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Weight Loss Guide

Why You Aren’t Losing Weight: The Link Between Undereating and Weight Gain

Published in Weight Loss

9 min read

November 16, 2021
March 8, 2023
a person eating a green salad, looking unsatisfied
a person eating a green salad, looking unsatisfied

Have you been trying to shed a few pounds to no avail? Do you find it challenging to manage your weight no matter how much—or how little—you eat? If you’re cutting down on calorie intake and portion sizes and still don’t see the results reflected on your weighing scale, you may be undereating! 

We know that overeating and cutting healthy foods out of our diets can be an issue for weight loss, but undereating is less commonly addressed. One of the signs of undereating is finding that you’re not only not losing body fat, but you may actually be seeing some weight gain. Shocking, right?

As with anything else, remember that this isn’t going to be true for everyone. So don’t start making any drastic changes to your food intake without asking a nutritionist or dietitian for some recommendations.

It’s also a good idea to track and monitor how foods affect your blood glucose levels with a CGM before making any changes to your diet. 

What is Undereating?

Undereating can cause a host of health issues and it can also be the reason some people can't shed the pounds, no matter how hard they try. These people may have been in a caloric deficit for too long.

Or they could be over-exercising without properly supplementing what their body is burning off. You may be wondering about this caloric deficit—shouldn’t it help you to lose weight? The answer is yes, and no. 

Calories are important to weight loss and so by that logic, if you eat fewer calories, you should lose weight. But there's more to it than that. Intermittent fasting and some amount of calorie restrictions can help some people lose weight and get healthy.

However, intermittent fasting and calorie deficit diets are not the same as undereating. Undereating means you’re not eating enough calories to sustain your daily metabolic functions, like breathing, digestion, and temperature regulation. It can lead to a long list of health problems stemming from nutritional deficiencies, including malnutrition. 

Focusing on what you eat is pretty important, too, rather than just how much or how little. Whatever your diet and fitness goals look like, it’s essential to ensure your body gets all the nutrients it needs to function optimally. If you aren’t sure how to do this in a safe, healthy manner, join a coaching program or speak to a registered dietitian. 

Can You Gain Weight From Not Eating Enough?

Undereating may cause weight gain for some people, but even if it doesn’t, it’s important not to eat so little that it adversely affects your health. From constipation to immune dysfunction, not eating enough can lead to a host of health issues.

One of the main reasons that undereating can lead to weight gain is because consuming too few calories can cause your resting metabolic rate to slow down. This means you may burn fewer calories throughout the day. According to research, a slower metabolic rate that can come as a result of calorie restriction can make it more difficult to lose weight.

While a caloric deficit is typically promoted to support weight loss, reducing calories may not help everyone lose weight. There are many other factors, such as activity levels, chronic condtions, genetics, and lifestyle habits that can affect someone's ability to lose weight.

What Are The Symptoms of Undereating?

a person holding a tape measure on a fork, looking unhappy

Here are some of the potential symptoms and side effects to look out for if you're worried that you may be undereating. 

Fluctuating Glucose Levels

Although rare in individuals without diabetes, undereating could also cause hypoglycemia or low blood sugar. If you experience dizziness, sweating, or sugar cravings, you may want to consider a blood test to check your glucose levels. Or, try out a CGM to see if your glucose levels are too low. 

Energy Loss and Chronic Fatigue

a person sitting, looking tired

Do you feel tired no matter how much you sleep? One of the earliest signs you’re not eating enough is having less energy than usual. Our bodies break down foods (mainly carbohydrate-rich foods) into glucose and then burn them for fuel. One of the side effects of not having enough fuel could be a dip in energy levels.

Think of it this way: if you don’t get enough nutrition, you could end up feeling tired all the time. If you undereat for a prolonged period, you can develop chronic fatigue. With chronic fatigue, you may begin to notice that even daily activities are tiring you out.

Over time, you may not have any energy to cook a meal or engage in hobbies you once enjoyed. 

Unhealthy Hair and Nails

Undereating sometimes leads to malnutrition, which will begin to show physically. Your hair and nails rely on proteins, healthy fats, and amino acids to grow strong and healthy. Without enough of those things, your hair will lose its shine, and your nails may become brittle. As malnutrition intensifies, you may even experience hair loss.

Mood Imbalances

a person having mood swings

Serotonin is a chemical in the brain that regulates your moods. When your glucose levels are too low, it can make it difficult for your body to produce enough of this serotonin. This can cause a cycle of mood swings and irritability.

Add to that the dissatisfaction about the weight you’re trying to lose, and you’re stuck in an infinite loop. You may end up eating too little to feel nourished but wondering if you should eat even less to lose weight. Undereating can quite literally make you “hangry!” 


A Slower Metabolism 

Eating too few calories can cause your metabolism to slow down, meaning you won’t burn as much fat off when you engage in physical activity. Your body requires energy when you walk, work out, think, breathe, and… well, just about everything!

When you deprive your body of the fuel it needs to burn calories, it will begin to store food and enter a sort of “survival mode.” So even when you exercise, your body will protect the fat that it has stored, and you may not be able to lose the weight you want to lose. 

Food Obsessions

a person looking on a cookies and a tape measure on a plate

During WWII, the University of Minnesota conducted a study showing that people experiencing a prolonged caloric deficit think about food constantly. Known as The Minnesota Starvation Experiment, the test found its subjects became obsessed with food over time.

These feelings can create an unhealthy obsession with food that can put you at risk of developing eating disorders. 


Muscle Loss

When your body goes too long without the appropriate amount of calories it needs to function, it can stop recognizing the difference between fat and body tissue. It will begin to burn lean body mass for fuel (which you don’t want—muscle is so important!).

But unless it’s an extreme case (like starvation), this is rare. Sufficient protein and exercise have shown to help prevent muscle deterioration and preserve lean body mass, so being in an extreme caloric deficit can also be detrimental for things like muscle growth. 

The Chills

a person sitting on a sofa covered with blanket

Do you ever reach for a sweater while everyone else is still in a t-shirt? Do you feel unnaturally cold, even on a warm day? This could be due to a lack of nutrients in your diet, which may be because you’re not eating enough.

Here’s an interesting piece of information about your body and heat: Thermogenesis is the dissipation of energy in the body, which occurs through heat production. When you don’t ingest enough calories, your body isn’t producing as much energy—aka heat!

So, the next time you feel chilly and can’t explain why, make sure that you’re getting enough to eat so that your body can regulate its temperature.

How to Regain a Healthy Caloric Intake if You’ve Been Undereating

a person smiling and eating

Intermittent fasting and caloric deficits may help many people reach their target weight and achieve their health goals. But as always, remember there’s no one-size-fits-all when it comes to staying healthy.

So, you have to make sure that whatever changes you make, you’re eating enough food for your body. The best way to regain a healthy caloric intake while still addressing your dietary and weight loss concerns is to get a nutrition coach (like a registered dietitian) to help with your specific needs. Here’s what else you can do:

Sleep Well

Sleep is crucial to maintaining a healthy metabolism as well as healthy hormonal levels. As you reintroduce more calories into your diet, make sure you’re forming a reliable sleep routine.

Try to create an environment in which you can sleep well through the night. Simple things like keeping the room dark, minimizing technology use, and not eating too close to bedtime can help you sleep better. 

Track Your Blood Glucose Levels

Does reintroducing calories, carbohydrates, and fats into your diet sound too intimidating? Do you find it difficult to balance healthy eating with your weight loss goals? An excellent way to create a “safety net” is to use a tool like a CGM.

CGMs track your blood glucose levels in real-time. So, when you eat and exercise, you have trackable data to see what foods and activities your body responds well to. This can help you get back on track in a way that keeps you in the driver’s seat when it comes to your health goals.

Eat Balanced Meals, Regularly

As your body readjusts to having a healthy caloric intake, eating small meals throughout the day can keep your glucose levels stable. It can also help your body understand that you’re no longer in “survival mode." It can also feel more manageable to eat small amounts if you’re worried about adding more calories into your diet. 

Introduce Gentle, Frequent Movement

two people doing yoga

Not only does movement help your body burn calories, but it also helps to regulate glucose levels and aids digestion. Try going for short walks or doing some light yoga before and after meals. If you work at a desk or sit for long periods throughout the day, introduce little stretching sessions or short walks to help kickstart your metabolism.

Get Advice From a Registered Dietitian 

Sometimes we all need a little extra help and encouragement to reach our health goals. Registered dietitians can help you understand your metabolic needs and interpret data if you’re using a CGM. A dietitian you trust can help you meet your weight loss goals by maintaining healthy eating practices, without sacrificing your overall health. 

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Engage with Your Blood Glucose Levels with Nutrisense

Your blood sugar levels can significantly impact how your body feels and functions. That’s why stable blood glucose levels can be an important factor in supporting overall wellbeing.

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When you join the Nutrisense CGM program, our team of credentialed dietitians and nutritionists are available for additional support and guidance to help you reach your goals.

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Kara Collier

Reviewed by: Kara Collier RDN, LDN, CNSC

Kara Collier is the co-founder and VP of Health at Nutrisense, one of America’s fastest-growing wellness-tech startups, where she leads the health team. She is a Forbes 30 under 30 recipient, frequent podcast guest & conference speaker.