable can change when another variable changes.

So unless we could measure your exact energy intake and expenditure minute by minute, we can’t know with complete certainty what your metabolism is doing or exactly how it is responding to the food you eat.

Most of the time, we therefore have to estimate, and our estimates are not perfect.

That said, having approximate data on your calorie intake and expenditure can still be extremely useful. It gives you a baseline, helps you track your progress, and can point you towards the changes you may need to make. More on that later in the article.

Not only are our estimates inaccurate, but the idea of “eating too little” is also subjective.

Think about it. When you say you’re “eating too little,” do you mean…

Eating less than usual?

Eating less than you were told to eat?

Eating less than feels right for you?

Eating less than you need to stay healthy?

Eating less than your estimated metabolic rate?

Eating less than your actual metabolic rate?

And how often does this apply? Are you…

Eating too little at one meal?

Eating too little for one day?

Eating too little every day?

Eating too little on most days, but too much on certain days?

Without being clear on some of these questions, you can see how easy it is to assume that you’re “eating too little” when in reality, you may still not be eating below your true energy expenditure.

Even if you’ve done a test to estimate your metabolic rate and it still seems like you’re eating less than the number the test gave you.

Most of the time, the problem is our interpretation.

As human beings, we’re not very good at accurately estimating what we eat and how much energy we expend. We tend to think we eat less and burn more than we actually do, sometimes by as much as 50%.

(Interestingly, lighter people who are trying to gain weight often have the opposite problem: they overestimate how much food they eat and underestimate how much energy they expend.)

It’s not that we’re lying to ourselves, although sometimes we can definitely be mistaken about what we consume. More than anything, we’re simply not very good at estimating portion sizes and calories.

And that’s especially difficult today, when plates and portion sizes have never been bigger. Energy-dense “foods” that taste incredible and are highly rewarding to the brain are everywhere, cheap, and socially encouraged.

(As we saw in the previous article: https://www.firefly-zone.be/en/post/the-good-crap-why-you-eat-too-much-of-it-5-strategies-to-fight-these-calorie-bombs).

When people start paying close attention to portion sizes using their hands, food scales, or measuring cups, they’re often shocked to discover that they’re eating much more than they thought.

A lot of my clients send me photos of their meals so we can look together at what might be going wrong with their diet!!

And often, the first thing I hear from them is that they’re controlling what they eat. I’m now less and less surprised when I realise that their actual food intake is much higher than what they think, compared with the measurements I use during my nutrition and fitness consultations.

At other times, we can do everything right at most meals, but energy, and therefore calories, can sneak in without us realising it.

Here’s a perfect story to illustrate that.

I remember a pretty surprising anecdote one of my experienced clients once told me. He was hungry and decided to grab some takeaway food. He was short on time and, guided by both a craving for something sweet and the desire to stay on track with his goals, he decided to get a ceasar salade some soup and a slice of carrot cake.

The surprise came later when, out of curiosity, he checked the estimated calorie content of what he’d eaten: it came to more than 2000 calories.

If the calorie content of that meal surprised someone with my client’s experience, how do you think most “normal” eaters would do? Good luck trying to estimate the calories in a meal just by looking at it.

Now imagine another scenario where you eat almost every meal of the week in a way that leaves you with an estimated negative energy balance of around -3,500 calories. Then, in just one meal, a “healthy” menu option plus dessert, you rack up 5,000 calories.

That single meal would theoretically put you into a positive energy balance for the week (+1,500 calories), leading to weight gain!

Seriously, how would you feel if, after eating 20 “perfect” meals in a row and just 1 “not so bad” meal, you gained weight?

You’d probably feel like your metabolism had broken.

You’d probably feel like it really is possible to gain weight while eating too little.

But once again, the laws of thermodynamics haven’t been broken. Instead, a whole bunch of calories simply slipped in without you realising it.

On top of that, the adaptable nature of metabolism can be confusing.

Another reason why it can be easy to believe that you’ve gained weight while eating too little, or at least that you haven’t lost weight despite eating less, is that your metabolism isn’t like a computer.

For example, you may have heard that one kilogram of fat equals around 7,000 calories, so if you cut 1,000 calories per day, you’ll lose one kilogram per week (7 x 1,000 = 7,000).

(Unless, of course, you consume 4,000 calories across two meals at the end of the week, in which case you might actually be heading towards weight gain.)

Except that human metabolism doesn’t work quite like that. The human body is a complex and dynamic system that reacts quickly to changes in its environment.

When you undereat, especially over a longer period of time, and that part is important, this complex system adapts.

Here’s an example of how that might happen:

You expend less energy through digestion because you’re eating less.

Your resting metabolic rate decreases because you weigh less.

The number of calories you burn through physical activity decreases because you weigh less.

Non-exercise activity thermogenesis, things like everyday movement and fidgeting, decreases and you expend less energy throughout the day.

Your digestion may slow down and your body may absorb more energy from the food you eat.

Your body will also adjust its hormonal responses. For example:

Appetite and hunger hormones increase, meaning we want to eat more, become more responsive to food cues, and may experience more cravings.

Satiety hormones decrease, meaning it becomes harder to feel full or satisfied.

Thyroid hormones and sex hormones, both of which are involved in metabolism, may decrease.

Your expected daily deficit of 500 calories can quickly become 400, 300, or even 200 calories, or less, even if you’re deliberately exercising just as much as before.

And speaking of exercise, the body has similar mechanisms when we try to compensate for excessive intake.

For example, research suggests that increasing physical activity beyond a certain point, by exercising more, can trigger:

More appetite and more calories consumed,

Increased energy absorption,

A lower resting or basal metabolic rate,

Reduced non-exercise activity thermogenesis (NEAT).

These are just two of the many examples we could talk about.

There are other factors too, such as the health of our gastrointestinal microbiome, our thoughts and feelings around eating less, for example whether we find eating less stressful, and so on.

The point is that metabolism is much more complicated, and much more interconnected, than most people think.

All of this means that when you eat less, you may lose less weight than expected.

Depending on how much you reduce your food intake and for how long, you may even regain weight over the long term because of these physiological and behavioural factors.

And on top of that, human variation is incredible!

And the same goes for our metabolisms.

Although the “average” responses described above are generally true, our individual responses, genetics, physiology, and so on mean that our calorie needs will differ from those of other people and from the needs predicted by laboratory tools and the equations they’re based on.

Imagine two people of the same sex, age, height, weight, and lean body mass. According to the calculations, they should have exactly the same energy expenditure and therefore the same energy needs.

However, we know that this isn’t necessarily the case.

For example:

Your basal metabolic rate , remember, this is the energy your body needs simply to keep your organs and basic biological functions running and keep you alive, can vary by around 15%. For an average woman or man, that can represent roughly 200 to 270 calories.

Genetic differences matter too. A single variation in a gene can create an additional difference of around 160 calories.

Sleep deprivation can lead to changes of around 5 to 20% in metabolism, potentially representing another 200 to 500 calories.

For women, the phase of the menstrual cycle can affect metabolism by around another 150 calories.

Even within the same person, metabolism can easily fluctuate by around 100 calories from one day to another, or even throughout the same day, depending for example on circadian rhythms, sleep, and wakefulness.

These differences can add up quickly, and this isn’t even an exhaustive list.

Ultimately, I hope you can see how equations designed to predict the calorie needs of an “average” person may not be completely accurate for you.

And this is why you might gain weight, or fail to lose weight, while consuming fewer calories than your measured or estimated energy expenditure suggests you should.

It’s also why some coaches, nutritionists, or experts who don’t fully understand the limitations of metabolic measurements may start looking for all sorts of complicated hormonal or environmental explanations for what they believe is a violation of thermodynamics.

The answer, however, can be much simpler than that, and often is.

The estimates simply weren’t very accurate.

Oh, and yes, water retention is very real.

Cortisol is one of our “stress hormones,” and it can affect how much fluid our body retains.

Restricting food and nutrition can itself become a source of stress, especially if you’re anxious about not eating enough or developing some kind of deficiency.

When we’re stressed, cortisol generally increases. And considering how many people today report feeling more stressed than ever, it’s pretty easy to go from “I’m a little stressed” to “well, shit, everything is falling apart.”

When cortisol rises, your body can retain more water. That means you may feel “puffier” and “less lean” than you actually are.

This water retention can hide the fat loss that is happening underneath, making it look as though you aren’t losing fat or weight when, in reality, you may still be progressing.

I’ve personally had moments where I was so stressed about an exam that, for two days, I didn’t exactly feel fatter, but I genuinely felt like my whole body had been padded out.

Here’s another example.

Someone I know once told me he was going away for a weekend with his girlfriend. He explained that he was so stressed about everything going well, entirely self-imposed stress, that the next morning, after eating a few homemade burgers with no fries or sauces, he was 2 kg heavier.

He’d worked hard on his physique for that weekend and suddenly went from 80 kg to 82 kg.

Thank you, stress, salt, and carbohydrates!

The funniest part? The day after the weekend, he was back down to 80 kg.

Aaaaah, good old water retention!

Got it, but what if I’m meticulously tracking my intake and expenditure?

By now, you’re probably nodding your head from side to side and starting to realise just how complex metabolism really is, how inaccurate calorie counting can be, how much we vary from one person to another, how strongly the body tries to maintain the status quo, and how limited our methods are for estimating our own intake and expenditure.

But what if you’re tracking everything meticulously? You log your meals? Count your steps? You’ve even knocked on the door of a local research lab to have your metabolism measured? And things still aren’t moving in the right direction?

Well, it comes back to what we’ve been discussing so far:

The calorie content of the foods you logged may be higher than expected, either because of inaccurate labelling or because of small errors in your own measurements.

Your energy needs may be lower than calculated, or even lower than measured. This could be because:

• You expend less energy through movement than your fitness tracker or exercise machine suggests;

• You have less lean mass than you think, or it may not use as much energy as you assume;

• You absorb more energy during digestion than you realise, for example if your gastrointestinal transit time is slow or your microbiome is particularly efficient at extracting nutrients.

Maybe you’re missing a few pieces of data.

As mentioned earlier, even if you’re probably not outright lying to yourself, you might simply “forget” to count those few chicken nuggets you finished from your kids’ plates because you didn’t want to waste them.

Or that extra spoonful of peanut butter.

Or the large glass of wine you mentally counted as a “medium.”

And in the same way, the calorie information on food labels can be, and often is, inaccurate.

Maybe you consider your workout to be high intensity, even though you spent a large part of it sitting on a bench between low-rep strength sets.

Maybe you were so hungry afterwards that you ate more than expected. But since you assumed it was all going towards building muscle, you didn’t think too much of it.

We’re human. It happens.

Importantly, carefully measuring and tracking your energy intake can help.

When we measure and track our food for a period of time, we become more aware of what we’re actually eating. We get a more realistic understanding of portion sizes, and it can help us stay consistent and accountable.

But measuring and tracking certainly isn’t a perfect strategy.

It can be stressful and time-consuming. Most people don’t want to do it forever.

And it can create the illusion that we know the “exact” number of calories we consume compared with the “exact” number of calories we burn, which can lead us to believe we’re eating less than we expend even when that isn’t actually the case.

(To be clear, I’m talking here about trying to measure and calculate your calories perfectly, right down to the gram.)

What about medical conditions?

Whenever we reach this point in the discussion, people usually start wondering whether underlying health conditions or medications can affect their metabolism, body weight, and/or appetite.

The answer is yes.

This can include things like polycystic ovary syndrome (PCOS), significant thyroid dysfunction, disruption of sex hormones, and so on.

However, these issues are less common than many people think, and even if you do have a medical condition, your body still isn’t breaking the laws of thermodynamics.

It simply means, as explained above, that your calorie expenditure may be lower than predicted. And that a few extra calories may still be sneaking in, Solid Snake style in Metal Gear.

The good news: weight loss is still possible, although it may happen at a slower pace.

If you genuinely feel that you’re estimating your intake accurately, exercising regularly for at least 5 to 7 hours per week, managing your sleep and stress, getting expert nutritional support, and covering all the fundamentals, then it may be worth discussing further testing with your doctor.

So, what can we actually do?

1. Choose mostly whole, minimally processed foods.

Foods that aren’t hyper-rewarding or hyper-palatable are generally harder to overeat. They’re also less likely to contribute to things like hypothalamic inflammation or leptin resistance, leptin being one of the hormones involved in regulating satiety.

They also contain plenty of useful nutrients, including vitamins, minerals, water, fibre, phytonutrients, and other compounds that support health, while usually being less calorie-dense.

And in general, they’re much better at helping you stay full and satisfied.

Choose whole foods that you actually enjoy and can realistically eat on a regular basis.

2. Measure your intake.

We’ve already seen that trying to measure your food with absolute precision isn’t especially useful because there are so many possible sources of error, and in some cases it can also contribute to unhealthy eating behaviours.

That said, having a general idea of how much you’re eating is still extremely useful. It gives you enough information to steer the ship in the direction you actually want to go.

Use whatever tools you prefer. Your hands, food scales, measuring spoons, whatever works for you.

Track your intake for a few days or for a full week and compare it with what you thought you were eating. We’re often surprised.

Sometimes, the simple act of tracking increases awareness of what we’re consuming, which can help us make better choices.

And those choices can start changing pretty quickly because tracking adds a practical layer to your awareness. You don’t just know that certain foods affect your calorie intake, you can actually see it happening.

That combination can be incredibly useful for changing your eating habits in the direction you want.

3. Experiment with your macronutrient intake.

Some people respond better to more carbohydrates and less fat. Others feel better with more fat and fewer carbohydrates.

There is no single perfect diet for everyone.

We all have different preferences and can respond differently to different foods. So experiment a little and find what works best for you.

The best diet is the one that is adapted to you.

If you’re still struggling, get some help.

Behaviour change and sustainable weight loss are difficult, especially when you’re trying to do everything alone.

Finding a qualified and compassionate coach or professional can help you navigate those murky waters.

So if you’d like some support, I invite you to book a discovery session with me so we can work together towards your goals.

I’d be more than happy to support you through your transformation.