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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
December 2, 2025
two people talking
two people talking

Key Takeways

Rapid weight loss from extreme diets often leads to serious side effects, including muscle loss, a slower metabolism, and nutritional deficiencies. These approaches can also disrupt hormones, cause fatigue, and even lead to painful gallstones, making sustainable results difficult to achieve.

Key Points:

  • Unhealthy weight loss can trigger "metabolic adaptation," where your body slows its metabolism to conserve energy, making it harder to keep weight off.
  • Work with a registered dietitian to build a sustainable plan based on your body’s unique data instead of following a one-size-fits-all diet.
  • Many insurance plans cover dietitian video calls, often with a $0 out-of-pocket cost for eligible members.

Why Can Rapid Weight Loss Backfire?

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?

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.

That 'Win' on the Scale That Feels Like a Loss

You’ve been incredibly strict all week, sticking to that new diet you read about. This morning, the scale finally showed a number you haven’t seen in years - a five-pound drop. But instead of feeling triumphant, you feel drained. A dull headache is setting in, and you snapped at your partner over nothing. 

Looking at your tiny, pre-portioned lunch, you wonder if this is the only way. Is losing weight supposed to feel this bad? It feels less like progress and more like punishment.

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 for 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.

How Does Metabolism Affect Weight Loss?

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.

What are the Side Effects of Unhealthy Weight Loss?

Unhealthy weight loss can differ from person to person, but it 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 a variety of health issues, including slower metabolism and the loss of muscle mass.

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 few 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 dietitianto set a safe pace.

Nutritional Deficiencies

Nutritional deficiencies lead to many health problems, including hair loss and fatigue. These deficiencies can also affect your immune system.If intake of iron, vitamin B12, vitamin D, calcium, phosphorus, or folate is low, bone health may be affected.

Gallstones

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.

Dehydration

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.

Headaches and Constipation

You could also experience headaches, constipation, hair loss, brittle bones, extreme cravings, and hormonal changes or menstrual irregularities. A healthy weightsupports healthy glucose patterns and overall metabolic health.

Micronutrients to Monitor During Weight Loss

Restrictive approaches can make it easy to miss specific micronutrients, so build simple checkpoints to keep intake balanced for your needs.

Micronutrients commonly reviewed during weight changes include thiamine, folate, vitamin B12, vitamin D, calcium, magnesium, zinc, potassium, vitamin C, vitamin E, and omega-3 fatty acids.

  • If you have a history of anemia, osteopenia or osteoporosis, or peripheral neuropathy, ask your clinician which labs and monitoring schedule make sense before adjusting intake.
  • Following very low-calorie, ketogenic, or fasting patterns can limit variety. Plan how you will maintain micronutrient coverage and pay attention to electrolytes such as magnesium and potassium.
  • For a food-first plan and to personalize supplements only when needed, book insurance-covered video calls to work with a registered dietitian.
  • Track day-to-day signals like energy, sleep, appetite, and mood. Make small, steady adjustments rather than abrupt cuts.

5 Ways to Prevent Unhealthy Weight Loss

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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Find the right Nutrisense program    to turn insight into progress.

FAQs about unhealthy weight loss and safe progress

Q1. How can I tell if my weight loss approach may be too aggressive?

A1. Red flags include skipping meals, cutting entire food groups, constant hunger, disrupted sleep, poor workout recovery, and sharp week-to-week drops on the scale. If you notice these patterns, slow down and reintroduce balanced meals. A registered dietitian can help set a plan that matches your needs and pace.

Source: Why Undereating Might Be the Reason for Your Weight Gain

Q2. How does glucose tracking help me avoid unhealthy weight loss methods?

A2. A continuous biosensor shows glucose measured in interstitial fluid 24/7, so you can see how meal timing, portions, and macros affect your responses. That insight helps you choose balanced meals, adjust pre- and post-workout fuel, and pace weight loss without extreme restriction. Pair the data with a dietitian's guidance to set sustainable targets.

Source: CGM for Weight Loss

Q3. What should I track besides scale weight to maintain a healthy pace?

A3. In the Nutrisense app, log meals, hunger, energy, workouts, and sleep alongside glucose trends. Review patterns over 7 to 14 days, then adjust meal timing, composition, and training recovery. Notes and experiments help you test one change at a time and avoid large calorie cuts that can backfire.

Source: Nutrisense App

Q4. If I start feeling lightheaded or fatigued during weight loss, what are the next steps?

A4. Pause intense training, prioritize regular balanced meals and fluids, and review your logs to identify large calorie gaps. Schedule a video call with a Nutrisense dietitian to adjust your targets and meal timing. If symptoms are severe or persistent, contact your healthcare provider for evaluation.

Source: Nutritionist video calls

Go Beyond Glucose Data with Nutrisense

Your glucose can significantly impact how your body feels and functions. That’s why stable levels are an important factor in supporting overall wellbeing. But viewing glucose isn't enough. Nutrisense, you’ll be able to learn how to use your body's data to make informed lifestyle choices that support healthy living.

One-to-one coaching

Sign up to access insurance-covered video calls to work with a glucose expert: a personal registered dietitian or certified nutritionist who will help tailor your lifestyle and diet to your goals.

Monitor and measure what matters

With the Nutrisense CGM Program, you can monitor your glucose with health tech like glucose biosensors and continuous glucose monitor (CGM)s, and analyze the trends over time with the Nutrisense App. This will help you make the most informed choices about the foods you consume and their impact on your health.

Find your best fit

Ready to take the first step? Start with our quiz to find the right Nutrisense program to help you take control.

Go Beyond Glucose Data with Nutrisense

Your glucose can significantly impact how your body feels and functions. That’s why stable levels are an important factor in supporting overall wellbeing. But viewing glucose isn't enough. With Nutrisense, you’ll be able to learn how to use your body's data to make informed lifestyle choices that support healthy living.

Personalized Nutrition Guidance

Sign up to access insurance-covered video calls with a glucose expert: a personal registered dietitian or certified nutritionist who will help tailor your lifestyle and diet to your goals.

See 24/7 Trends in One Place

With the Nutrisense Program, you can monitor your glucose with health tech like glucose biosensors and continuous glucose monitors (CGMs), and analyze the trends over time with the Nutrisense App. This will help you make the most informed choices about the foods you consume and their impact on your health.

Ready to Begin?

Start with our quiz to find the right Nutrisense program to help you take control.

Reviewed by: Cheri Bantilan

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 exploring and using your own body (symptoms, energy, mood, labs, CGM data) to find what your personal optimal is.

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