Fatigue and Weight Loss: Can Losing Weight Make You Tired?

Weight loss can lead to a whole host of benefits, like better sleep, less joint pain, and improved long-term health. But it can also cause fatigue.

What’s up with fatigue and weight loss? It could be due to eating fewer calories, nutritional deficiencies, exercising more, or side effects from weight loss medication, if you’re on it.

You might find you feel more energetic in the long run, though, so it’s worth sticking with your weight loss plan.

Read on for everything you need to know about fatigue and weight loss, including what can cause it and how to get more energy while losing weight (and beyond).

Losing weight can cause fatigue and leave you feeling weak and sluggish. Though it’s common and usually nothing to worry about, it’s not exactly what you want when making positive steps to improve your health.

Here are some common causes of weight loss-related fatigue:

• Reduced calorie intake

• Nutritional deficiencies

• Dehydration and electrolyte imbalance

• Hormonal changes

• Increased exercise

• Stress

• Side effects from weight loss medication

Keep reading for detailed insight.

1. Reduced Calorie Intake 

You might have reduced your calories to lose weight. This can be an effective weight loss method, but the reduction in calories means your body is getting less fuel from food.

So, if you’ve cut your calories dramatically, you might experience fatigue. 

And a big reduction in calories may mean you’re not eating enough, which can lead to hypoglycemia (low blood glucose, aka blood sugar). People with diabetes are more at risk.

Hypoglycemia can cause symptoms like:

• Tiredness

• Irritability

• Headache

• Dizziness

• Lightheadedness

• Feeling shaky or jittery

The timing of your meals can impact your energy levels, too. Does fasting make you tired? It sure can. 

When fasting, you may not be getting enough calories or nutrients to fuel your body throughout the day. And you might experience low blood sugar, especially if you’re skipping meals or going too long between meals.

2. Nutritional Deficiencies 

If you’ve made big changes to your eating habits or reduced your calories dramatically, you might not be getting enough nutrients for your body to function properly and feel its best.

In particular, protein is vital for muscle recovery and growth. Not eating enough protein may lead to muscle loss and decreased strength. And extreme protein deficiency can cause weakness.

That’s not the only important nutrient, though. Healthy fats give your body energy, promote proper cell function, and help with nutrient absorption. As for carbohydrates, your body breaks down carbs into glucose, a primary source of energy for your body.

It’s not just these macronutrients, either. Micronutrient deficiencies (not getting enough of certain vitamins and minerals like iron, folate, and vitamin B12) can also lead to fatigue.

2. Dehydration and Electrolyte Imbalance 

If you’re not drinking enough water, you can become dehydrated, which can lower blood pressure and cause fatigue.

You may also have an electrolyte imbalance, which can be caused by:

• Not drinking enough water, especially during intense workouts or being outside in hot weather

• Heavy sweating (perhaps from your new workout routine)

• Drinking too much water

• Vomiting and diarrhea

If your electrolytes are out of whack, you might experience fatigue, weakness, and sluggishness.

3. Hormonal Changes 

Being in a calorie deficit and losing weight can lead to hormonal changes, like a decrease in leptin (the satiety or fullness hormone) and an increase in cortisol (the main stress hormone).

These hormonal changes might impact your appetite, metabolism, and sleep, which can lead to fatigue.

Check out our guide to hormones and weight loss to learn more.

4. Increased Exercise or Not Recovering From Exercise  

You might have started working out more in your efforts to lose weight. This could result in fatigue and muscle soreness, especially if you’re not eating or sleeping enough to recover properly from your workouts.

And that’s not all. During physical activity, your body turns to glycogen for energy, and eating carbs tops up your glycogen stores.

If you’re not eating enough carbohydrates, your glycogen stores might be running low, leading to poor recovery and feeling tired and weak during your workouts.

5. Stress 

It can sometimes feel like you can’t avoid stress, no matter what you do. It’s a normal part of life, but chronic stress can disrupt your sleep, and it’s directly linked to fatigue.

To make matters worse, weight loss itself can be stressful. You might feel stressed when you’re making changes to your lifestyle, starting weight loss medication, following a restrictive routine, or feeling any kind of pressure to lose weight.

6. Side Effects From Weight Loss Medications 

Anecdotally (like on Reddit forums), some people taking GLP-1 weight loss meds have mentioned fatigue. If you’re taking weight loss medication to help on your journey, common side effects could contribute to fatigue.

For example, Ozempic^® side effects include:

• Nausea

• Vomiting

• Constipation

• Diarrhea

• Abdominal pain

Dealing with these side effects can be mentally draining and disruptive to your sleep. Vomiting and diarrhea can also cause dehydration and electrolyte imbalances, both of which can lead to fatigue.

Weight loss meds can suppress your appetite (this is one way they help you lose weight). But if you’re not careful, you might find yourself eating too few calories, not eating enough protein, or not drinking enough water because you’re feeling full.

Check out our guide to Ozempic and fatigue to learn more.

So, fatigue and weight loss are an unfortunate pair. But what can you do about it? Here’s how to increase your energy levels and feel your best when you’re losing weight.

Eat Enough Nutritious Foods

Make sure you’re eating enough food to fuel your body, even while you’re losing weight. And aim to eat a variety of nutrients to avoid deficiencies.

Here’s what to go for:

• Plenty of fresh fruits and vegetables

• Lean protein like chicken, turkey, eggs, tofu, tempeh, fish, or lean beef

• Healthy fats like olive oil, avocado, nuts, and seeds

• Whole grains like brown rice, quinoa, oats, and whole-wheat bread

• Legumes like chickpeas, lentils, beans, and peas

• Low-fat or fat-free dairy products

We’ve got a weight loss meal plan if you need some inspiration. 

It can be tough to strike a balance between reducing your calories to promote weight loss and eating enough calories to avoid fatigue.

Speaking to a registered dietitian or nutritionist can help. You can also try to tune into your hunger and fullness cues for a better idea of when you need to eat more and when you’re full.

It might also be worth asking a healthcare professional about supplements if you think you have a nutrient deficiency.

If you’re on weight loss medication, make sure you’re eating enough (especially protein), even with a smaller appetite. Eating smaller, more regular meals can help if your appetite just isn’t there or if nausea or other side effects crop up.

Drink Enough Water

Avoid dehydration by drinking plenty of water. In general, guidelines state women should get about 91 ounces of water a day, and men should shoot for 125 ounces a day. This is from both food and water.

If you’re struggling to drink enough water throughout the day, try:

• Getting a reusable water bottle and taking it with you everywhere

• Drinking a glass of water before each meal (this can also keep your appetite in check)

• Swapping plain water for sparkling water or water flavored with mint, raspberries, or cucumber

Check out our guide to drinking more water for more advice.

Prioritize Sleep

If you’re feeling low on energy, take a look at your sleep. Aim for seven to nine hours of good-quality sleep a night.

If those numbers feel like a far-off dream, you can improve your sleep by:

• Keeping a regular sleep schedule, even on weekends and days off

• Avoiding screens around bedtime — try turning off devices at least 30 minutes before lights out

• Keeping your bedroom for sleep and sex only

• Making your bedroom cool, dark, and relaxing

Our guide has more pointers for how to sleep better.

Get More Movement

Counterintuitively, exercise can actually boost your energy levels when done right. Research shows it can decrease fatigue and increase energy and feelings of vitality.

If you don’t feel up for a full gym sesh, still try to get some movement in every day, like a brisk walk around the block or a gentle yoga flow. Every little bit counts.

As for overtraining, listen to your body and take rest days when you need them. Make sure you’re eating nutritious foods and drinking enough water to fuel your movement.

We have tips on how to get back into working out if you’re making a comeback.

Manage Your Stress Levels

Try to lower your stress levels to improve your energy and sleep.

You can keep stress in check by: 

• Finding relaxing activities you enjoy, like yoga, journaling, reading, or crafts 

• Spending time in nature and with loved ones (bonus points if you do both at the same time)

• Practicing meditation or doing deep-breathing exercises 

• Prioritizing sleep and regular exercise

• Reaching out to a therapist if needed 

If weight loss is stressing you out, reach out to friends, family, or a therapist for support.

Remember, healthy weight loss is slow and steady, and your goals and expectations should reflect that. You’ve got this.

We’ve focused on the link between planned weight loss and fatigue. But if you’ve noticed unexpected weight loss and fatigue, reach out to a healthcare provider to find out if there’s an underlying health issue at play.

Unintentional weight loss is characterized as losing at least 10 pounds or five percent of your usual body weight over six to 12 months without knowing why.

Some health conditions can cause a loss of appetite or weight loss in other ways.

Possible causes of unexplained weight loss include:

• Mental health conditions like anxiety and depression 

• Hyperthyroidism (overactive thyroid gland)

• Celiac disease

• Crohn’s disease

• Ulcerative colitis

• Undiagnosed diabetes

• Undiagnosed eating disorders like anorexia

• Chronic pancreatitis (inflammation of the pancreas)

• Irritable bowel syndrome (IBS)

Similarly, if you’re intentionally losing weight but experiencing extreme fatigue and weight loss, reach out to your primary care provider or another healthcare professional to check for medical conditions that could be affecting your energy levels.

Weight loss can lead to lots of positives in life, but it can also cause fatigue. Although, you might find you have more energy soon enough.

Here’s a final rundown of what you need to know: 

• Weight loss can cause fatigue. This might be due to factors like eating fewer calories, developing a nutritional deficiency, or exercising more. Side effects from weight loss meds may also contribute to fatigue if you’re on them. 

• Small changes can improve your energy. Make sure you’re eating a variety of nutritious foods and getting enough protein, carbs, and healthy fats. Drink plenty of water, get enough sleep, and do some movement if you can. 

• Reach out to a healthcare provider if you’re concerned. If you have fatigue and unintentional weight loss or severe fatigue and weight loss, speak to a healthcare provider. They can check for underlying health conditions that could be to blame.

If you’re considering weight loss medication, you can take our free online weight loss assessment. A healthcare provider will go over your answers and let you know if you’re a good candidate. You can also discuss any worries about fatigue if it’s something that crops up for you.