Can You Gain Weight By Eating Too Little?

Weight loss is a tricky thing. There are so many diets, fasts, cleanses, etc out there that claim to help with loss that it can become confusing as to what actually works and what doesn’t. One common frustration is that some people cut back on what they are eating and somehow gain weight (or stay the same). After a quick search you might find something that speaks about how eating too little might be slowing down your metabolism and putting you in a starvation mode where your body tries to hang on to everything it has. Others may say people are tracking their food improperly or claim something is going on hormonally.

The truth is, humans do not defy the laws of thermodynamics. The first law applies most directly to nutrition – that energy cannot be created or destroyed. Simply put, we take in energy in the form of food, and expend it the form of activity. If you expend less energy that you take in, your body stores the energy in the form of body fat.

Here are some examples of processes that expend energy

  • Breathing
  • Blood circulation
  • Movement
  • Digestion
  • Heat production
  • Organ function
  • Excretion
  • Muscle recovery

At the end of the day if you absorb more calories from food than you expend through processes like the ones above, you will gain weight. It has been proven over and over again in countless studies. The problem is that there are too many factors too 100% accurate to determine how much energy someone expends and how much energy someone is absorbing. You can see from the short list above that every human would operate differently and expend a different amount of calories. Not to mention that you will expend dramatically different amounts of calories from day to day depending on what you’re doing.

On the other side of the equation is how much energy is coming into the system (your body). Did you know that if you eat the same meal as your friend next to you, you may be getting a different amount of calories from that meal? That is because some people break down and absorb the food they eat better than others. If some food is simply passing through you, you will not get calories from it (by the way, this is NOT a good thing).

With all of this said, it is nearly impossible to determine someone’s BMR (amount of calories it takes to keep you alive at rest) or TDEE (Total Daily Energy Expenditure) with a machine. It may spit out a number, but it should only be used as a reference point to get you started.


So how come you’re still gaining weight if you’re eating less calories than you’re expending? 

You’re not eating less calories than you’re expending. The most common reason for that is perception.

Humans are terrible at judging how much they eat. I once did a seminar where I had everyone take a scoop of peanut butter that they thought was 1 serving (2 tablespoons/ 32 grams) and then weigh it. Out of 15 people in the seminar, zero people were under 32. Most were around 40-50g. One client weighed his scoop at 76 grams. That’s more that DOUBLE the serving size. 

Dr. Baradi of Precision Nutrition talks about a time where we went out to eat, got one of the menu’s low carb “healthy” options, and a piece of cheesecake for dessert. After he decided to check to see how many calories it had. 5,000. FIVE THOUSAND! 

So now imagine you’ve been in a perfect 300 calorie deficit for 6 days in a row (1,800 calories) and today you just went 5,000 calories over. You’re now net + 3200 calories. And this isn’t unrealistic. It happens all the time, even to professional nutritionists and dietitians. 

With that said, under eating for a LONG TIME can definitely have a negative impact on your metabolism. This is especially true for people who are chronic dieters and have lost weight in the past, without a phase where they can build lost muscle. Here’s how.

  • Eating less = digesting less = less calories burned.
  • BMR goes down because you weigh less = less calories burned
  • Calories burned through movement go down because you weigh less = less calories burned
  • Digestion slows down = you absorb more calories
  • This is just one simple example of how eating less for a long time could cause you to be expending less energy now and absorbing more of it.

The point is, you are still absorbing more calories than you are expending in this case if you’re not losing weight.

The human metabolism is incredibly complex. This didn’t even talk about how sleep deprivation, genetics, or menstruation can affect weight change.

So what can you do?

    1. Measure your intake. This isn’t a perfect strategy, and can sometimes be stressful for people, but if you don’t know what your normal intake is, you’ll have a hard time adjusting properly. Use a scale, your hands, measuring cups, etc. Get a baseline, and make changes as needed.


  • Choose healthy Options. Whole, non processed foods are simply much harder to eat. Try to get a full head of broccoli and you’ll know what I mean.
  • Mindset. If you’re someone who has been dieting for a LONG time, take a step back, add some calories and focus on building a bit of muscle. This way, when you go to lose weight next time you can do so eating more calories, making it easier for you to lose weight. 
  • Forgive yourself. You’re going to have days that aren’t perfect. Maybe your weight goes up from water retention. Realize this isnt a short term fix. Forgive yourself for days that aren’t perfect, and get right back on track!
  • BODY WEIGHT AND COMPOSITION ARE NOT THE SAME. BOLD, UPPERCASE, ITALICS for reason. This blog focused on body weight. You may be getting closer to how you want to look, but the scale is going the opposite direction. That is OK! The scale is just a number. At the end of the day, body composition is what you’re after.


If you’re still having trouble, get coaching. Weight loss or gain can be hard. Having a coach to lean on, keep you accountable, and do the hard thinking for you can help tremendously.


  • Coach Andrew