Weight loss no longer has to be an exhausting uphill battle. Glucagon-like peptide-1 (GLP-1) drugs – sold under brand names such as Ozempic, Wegovy and Mounjaro – have revolutionised how we tackle obesity, delivering dramatic results through a simple injection.
Adults on the highest doses have been shown to lose more than a fifth of their body weight in 16 months, while gaining benefits for their liver, heart and even brain.
Little wonder these treatments are ferociously popular. In the US, approximately 12 per cent of adults say they’ve tried GLP-1s – that’s 41 million people. Across the Atlantic, an estimated 1.6 million adults in the UK took a weight-loss drug in the last year.
The surge in GLP-1 use might look like a clear win for our waistlines. But beneath the excitement, a less visible problem could be taking shape.
Emerging research suggests that within months of starting these drugs, many people begin to fall short on essential nutrients their bodies need to function properly.
For instance, in a Cleveland Clinic study of 460,000 adults prescribed a GLP-1 drug, nearly 1 in 5 developed a nutrient deficiency within a year – many without realising it.
A separate study of 480,000 GLP-1 users by researchers in Mexico points to a similar pattern: 13.6 per cent developed a significant vitamin D deficiency within a year of starting the drugs, while around 60 per cent weren’t getting enough iron or calcium from their diets.
The problem runs deeper still. Other research suggests GLP-1 drug use can deplete a wide range of essentials – magnesium, potassium, and vitamins A, C, D and E among them.
“You don’t actually need large quantities of these nutrients in a healthy diet,” says Prof Giles Yeo, a neuroendocrinology and obesity expert at the University of Cambridge.
“They’re micronutrients – you need really small amounts of them. It’s just that these small amounts are absolutely crucial for the running of the body.”
And it’s not just micronutrients – meaning vitamins and minerals – that are an issue here. Adults on GLP-1s may also struggle to consume enough protein and fibre.
But, despite their prevalence, these deficiencies are unlikely to be spotted by a medical professional. Many people take GLP-1 drugs without their nutrition being monitored – something described as a “critical oversight” by a 2025 article in the International Journal of Obesity.
Dr Steven Heymsfield – director of the Metabolism and Body Composition Laboratory at Louisiana State University, in the US, and senior author of the Cleveland Clinic study – agrees.
“Obesity is a chronic disease and it should be managed like one,” he says. “If your doctor prescribes these drugs for you, that doctor should assess you with nutrient blood tests – just like they do for other diseases.”
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Smaller portions, bigger problems
So why do these deficiencies form in the first place? It largely comes down to how these drugs reshape how much you eat.
GLP-1 drugs mimic a natural fullness hormone, helping you feel satisfied on smaller portions. That’s what drives the weight loss.
But it also means fewer opportunities to consume the full range of nutrients your body needs each day.
“For many people on these drugs, their diets may not have been great to begin with,” says Yeo, noting that they’re often low in fresh fruit and veg, limited in variety, or heavy in ultra-processed foods that lack essential nutrients.
“If you just take the drugs and don’t make any behavioural changes, then you’re going to eat less of a not-great diet,” he continues.
“Unless you improve your diet, you might end up with nutritional deficiencies.”
That’s why the deficiencies that crop up among GLP-1 users often mirror those that doctors routinely spot in the general population – vitamin B12 and iodine in vegetarians and vegans, iron in women due to menstruation, and vitamin D in anyone living somewhere short on sunshine.
“Those are the deficiencies we already see relatively commonly,” says Yeo. “Those are only going to be worse [with GLP-1s].”
But these drugs don’t just increase our risk of deficiencies by shrinking our meals. Other side effects can play a part too.
“When people are treated with GLP-1s, they reduce their food intake – but they also get gastrointestinal symptoms, like vomiting or diarrhoea,” says Heymsfield.
“If you have diarrhoea, you’re going to lose nutrients through that pathway.”
In other words, some of the nutrients you do eat while on this medication might leave your body before they’re absorbed.
These side effects aren’t rare. Nearly a third of GLP-1 users experience diarrhoea, and almost a quarter report nausea and vomiting.
Tipping the scales
For experts, the size of the problem is alarming. With millions now using GLP-1 drugs, huge swathes of the population could be developing nutrient gaps without realising.
“I think this will become very important,” says Heymsfield. “I’ve been telling people, don’t write off GLP-1s as completely safe. You’ve got to wait and see how the deficiencies play into long-term impacts.”
At their most extreme, nutrient deficiencies can be medical emergencies. For example, some people have developed severe neurological conditions – characterised by slurred speech, disorientation, weakness and other symptoms – due to vitamin B deficiency, after taking GLP-1 drugs.
Such instances are rare. But among the general population, even vitamin D deficiencies – the most common – can have a very real impact on future health.
Heymsfield explains that low levels of vitamin D could lead to bone loss (osteoporosis) and increase the risk of frailty in old age. That danger is heightened if you’re also missing out on protein, and therefore losing a lot of muscle as you lose weight, known as sarcopenia.
“There are people who are 70 years old, who are obese, who are going on these drugs, and that puts them on a path to sarcopenia and frailty,” says Heymsfield. This can increase someone’s risk of falls and fractures in later life.
A tale of two diets
The growing problem is “a real worry” for Yeo. “It’s absolutely crucial to remember that, at the moment, the vast majority of people getting [GLP-1s] are privileged, so there’s a floor to how poor their diets were to begin with,” he says.
Indeed, most GLP-1 users in the UK currently access the drug privately – and in the US, monthly bills are often in the hundreds of dollars. These groups are more likely to be affluent, and able to afford healthy food.
But recent innovation, such as new pill versions of GLP-1s, is expected to gradually bring down the price of the drugs and enable more people to access them.
“At some point, huge amounts of people who are not as privileged – and therefore their diets, often not by choice, are far poorer – will have access to these drugs. That, I think, is going to be a serious situation,” says Yeo.
While supplements can help people avoid specific micronutrient deficiencies, Yeo is clear that they’re no substitute for proper medical support for anyone using GLP-1s.
“We don’t want to overstate and scaremonger, because these drugs are still powerful tools,” he adds. “But we have to watch out for the micronutrients that we know are common deficiencies. Those will only become worse when people eat less.”
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