Thinking About Starting Wegovy or Mounjaro? What to Know Before You Start

Thinking about starting Wegovy or Mounjaro? Here's what dietitians wish more people knew first

Man preparing Semaglutide injection

If you're considering a GLP-1 medication like Wegovy, Mounjaro, or Ozempic, the most important thing to understand is that these medications are a powerful tool - but they work best alongside dietary and lifestyle changes, not instead of them.

GLP-1 medications work partly by significantly suppressing appetite, which is exactly why they're so effective for weight loss. But that sharp drop in food intake creates a real nutritional challenge: when you're eating substantially less, your body may not get enough protein, vitamins, or minerals - even while the number on the scale is moving in the right direction. What you eat matters more than ever when you're eating less of it.

How common are nutrient deficiencies, really?

More common than most people expect. A large claims-based study following 461,382 adults who started GLP-1 receptor agonists found that 12.7% were newly diagnosed with a nutritional deficiency within 6 months, most commonly vitamin D deficiency.

By 12 months, vitamin D deficiency alone had risen to 13.6%, alongside increasing diagnoses of muscle loss, B vitamin deficiencies, dehydration, and mineral deficiencies including calcium, selenium, and zinc.

This is why a major 2025 joint advisory, published in the American Journal of Clinical Nutrition and backed by the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society, now treats structured nutrition support as central to safe GLP-1 use, not as an optional extra.

The advisory specifically prioritises preventing nutrient deficiencies, preserving muscle and bone mass through resistance training and appropriate diet, and managing gastrointestinal side effects throughout treatment.

What about muscle loss?

This is genuinely one of the most misunderstood parts of GLP-1 treatment, so it's worth being precise about what the evidence actually shows.

A 2024 meta-analysis of 22 randomised controlled trials (2,258 participants) found that lean mass loss averaged 0.86kg, making up approximately 25% of total weight lost. A separate, earlier meta-analysis of 9 trials found a somewhat higher figure, with lean mass comprising around 31% of total weight lost. Both reviews agree on the broader picture: most of the weight lost is fat, but a meaningful minority is muscle.

Some research using MRI imaging suggests part of this reduction may reflect the body adapting to carrying less weight overall, rather than a harmful loss of function. But this doesn't mean the risk should be dismissed.

Older adults specifically have been shown, in post-hoc analysis of the STEP trials, to lose a larger proportion of their weight as lean mass than younger participants, and to recover that muscle more slowly. People who are prefrail or already at risk of sarcopenia are the groups most often flagged for closer monitoring of strength, function, and body composition during treatment.

Weighing scales

Why appetite changes before anything else does

A 2025 scoping review examined 129 clinical trials of GLP-1 medications published through the end of 2024. Around 44% mentioned some form of general lifestyle counselling, but only 28% included any dietary assessment at all, and just 8% reported actual dietary intake data.

For semaglutide specifically, the most widely prescribed of these medications, only 2 trials reported any dietary intake data whatsoever.

A separate 2025 study of 69 adults already using a GLP-1 medication found that only 51% had received any information on managing side effects, and just 20% had been referred to a registered dietitian. In other words, appetite suppression is happening to most people without anything close to matching nutritional support.

This gap matters because many people start GLP-1 treatment with pre-existing shortfalls in nutrients like vitamin D, B12, folate, copper, or zinc, and a sudden drop in overall food intake can make these shortfalls considerably worse.

What dietitians actually prioritise

The evidence points to a handful of consistent priorities - adequate protein to protect muscle, managed fibre intake to support gut tolerance, appropriate hydration, and food choices adapted to your changing appetite and tolerance. But the right targets for each of these depend heavily on your individual situation, your starting point, your other health conditions, and how your body responds to the medication. This is exactly why generic targets often miss the mark, and why personalised dietitian support makes a meaningful difference to outcomes.

The side effects that cause people to stop treatment

Gastrointestinal symptoms are the most common reason people discontinue GLP-1 medications before reaching their goals - nausea in particular affects a significant proportion of people, especially in the early weeks of treatment or after a dose increase.

The good news is that dietary strategies can make a real difference to tolerability. But how much, and which approaches work, varies considerably from person to person. What helps one client through the first few weeks may not work for another, which is exactly why this is one of the areas where individual support matters most.

What happens after you stop the medication?

This is the part of the conversation that gets the least attention before people start treatment — and arguably the most important.

A 2025 meta-analysis of 8 randomised trials found that weight regain after stopping GLP-1 treatment was proportional to how much had been lost. On average, people regained 2.20kg after stopping liraglutide, compared to 9.69kg after stopping semaglutide or tirzepatide. The medication suppresses appetite. It does not build the habits, food skills, or relationship with eating that sustain results once it stops.

That foundation has to be built deliberately - and ideally from the very start of treatment, not after the weight has already come back.

The bottom line

GLP-1 medications are a genuinely powerful tool. But the research is clear that they work best — and produce results that last - when combined with proper nutritional support from the beginning.

If you'd like to talk through what that could look like for your situation, I offer a free 15-minute discovery call. Or if you're ready to get started, find out more about my 12-Week Nourish Me Programme - designed specifically to support people on GLP-1 medications from day one.