Can you gain weight by eating too little?

By Marwan / June 3, 2018


You’re tracking your eating and exercise meticulously but not seeing results. Has your metabolism slowed to a crawl? Are your hormones off? Is it really possible to GAIN weight from eating too LITTLE? Here’s what’s really going on — and how to solve it.

So the question is…How can I be gaining weight by eating too little?

Here are some things to consider..

The law of Thermodynamics

So you’ve heard of Thermodynamics, energy balance, calories in and calories out but what do all these really mean?

Thermodynamics is a way to express how energy is used and changed. Put simply, we take in energy in the form of food, and we expend energy through activities like:

  • basic metabolic functions (breathing, circulating blood, etc.)
  • movement (daily-life activity, purposeful exercise, etc.)
  • producing heat (also called thermogenesis)
  • digestion and excretion

Energy balance (calories in, calories out) does determine bodyweight.

  • If we absorb more energy than we expend, we gain weight.
  • If we absorb less energy than we expend, we lose weight.

However, humans do not defy the laws of thermodynamics.

But what about unexplained weight changes you ask? That one time that you had a “big” dinner, or a big cheat meal and you woke up lighter on the scale?

Here is the real reason and it’s the dreaded word,


Everyone’s metabolism is different therefore it is tricky to measure metabolism.

It is possible to approximate your basal metabolic rate — in other words, the energy cost of keeping you alive. But measurements are only as good as the tools we use.

You can measure your metabolism in a hermetically sealed metabolic chambers but not many of us are hanging out in one of them!!

You can estimate your calorie expenditure at the gym, you can use different apps to count total calories in like Fat Secret or MyFitnessPal but even those “estimates” can be anywhere from 20-30% off in young people possible way more in other population.

Most times our problems are purely perception.  As human beings we are pretty bad at estimating!!! We like to think we burn more than we take in.  The guys (or gals) that are trying to put on weight typically underestimate how much they’re burning and overestimate how much they’re eating.

We can be as far out as 50%!!!

I will give you a personal example.  I eat a lot of chicken so I know the calories of a normal chicken breast are around 150 odd calories OK?

For those of you that live in USA and know of the Cheesecake Factory will know what I mean 🙂

Luckily it had calories next to each meal.

So I looked through the menu and found chicken, I thought yay!

But a brief glance at the calorie content next to the meal my face turned to horror!!!


2000 CALORIES???? What did they feed the chicken?!!!

So it’s not that we’re lying (though we can sometimes deceive ourselves, and others, about our intake). More than anything, it’s that we struggle to estimate portion sizes and calorie counts.

This is especially difficult today, when plates and portions are bigger than ever. And energy-dense, incredible tasting, and highly brain-rewarding “foods” are ubiquitous, cheap, and socially encouraged.

When people start paying close attention to their portion sizes using their hands or food scales and measuring cups, they are frequently shocked to discover they are eating significantly more than they imagined.

Another reason it can be easy to believe you gained weight eating too little (or at least didn’t lose weight when eating less) is because your metabolism isn’t like a computer.

For instance, you might have heard that one pound of fat is worth 3,500 calories, so if you cut 500 calories per day, you’ll lose one pound per week (7 x 500 = 3,500).

Except this isn’t how human metabolism works. The human body is a complex and dynamic system that responds quickly to changes in its environment.

When you undereat, especially over a longer period (that part is important), this complex system adapts.

Here’s an example of how this might play out:

  • You expend less energy in digestion because you’re eating less.
  • Resting metabolic rate goes down because you weigh less.
  • Calories burned through physical activity go down since you weigh less.
  • Non-exercise activity thermogenesis (daily-life fidgeting, movement) goes down and you expend less energy through the day.
  • Your digestion slows down, and you absorb more energy from your food.

our body will also adjust hormonal feedback and signaling loops. For instance:

  • Appetite and hunger hormones go up (i.e. we want to eat more, are more stimulated by food cues, may have more cravings).
  • Satiety hormones go down (which means it’s harder for us to feel full or satisfied).
  • Thyroid hormones and sex hormones (both of which are involved in metabolic rate) go down.

Your planned 500 calorie daily deficit can quickly become 400, 300, or even 200 calories (or fewer), even if you intentionally exercise as much as you had before.

And, speaking of exercise, the body has similar mechanisms when we try to out-exercise an excessive intake.

For example, research suggests that increasing physical activity above a certain threshold (by exercising more) can trigger:

  • More appetite and more actual calories eaten
  • Increased energy absorption
  • Lowered resting or basal metabolism
  • Less fidgeting and spontaneous movement (aka NEAT)

When it comes to genetics, here are some differences between individuals, and they can be the same sex, weight, height etc..

  • Your basal metabolic rate — remember, that’s the energy you need just to fuel your organs and biological functions to stay alive — can vary by 15 percent. For your average woman or man, that’s roughly 200-270 calories.
  • Genetic differences matter too. A single change in one FTO gene can be an additional 160 calorie difference.
  • Sleep deprivation can cause a 5-20 percent change in metabolism, so there’s another 200-500 calories.
  • For women, the phase of their menstrual cycle can affect metabolism by another 150 calories or so.

And yes there is such a thing called “water retention”.

Cortisol is one of our “stress hormones”, and it has effects on our fluid levels.

Food and nutrient restriction is a stressor (especially if we’re anxious about it). When we’re stressed, cortisol typically goes up. People today report being more stressed than ever, so it’s easy to tip things over into “seriously stressed”.

When cortisol goes up, our bodies may hold onto more water, which means we feel “softer” and “less lean” than we actually are. This water retention can mask the fat loss that is occurring, making it seem like we aren’t losing fat and weight, when in fact we are.

So what about recommendations?

You probably think that I am going to tell you to count calories.

Yes and No.

I want you to track your calories for ONE WEEK and one week only ok?  Whatever foods you were going to eat log them with portion sizes using scales.

You can find hundreds of calorie counting apps on mobile phones such as MyFitnessPal and Fat Secret.  Some even have a barcode scanner that instantly recognise the food.

At the end of the week tally everything up and see if it matches with whatever number you thought you were consuming.

You might be surprised!!!

This achieves another thing too, it teaches you about portion sizes and estimates.  You get to know how many grams one full plate of rice is etc.

Sometimes just the act of tracking increases our awareness of our intake, which helps us make better choices.


About the author


Hi, my name is Marwan and I set up this site to help skinny guys and girls just like you to end their frustrations and have the body of their dreams. I believe there are no limitations to what mankind can achieve.