GLP-1 receptor agonists — sold under brand names including Ozempic, Wegovy, and Mounjaro — are now the most prescribed drugs in the UK. Millions of people are using them, millions more are considering them, and the conversation around them dominates health media, social platforms, and GP waiting rooms in equal measure.
The drugs work. The evidence for their effectiveness in weight loss and blood glucose management is robust, and for many people they represent a genuinely significant health intervention. But there is a conversation happening around GLP-1s that almost nobody in the mainstream is having — and it's the one that matters most if you're currently taking them or thinking about starting.
GLP-1 drugs significantly reduce appetite and food intake. They also significantly reduce the amount of nutrients your body is absorbing. And for the vast majority of people taking them, nobody has tested whether those nutrients were adequate in the first place — or whether they're becoming depleted as a result of the medication.
This blog is for anyone currently on Ozempic, Wegovy, or Mounjaro, and for anyone considering starting. Not to discourage you — but to make sure you're doing it as safely and effectively as possible.
What GLP-1 drugs actually do to your nutrition
GLP-1 receptor agonists work primarily by mimicking the GLP-1 hormone — slowing gastric emptying, increasing satiety signals to the brain, and reducing appetite. The result is significant caloric restriction, often without the person consciously restricting food — the medication simply removes the desire to eat as much.
This is powerful for weight loss. It is also nutritionally significant. Eating substantially less means absorbing substantially less — of protein, of vitamins, of minerals, of everything your body needs to function. Most people on GLP-1 drugs are in a state of significant caloric restriction for months or years, often without any monitoring of what this restriction is doing to their nutritional status.
The most commonly reported nutritional concerns in people on GLP-1 medications include protein intake and muscle mass, B12 and folate absorption, iron and ferritin levels, vitamin D, zinc, magnesium, and electrolytes. Each of these has specific implications for how you feel, how you perform, and what happens to your body composition during and after the weight loss.
The muscle loss problem
This is the most clinically significant nutritional issue with GLP-1 medications and the one that receives least attention in mainstream coverage. When the body loses weight rapidly through caloric restriction — with or without medication — it loses both fat and muscle. Research on GLP-1 medications suggests that a significant proportion of the weight lost — sometimes as high as 30-40% — can be lean muscle mass rather than fat, particularly in people who are not consuming adequate protein and not engaging in resistance training.
Muscle loss matters far beyond aesthetics. Muscle is metabolically active tissue — it burns calories at rest, regulates blood glucose, supports bone density, and maintains physical function and independence as we age. People who lose significant muscle mass during GLP-1 treatment often experience a rebound weight gain when they stop the medication, because their reduced muscle mass means their basal metabolic rate has decreased.
Understanding your baseline muscle mass and protein status before and during GLP-1 treatment — and supplementing protein strategically — is essential for protecting against this outcome. A blood panel that includes markers of protein status alongside body composition assessment gives you the information you need.
B12 and folate — the absorption issue
Reduced food intake means reduced B12 and folate intake. This matters significantly because B12 deficiency is already extremely common — particularly in people over 40 — and GLP-1 medications add nutritional restriction on top of an already potentially inadequate baseline.
B12 is essential for neurological function, red blood cell production, energy metabolism, and the regulation of homocysteine. Folate works alongside B12 in the methylation cycle. Deficiency in either causes fatigue, brain fog, mood disturbances, and neurological symptoms — all of which can be misattributed to other aspects of the GLP-1 experience rather than identified as a correctable nutritional deficiency.
Testing B12 — both total and active — alongside folate before starting a GLP-1 medication establishes your baseline and allows monitoring for decline over time. If deficiency is identified, therapeutic supplementation — or IV B12 for faster repletion — can address it directly.
Iron and ferritin
Reduced food intake reduces dietary iron intake. Reduced gastric acid production — which GLP-1 medications can also contribute to — reduces iron absorption. The combination creates a significant risk of iron and ferritin depletion over time, particularly in women who already have lower iron stores and who may be managing heavy periods alongside their GLP-1 treatment.
Low ferritin causes the kind of fatigue that is indistinguishable from the fatigue people sometimes experience as a side effect of GLP-1 medications — making it easy to attribute to the wrong cause and miss entirely. Testing ferritin at baseline and monitoring it over the course of GLP-1 treatment allows timely intervention before deficiency becomes significant.
Vitamin D
Vitamin D is a fat-soluble vitamin — it requires dietary fat for absorption. People on GLP-1 medications, who are eating significantly less fat alongside less food overall, are at increased risk of vitamin D decline over time. Given that the majority of UK adults are already deficient in vitamin D — and that deficiency contributes to fatigue, low mood, poor immune function, muscle weakness, and bone density loss — monitoring and maintaining vitamin D levels during GLP-1 treatment is straightforward and important.
Magnesium, zinc, and electrolytes
Reduced food intake reduces the intake of all micronutrients. Magnesium and zinc are particularly important because they are involved in hundreds of enzymatic reactions — energy production, protein synthesis, immune function, and hormonal regulation among them. Electrolyte balance can also be affected, particularly in people who experience nausea or vomiting as GLP-1 side effects.
Thyroid function
This is a specific concern that applies before starting GLP-1 medications rather than during. Thyroid dysfunction — particularly hypothyroidism — causes weight gain and difficulty losing weight through mechanisms that overlap with the metabolic dysfunction GLP-1 drugs are used to treat. For some people, what they believe is obesity driven by lifestyle factors is actually largely driven by an underactive thyroid — a condition that responds to thyroid treatment rather than appetite suppression.
Identifying and treating thyroid dysfunction before starting a GLP-1 medication is clinically important — both because thyroid treatment may be the more appropriate primary intervention, and because undiagnosed hypothyroidism will limit the effectiveness of the GLP-1 and leave the underlying cause unaddressed.
Cardiovascular and metabolic baseline
GLP-1 medications have well-documented beneficial effects on cardiovascular risk markers — they reduce HbA1c, reduce triglycerides, and have been shown to reduce cardiovascular events in high-risk populations. Establishing a baseline cardiovascular and metabolic panel before starting — HbA1c, fasting glucose, full lipid panel, hsCRP, homocysteine — allows you to track the genuine physiological improvement the medication is producing, separate from weight loss alone. This is clinically motivating and practically useful for understanding your true health trajectory.
Kidney function
Rapid weight loss and significant dietary restriction both place demands on the kidneys. Monitoring kidney function — creatinine, urea, eGFR — during GLP-1 treatment is sensible, particularly for anyone with pre-existing kidney considerations.
IV therapy for people on GLP-1 medications
IV vitamin therapy has a specific and valuable role for people on GLP-1 medications, for precisely the reason that makes nutritional monitoring important in the first place. When food intake is significantly reduced, oral supplementation has to do a great deal of work that diet would normally handle — and in people whose gut absorption may already be suboptimal, oral supplementation is further limited.
IV delivery bypasses the gut entirely. For people on GLP-1 medications who are at risk of B12, iron, vitamin D, or magnesium depletion, IV therapy delivers these nutrients at therapeutic concentrations regardless of what's happening with oral intake or absorption. A regular IV therapy schedule — monthly or fortnightly — provides nutritional insurance for the period of significant caloric restriction.
Nutrition planning for GLP-1 users
Sam's personalised nutrition planning at Boost & Glow is specifically relevant for people on GLP-1 medications because the dietary strategy that supports health during GLP-1 treatment is very different from a generic calorie-restricted diet. Maximising protein density within reduced caloric intake to protect muscle mass. Prioritising the most nutrient-dense foods within a smaller eating window. Strategic supplementation based on blood test results to address specific deficiencies. Meal timing and composition strategies to manage GLP-1 side effects while maintaining nutritional adequacy. These are not generic recommendations — they depend on your individual results, your medication dose, your activity level, and your specific health goals.
What to test and when
Before starting a GLP-1 medication — full thyroid panel to rule out thyroid dysfunction as a primary driver, full metabolic and cardiovascular panel as a baseline, full nutritional panel including B12 and active B12, folate, ferritin and iron studies, vitamin D, zinc, and magnesium, full hormone panel, kidney function.
During GLP-1 treatment — repeat the nutritional markers every three to six months to identify depletion before it becomes symptomatic. Track the metabolic and cardiovascular markers to document genuine physiological improvement. Monitor kidney function if there are any concerns.
At Boost & Glow
Lucy carries out all blood testing at our clinic in Epping or at your home across Essex, Hertfordshire, and London. A comprehensive GLP-1 monitoring panel can be arranged as a one-off baseline or as a regular monitoring programme — giving you the information to take GLP-1 medications as safely and effectively as possible, with the nutritional support in place to protect your health during and after treatment.
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