Mounjaro, iron, B12 and vitamin D: nutrient gaps when you eat much less

Why it drops on Mounjaro: red meat is the richest source and one of the foods people most often go off. Menstruating women lose iron every month regardless.

What low iron looks like: the NHS lists tiredness, lack of energy, shortness of breath, noticeable heartbeats and pale skin as symptoms of iron deficiency anaemia.^[2] Hair shedding and restless legs are also associated with low iron stores.

Food sources: red meat, liver, beans and lentils, tofu, fortified breakfast cereals, dark green vegetables, dried fruit. Vitamin C with a meal helps absorption; tea and coffee with a meal hinder it.^[2]

Do not self-treat. Iron supplements can cause constipation, which you already have enough of, and taking iron without a confirmed deficiency can be harmful. A GP blood test tells you whether you need it.

Why it drops: B12 comes almost entirely from animal foods: meat, fish, eggs, dairy. A diet that has narrowed to toast and soup, or a plant-based diet without fortified foods, runs short over months because the body’s stores take time to deplete. If you take metformin for type 2 diabetes, it is a known cause of low B12 in its own right and your GP should already be checking.

What low B12 looks like: the NHS lists extreme tiredness, lack of energy, pins and needles, a sore red tongue, mouth ulcers, muscle weakness, disturbed vision and problems with memory or mood.^[3] The mood and memory element is one reason a flat feeling on treatment deserves a blood test rather than an assumption.

Food sources: meat, fish, eggs, milk and cheese; fortified plant milks, fortified cereals and yeast extract for vegans.

Why it matters here: vitamin D comes mainly from sunlight on skin, and from a small number of foods such as oily fish and egg yolks. UK government advice is that everyone should consider a daily supplement between October and March because sunlight is too weak, and that people who are rarely outdoors or who cover their skin should consider it all year.^[4] Reduced food intake removes the small dietary contribution as well.

What low vitamin D looks like: often nothing obvious; bone and muscle aches and tiredness in more marked deficiency.

Vitamin D is the one nutrient on this page where the public health advice is to supplement without a test, at the standard daily amount stated in the NHS guidance, and to avoid high-dose products unless a doctor has prescribed them.^[4] Our multivitamin guide covers the wider question.

Say you are on Mounjaro, how long, roughly how much less you are eating and which foods you have cut, and what symptoms you have. Ask whether a full blood count, ferritin, B12 and folate, and vitamin D are worth checking. It is a routine request and your GP will decide which are appropriate. If you have type 2 diabetes, or a history of anaemia, coeliac disease or bariatric surgery, mention it; those change the picture.

Pharmacies that prescribe Mounjaro cannot run bloods, but many will write a note for your GP if you ask. Our guide explains what your pharmacy has already told your GP.

Does Mounjaro cause vitamin deficiency?

Not directly; it does not block absorption and deficiency is not a listed side effect. Eating much less for months can lead to shortfalls of iron, B12 and vitamin D, particularly if you have gone off meat or eat very narrowly.

Should I take iron on Mounjaro?

Only if a blood test shows you need it. Iron supplements can worsen constipation and unnecessary iron can be harmful. Ask your GP for a test if you have tiredness, breathlessness or hair shedding.

Should I take vitamin D on Mounjaro?

UK guidance is that everyone should consider a standard daily vitamin D supplement from October to March regardless of medicines. Follow the amount in the NHS guidance and avoid high-dose products unless prescribed.

Can low B12 explain feeling flat on Mounjaro?

It can contribute; low B12 is linked to mood and memory changes. It is one of several things a GP can rule out with a blood test when someone feels flat on treatment.