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The Side Effects of Unhealthy Weight Loss

Yvonne Mahl, MHSC, RDN, LDN, IFNCP

Published in Weight Loss

7 min read

February 16, 2022
two people talking
two people talking

Fad diets, weight loss supplements, and rapid weight loss programs may want you to believe you can lose weight in an instant without any side effects. Whatever your reason, you’ve likely thought of shedding a few pounds at some point in your life. And while several types of weight loss can be a good thing, it’s essential to be aware of how your body will react to any significant changes. 

With any lifestyle change, it’s best to track and monitor your progress, make changes incrementally and ask a professional for help whenever possible. Some of the mistakes people make when they’re trying to lose weight are undereating and overexercising in a bid to lose as much weight as quickly as possible. It’s a good idea to listen to your body—if you overexercise, you can put yourself at risk of injuries. Undereating can hamper weight loss plans even more and sometimes prevent you from losing weight altogether.

If you deprive your body of essential nutrients, you may still achieve your weight loss goals, but your weight loss could be unhealthy. And, unhealthy weight loss can lead to health issues and conditions like hair loss, bad skin, and heart disease. But, what is unhealthy weight loss, and how can you tell if you’re experiencing it? Read on to find out more. 

What is Unhealthy Weight Loss?

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If you’re trying to lose weight, it’s not uncommon to want to do so as quickly as possible. And barring quick fixes like diet pills and weight loss supplements, you may not realize what amount is too much too fast. While some experts (like the Centers for Disease Control) suggest a target weight loss goal of one to two pounds a week, everyone’s body differs. So, it’s difficult to pinpoint how much weight you can lose and even more challenging to put a number on how quickly you can lose it without affecting your health.

It’s why it may be even more important to listen to your body and find out what your specific needs are. Just like gaining a lot of weight quickly can put you at risk of health problems, fast weight loss can also put you at risk. Several things will factor in here. How much carbohydrate, protein, and fat intake do you need in your diet? What’s your target weight, and is it the right one for your body? Does a healthy lifestyle for you involve weight gain rather than weight loss? 

Even if you need to lose weight quickly, looking to one-size-fits-all or extreme weight-loss programs isn’t the best way. Instead, consider working with a health care professional like a registered dietitian to see how to get to your target weight healthily. 

Metabolism and Weight Loss

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You likely already know that your body gets energy from three primary nutrients: fats, carbohydrates, and proteins. They travel through your bloodstream until they reach your organs, where they turn into what’s known as adenosine triphosphate, or ATP. It’s a molecule that stores and transfers energy in your cells. 

All these processes require energy, which is measured in calories. When your body gets enough energy from nutrients, the energy begins to turn into heat. When you exercise, your body burns these calories, maintaining your weight and getting the energy it needs to stay healthy.

When we talk about metabolism, we talk about energy balance, which means your energy intake should equal your energy output. For example, a nutritious meal can give your body energy, and an hour of exercise can produce energy expenditure. If you want to lose weight healthily, you want to make sure that you are nourishing your body appropriately. 

The Side Effects of Unhealthy Weight Loss

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Unhealthy weight loss can differ from person to person but will generally include getting too few of the nutrients your body needs to stay healthy. Depriving your body of the calories and nutrients it needs will affect your metabolism and overall wellness. 

It can lead to various health issues, including slower metabolism and the loss of muscle mass. If you’re experiencing any of these weight loss effects, you may want to take a step back and reconsider your weight loss program.

Losing Muscle Mass

A low-calorie diet helps the body get rid of excess water, fat, and sometimes, muscles if it is too extreme! If there is no energy balance, your muscles can weaken over time. So you may eventually be losing muscle instead of fat! 

Slow Metabolism

Crash diets and fast weight loss, especially the sort that deprives your body of essential nutrients, can sometimes slow down your metabolism. If there are little to no calories to burn, your metabolism slows down. Another cause for slower metabolism may also be the loss of muscle mass caused by extreme or yo-yo dieting. Your metabolism recovers very slowly after unhealthy weight loss. So, it’s best to consult with a doctor or dietitian to ensure you’re on the right track when you start your weight loss journey. 

Nutritional Deficiencies

Nutritional deficiencies lead to many health problems, including hair loss and fatigue. These deficiencies can also affect your immune system. And, if you have a lack of iron, vitamin B12, vitamin D, calcium phosphorus, and folate, your bones could become weaker too.


Gallstones are a painful side effect of unhealthy weight loss. They form due to a lack of nutrients—if digestive juices like bile sit in the gallbladder for too long, they can turn into a stone-like material. These gallstones can get stuck inside the opening of the bladder and cause severe pain and indigestion.


During the first week of a fast weight loss plan, your body will lose a lot of water. It can cause some water weight loss, but it can also cause dehydration. Keeping yourself hydrated is vital for healthy metabolic function. And in the long run, drinking more water can help with your weight loss goals. 

You could also experience headaches, constipation, hair loss, brittle bones, extreme cravings, and hormonal changes or menstrual irregularities. A healthy weight is vital for good glucose management and reducing the risk of developing diabetes.

5 Ways to Prevent Unhealthy Weight Loss

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Any weight management journey can be a challenge, whether it’s weight loss, weight gain, or simply staying motivated after reaching your target weight. So, even if you don’t pick extreme weight loss methods, you may unintentionally find yourself experiencing some of the side effects of unhealthy weight loss. Here are a few hints and tips to help you try to prevent that as far as possible on your journey to reaching your target weight.

Work with a Qualified Nutritionist

Remember that a weight loss plan is an excellent way to make sure you stay on track. Work with a dietitian instead of just cutting entire food groups out of your diet. They will guide you to a healthy weight while also helping you get the proper energy intake for your specific needs.

Don’t Cut Out Nutrients

Whatever type of diet you choose, it’s a good idea to consider adding some protein to it, but don’t forget about your carbs, fat... you get the drift! The key to healthy weight loss is balance. Protein helps preserve muscle mass and boosts metabolism, carbs are essential for energy levels, and fats help with the absorption of vitamins and minerals. Don’t cut anything out of your diet unless recommended by a healthcare professional for a specific reason. 

Tailor Physical Activity to Your Body

Engage in physical activity according to what suits your body rather than trying to follow a one-size-fits-all exercise routine. Consider everything from 30-minute workouts to mini workouts, HIIT to aerobic activity. You never know what will work best for you.

Focus on Maintaining a Target Weight

Losing body fat quickly can help you meet weight loss goals in the short term, but what about weight maintenance? Studies suggest that people who lose weight faster than usual (with rapid weight loss) are more likely to regain weight soon after the end of the diet. So, focus on long-term weight maintenance over quick weight loss, even if that means your weight loss journey is a little slower in the short term!

Get Good Quality Sleep

Don’t forget about getting good sleep! Sleep is an essential part of a healthy lifestyle. Lack of sleep can also produce high amounts of ghrelin, the hunger hormone, making it hard for you to lose weight.

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Cheri Bantilan, MS, RD, CD

Reviewed by: Cheri Bantilan, MS, RD, CD

Cheri is a registered dietitian and Health Co-Ordinator Product Development at Nutrisense, with a Master's degree from the University of Utah in Integrative Physiology and Nutrition. She has a strong interest in functional and integrative nutrition and emphasizes the importance of exploration and using your own body (symptoms, energy, mood, labs, CGM data) to find what your personal optimal is.

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