Micronutrients on Mounjaro and Wegovy: Which Deficiencies to Watch For and How to Prevent Them

Updated 28/07/28

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GLP-1 medications like Mounjaro and Wegovy produce significant, rapid weight loss by reducing appetite and food intake substantially. The clinical results are well-documented. What is less commonly discussed is what happens to nutrient intake when the amount of food you eat reduces dramatically. Vitamin B12, iron, calcium, vitamin D, zinc, magnesium and folate can all become harder to obtain in adequate quantities when meals are small and irregular. Left unaddressed, deficiencies in these nutrients produce symptoms - fatigue, poor immunity, hair loss, muscle weakness, bone loss - that are frequently attributed to the medication when the root cause is nutritional. This guide covers the most clinically relevant deficiencies for women on GLP-1 treatment, how to identify them early, and what to do about them.

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Why GLP-1 Medications Increase the Risk of Micronutrient Deficiency

Understanding the mechanism makes the preventive approach make more sense.

GLP-1 medications suppress appetite significantly and slow gastric emptying. The direct consequence is reduced food intake - often dramatically so in the first few months of treatment. Most women eat 30 to 50% fewer calories than before starting treatment. Since the majority of micronutrient intake comes from food, a proportional reduction in food intake means a proportional reduction in vitamins and minerals unless specific attention is paid to dietary composition.

The gastric emptying effect also changes how some nutrients are absorbed. Slower transit through the stomach affects the intrinsic factor production that is essential for vitamin B12 absorption, and changes the gut microbiota in ways that affect the absorption of several fat-soluble vitamins.

Rapid weight loss itself adds a third layer. As fat mass reduces quickly, the body mobilises stored nutrients from fat tissue that then need to be replaced through diet. This is one of the reasons deficiencies can develop even when women believe they are eating reasonably well.

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Vitamin B12

B12 is the micronutrient most specifically associated with GLP-1 treatment, and the mechanism is direct rather than simply a function of reduced food intake.

Vitamin B12 absorption requires intrinsic factor, a protein produced by the stomach's parietal cells. GLP-1 medications slow gastric emptying and alter the stomach environment in ways that can reduce intrinsic factor production. Studies suggest that 10 to 30% of people on long-term GLP-1 treatment show reduced B12 levels, with higher rates in those on vegetarian or vegan diets where dietary B12 is already limited.

The symptoms of B12 deficiency develop gradually and are easily attributed to other causes: fatigue, numbness or tingling in the hands and feet, cognitive fog, mood changes, and in more advanced deficiency, macrocytic anaemia. By the time neurological symptoms develop, the deficiency has typically been present for a significant period.

B12 is found almost exclusively in animal products - meat, fish, eggs, dairy. Women who have reduced their consumption of these foods alongside weight loss are at higher risk. Fortified plant-based products provide some B12 but often in forms with lower bioavailability.

Testing at baseline and every six months during active GLP-1 treatment is advisable. Supplementation with methylcobalamin or cyanocobalamin is straightforward and inexpensive, and should be started at the first sign of falling levels rather than waiting for frank deficiency.

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Iron

Iron deficiency is the most common micronutrient deficiency globally, and women of reproductive age are at baseline higher risk due to menstrual blood loss. GLP-1 treatment compounds this through reduced dietary intake.

The best dietary sources of iron are red meat, poultry and fish, which are often among the foods that women eat less of on a suppressed appetite. Plant-based iron from legumes, spinach and fortified cereals is less bioavailable than haem iron from animal sources, and absorption is further affected by several factors common in GLP-1 patients - reduced stomach acid, competition with calcium, and low vitamin C intake.

Symptoms include fatigue, pallor, breathlessness on exertion, brittle nails, hair loss and reduced concentration. Iron deficiency anaemia develops when stores are sufficiently depleted and haemoglobin production is affected. Regular ferritin testing catches depletion early, before anaemia develops.

Practical dietary measures to maintain iron status: include a source of haem iron at least three times a week where possible, pair plant-based iron sources with vitamin C-rich foods at the same meal, and avoid tea, coffee and calcium-rich foods within an hour of iron-rich meals as these inhibit absorption.

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Calcium and Vitamin D

These two nutrients are covered together because they are interdependent - vitamin D is required for calcium absorption, and addressing one without the other produces suboptimal results.

Calcium requirements for women are 700mg daily for those under 50, rising to 1,200mg for postmenopausal women. On a significantly reduced diet, these targets are hard to hit without conscious effort. The main dietary sources are dairy products, fortified plant-based milks, leafy greens, tofu and almonds. Women who are eating smaller meals and avoiding dairy may find their calcium intake has fallen substantially.

Vitamin D deficiency is common in the UK population year-round due to limited sunlight and is higher in women with more body fat because vitamin D is sequestered in adipose tissue. As weight reduces on GLP-1 treatment, stored vitamin D can become more bioavailable, which is a benefit - but baseline deficiency needs to be corrected before this can help. The NHS recommends 10 micrograms of vitamin D daily for all adults between October and March.

The consequences of sustained deficiency in both nutrients include reduced bone density, increased fracture risk, muscle weakness and immune suppression. Postmenopausal women are at significantly elevated risk given declining oestrogen's effect on calcium absorption.

A combined calcium and vitamin D supplement is the most practical option for women on GLP-1 treatment whose dietary intake is consistently insufficient.

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Other Micronutrients Worth Monitoring

These are less commonly discussed but clinically relevant for women on long-term GLP-1 treatment.

Folate. Essential for cell division, DNA synthesis and red blood cell formation. Particularly important for women of reproductive age given the consequences of deficiency in early pregnancy. Found in leafy greens, legumes and fortified grains. Reduced vegetable and grain intake on a suppressed appetite can lower levels.

Zinc. Supports immune function, wound healing and hormonal balance. Deficiency is associated with hair thinning, impaired immunity and reduced taste sensation - the last of which can worsen appetite suppression and create a difficult cycle. Found in meat, shellfish, nuts and seeds. Women on plant-based diets are at higher risk due to lower bioavailability of plant-based zinc.

Magnesium. Required for over 300 enzymatic reactions including muscle contraction, nerve function and blood sugar regulation. Deficiency produces muscle cramps, fatigue, headaches and sleep disruption - symptoms commonly reported on GLP-1 treatment that may be partly nutritional rather than medication-related. Found in whole grains, nuts, seeds, dark chocolate and leafy greens. Magnesium glycinate is the best-tolerated supplemental form for women on GLP-1 treatment.

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Food Pairing and Absorption: What Matters Practically

Getting enough of a nutrient from diet is only half the picture - absorption matters as much as intake.

Iron absorption is significantly improved by eating plant-based iron sources (lentils, spinach, chickpeas) alongside vitamin C-rich foods (tomatoes, bell peppers, citrus). Conversely, calcium, tea, coffee and phytates in whole grains all reduce non-haem iron absorption. Separating iron-rich meals from calcium-rich foods by at least an hour is worth doing consistently.

Calcium absorption requires vitamin D. Taking a calcium supplement without adequate vitamin D produces less than half the absorption benefit. Most combined supplements include both for this reason.

Fat-soluble vitamins - A, D, E and K - require dietary fat for absorption. Taking vitamin D supplements with a meal that contains some fat improves absorption compared to taking it on an empty stomach or with a very low-fat meal.

Cooking method affects micronutrient retention. Steaming preserves water-soluble vitamins including vitamin C and folate better than boiling. Overcooking vegetables destroys B vitamins. Raw or lightly cooked leafy greens retain more folate than cooked-down versions of the same volume.

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Monitoring: What to Test and How Often

The following blood markers are worth checking at baseline before starting GLP-1 treatment and then at regular intervals during treatment:

Vitamin B12 and folate. Ferritin (iron stores) and haemoglobin. Vitamin D (25-hydroxyvitamin D). Calcium and magnesium. Full blood count.

Every six months during active treatment is a reasonable monitoring frequency for most women. Women with pre-existing risk factors - postmenopausal, vegetarian or vegan, history of anaemia, diagnosed malabsorption conditions — should consider quarterly testing or as advised by their clinician.

Your SheMed clinician can advise on which tests are most relevant given your individual medical history and dietary pattern, and can refer you for blood testing or interpret results in the context of your treatment.

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Supplementation on GLP-1 Treatment: What Is Worth Taking

A targeted approach to supplementation based on your specific deficiencies is more useful than taking a broad multivitamin and assuming all bases are covered.

A quality multivitamin can serve as a useful baseline during GLP-1 treatment, particularly in the early months when food intake is most significantly reduced. Look for one that contains methylcobalamin (the active form of B12), adequate iron if you are not getting sufficient through diet, vitamin D3, and a meaningful dose of zinc.

Beyond a multivitamin, the supplements most commonly warranted on GLP-1 treatment are: vitamin D3 at 10 to 25 micrograms daily depending on baseline levels, magnesium glycinate at 200 to 400mg in the evening (this form is best absorbed and least likely to cause digestive discomfort), and a combined calcium and vitamin D supplement if dairy intake is consistently low.

Iron supplementation should only be started after blood testing confirms deficiency. Taking iron supplements without a confirmed deficiency can cause digestive discomfort and interfere with the absorption of other minerals.

Always discuss supplementation with your SheMed clinician or GP before starting, particularly if you are on other prescription medications, as several supplements interact with common medications.

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Frequently Asked Questions

Do GLP-1 medications cause vitamin deficiencies?

They increase the risk of certain deficiencies, particularly vitamin B12, iron, vitamin D and calcium, primarily through reduced food intake and changes to gastric function. They do not directly cause deficiencies but create conditions where deficiencies are more likely to develop if nutrition is not actively managed.

Should I take a multivitamin on Mounjaro or Wegovy?

A quality multivitamin is a reasonable baseline during GLP-1 treatment, particularly in the early months when food intake is most significantly reduced. It is not a substitute for a varied diet or for addressing specific confirmed deficiencies, but provides useful nutritional insurance during a period when dietary intake may be insufficient.

How often should I have blood tests on GLP-1 treatment?

Every six months for most women. Women with higher risk factors - postmenopausal, vegetarian or vegan, history of anaemia or malabsorption - should consider quarterly testing. Your SheMed clinician can advise on the right monitoring schedule for your circumstances.

What are the signs of vitamin B12 deficiency on Mounjaro?

Fatigue, numbness or tingling in the hands or feet, cognitive fog, mood changes and in later stages macrocytic anaemia. These symptoms develop gradually and are easily attributed to other causes. B12 testing is the only reliable way to confirm whether low B12 is contributing to symptoms.

Can I get enough micronutrients from food on GLP-1 treatment?

With careful dietary planning, yes - but it requires intentional choices at every small meal rather than relying on general healthy eating. Prioritising nutrient-dense foods including meat, fish, eggs, dairy, leafy greens and legumes within whatever appetite allows, and paying attention to food pairing for absorption, is essential. Many women find targeted supplementation necessary alongside dietary effort rather than as a replacement for it.

Does iron deficiency cause hair loss on GLP-1 treatment?

Iron deficiency is one of several potential causes of hair loss during rapid weight loss, alongside the telogen effluvium (stress-related hair shedding) that occurs with significant calorie restriction and physical change. Testing ferritin, B12, zinc and thyroid function alongside iron if hair loss is significant will identify which, if any, nutritional factors are contributing.

What foods are highest in B12?

Liver and kidney (very high), clams and oysters, oily fish including salmon and mackerel, beef and lamb, eggs, and dairy products. Fortified plant-based milks and cereals provide B12 but typically in lower amounts and sometimes in less bioavailable forms. Women who eat limited animal products should discuss B12 supplementation with their clinician rather than relying on dietary sources alone.

Can too much supplementation be harmful?

Yes. Fat-soluble vitamins including A, D, E and K accumulate in the body and can reach toxic levels if over-supplemented. Vitamin D toxicity produces hypercalcaemia with symptoms including nausea, weakness and kidney damage. Iron over-supplementation causes digestive discomfort and can interfere with zinc and copper absorption. Always supplement based on confirmed deficiency or clinical guidance rather than on precautionary high doses.

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References

National Health Service. Vitamins and minerals overview. nhs.uk

National Health Service. Vitamin B12 or folate deficiency anaemia. nhs.uk

National Health Service. Iron deficiency anaemia. nhs.uk

National Institute for Health and Care Excellence. Vitamin D deficiency in adults: treatment and prevention. nice.org.uk

Carvalho MC, Ariza AC, Coutinho IG, et al. Vitamin B12 deficiency in patients using glucagon-like peptide-1 receptor agonists. Diabetes, Obesity and Metabolism. 2023.

World Health Organization. Micronutrients. who.int

Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity. SURMOUNT-1. New England Journal of Medicine. 2022.

Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. STEP 1. New England Journal of Medicine. 2021.

McGowan L, Sheard L, Abner S, et al. NHS health check attendance is associated with reduced multiorgan disease risk. BMC Medicine. 2023.

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