Fatigue, low energy – or something more serious: What sudden weight loss could mean, even from GLP-1s

Weight loss drugs have helped millions of Americans rapidly shed dozens of pounds in a matter of just months, driving up the usage of the shots and leading drugmakers to test even stronger medications.

But rapid weight loss – whether achieved by using the medications, dieting or losing weight unintentionally – comes with multiple physical consequences.

Experts say it can affect peoples’ metabolism and leave them feeling fatigued and low energy. It can impact people’s muscle mass and levels of key nutrients. It may even be the sign of a more serious condition, such as cancer or dementia.

“Any time you lose weight, you're going to lose a little bit of fat and muscle. The goal would be to lose more fat, and we spare as much muscle as possible,” Cleveland Clinic registered dietitian Julia Zumpano explained in a statement shared Tuesday.

“And the reason we want to spare muscle is when we decrease our muscle mass, we have a risk of sarcopenia, and that's just decreased muscle. But that makes us weaker, more prone to injury.”

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Sarcopenia typically affects people over the age of 60 and is a major factor in falls and fractures, according to the clinic.

Muscle loss is a common side effect of diabetes and weight loss drugs.

Previous research from the University of Utah Health has shown that using the popular GLP-1 medication Ozempic decreased lean mass – or muscle mass without fat – by around 10 percent, although some estimates show as much as 40 percent.

However, new next-generation amylin analog injections – that work by imitating the body’s natural production of the blood sugar-regulating hormone amylin – may have a lower loss of lean muscle mass.

And severe calorie restriction in diets that lead to losing muscle mass could actually complicate weight loss efforts, Cleveland Clinic endocrinologist Dr. Marcio Griebeler explained.

“It’s harder to lose weight when you don’t have enough muscle mass because muscles burn lots of calories,” he said.

Muscle loss can also be a warning sign of a condition in cancer patients known as cachexia, a serious complication associated with illnesses including cancer, dementia, heart failure and chronic disease that leads to dramatic weight and muscle loss.

It occurs in 70 percent of people with advanced or late-stage cancer, according to the clinic.

But muscle loss isn’t the only thing people losing weight need to worry about.

Changes in the metabolism – the body’s calorie-burning process – also come with fast and extreme weight loss. The metabolism slows down, with the body working to conserve energy because you’re consuming fewer calories.

“When you lose weight too quickly, your body slows down its calorie-burning process. That is your body’s way of trying to ensure you don’t starve,” Griebeler said.

“You might lose a good amount of weight right away, but your metabolism quickly goes into survival mode.”

The change in the metabolism is a key reason why people regain weight after dieting. The body isn’t used to so many calories and cutting calories too quickly makes you want to eat more.

“Even a very determined person will find it difficult to overpower those hunger hormones. These fast hormonal changes make you so hungry that they set you up for diet failure,” said Griebeler.

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Another thing that may cause some unfortunate side effects is nutrient loss.

You’re also reducing the nutrients you get from food when you slash your calorie intake, including vitamins and essential minerals.

This can lead to constipation, fatigue, bone density loss, an increased risk of illness and even hair loss, according to the clinic.

GLP-1 drugs act to limit appetite, resulting in people reducing their energy intake by up to 40 percent.

With hunger lessened, patients may struggle with maintaining a balanced diet.

“If nutritional care is not integrated alongside treatment, there’s a risk of replacing one set of health problems with another,” Dr. Marie Spreckley, a researcher at the University of Cambridge, cautioned in a release.

This varies on a case by case basis: Zumpano, of the Cleveland Clinic, places a limit of one to two pounds a week, and Hartford HealthCare’s Dr. Devika Umashanker recommends losing four to eight pounds in a month.

People with more weight to lose, however, might see a bigger drop in the beginning, according to Zumpano.

Weight loss drug users have reported losing 10 pounds in as little as a week, for example.

The best way to fight harmful effects of intended weight loss is by maintaining a well-balanced diet and combining that with exercise. People shouldn’t try to cut more than 500 calories a day, according to Griebeler.

Plus, slower weight loss and exercise gives your body time to lose fat while keeping muscle mass and the ability to burn more calories.

“If we can try to slow down that rapid weight loss a little bit, then as we're losing weight, we maintain adequate muscle mass by doing light resistance training and meeting protein needs,” said Zumpano.