GLP-1 Medication and Nutrient Deficiencies: What to Watch For

If you have been prescribed a GLP-1 medication, you may be wondering whether it can cause nutrient deficiencies. The honest answer is that eating much less makes gaps more likely, and that one large study found deficiencies were diagnosed in roughly one in five people within a year of starting. It is not a reason to panic, and it is something you can plan for. This guide explains which nutrients are most often flagged, what that study did and did not show, and how to look after your intake on a small appetite. It is written for people already on treatment, alongside their prescriber.

Why a small appetite can mean nutrient gaps

These medicines slow how quickly your stomach empties and quieten hunger, so you eat less. That is the point of them, but it also means you take in less of every nutrient, not only energy. Nausea, constipation and food aversions can narrow what you eat even further, and a plate that is mostly one or two foods gives less variety than a mixed one.

Some people also begin treatment with gaps already there. A 2026 consensus statement notes that people living with obesity are often at risk of low vitamin D, B12, folate, thiamine, iron, calcium and magnesium, because an eating pattern high in energy and low in nutrients can sit alongside a shortfall of vitamins and minerals. A much smaller appetite can then make an existing gap larger.

If you also take metformin, it is worth knowing that it can lower vitamin B12, which one review highlights as a reason to pay attention to B12 on these medicines.

What the research shows, and what it does not

The study people quote most is an analysis of US health insurance claims for more than 460,000 adults who were newly prescribed these medicines and had no earlier diagnosis of a nutritional deficiency. Within six months, a deficiency had been diagnosed in 12.7% of them, and within 12 months in 22.4%. Vitamin D was the most common (13.6% at 12 months), followed by thiamine and other B vitamins, and nutritional anaemia affected about 4% by the end of the year. Dietitians Australia cites a 2025 study with a similar finding in its advocacy for “nutrition first” care.

It is worth reading with some care:

• It is observational. It shows an association, not that the medicine caused the deficiencies.
• It counts diagnoses. A deficiency has to be tested for and recorded, so the figures depend on how often people were tested.
• Most people in it had type 2 diabetes, so it may not describe everyone prescribed these medicines.
• Some of the authors work for a nutrition company, which is worth knowing even though the data come from a large claims database.

There are no trials yet showing that seeing a dietitian prevents these deficiencies. What professional bodies recommend is nutrition support alongside treatment because the risk is plausible and the cost of looking after your intake is low.

Nutrients worth knowing about

This list is a guide to what reviews flag and where the nutrients are found. It is not a way to diagnose a deficiency, which needs a doctor and, sometimes, a blood test.

• Protein. Intake is frequently inadequate on a small appetite, and it matters for muscle (see protecting your muscle). Found in: eggs, dairy, fish, chicken, lean meat, tofu, lentils, beans.
• Vitamin D. The deficiency diagnosed most often in the large study. Sunlight is the main source, and food gives only part of what you need. Found in: oily fish, eggs, some fortified foods.
• Vitamin B12. Found mainly in animal foods, so a small appetite or plant-based eating both matter. Metformin can lower it. Found in: meat, fish, eggs, dairy, fortified plant milks.
• Thiamine and other B vitamins. The next most common after vitamin D in the large study. Found in: wholegrain bread and cereals, legumes, nuts and seeds.
• Iron. Intakes are often low before treatment, and a smaller appetite can lower them further. Found in: red meat, chicken, fish, legumes, tofu and fortified cereals (with a vitamin C food to help absorb plant iron).
• Calcium. Reviews flag calcium. Dairy foods are the main source, so a smaller appetite can lower intake. Found in: milk, yoghurt, cheese, calcium-set tofu, fortified plant milks, tinned fish with bones.
• Magnesium and potassium. Intakes were often below recommendations in the studies reviewed. Found in: nuts, legumes, wholegrains, leafy greens, bananas, potatoes.

Signs such as unusual tiredness, weakness, dizziness, pale skin or pins and needles can have many causes, including a nutrient gap. They are a reason to see your doctor, not a way to self-diagnose.

Eating for nutrients when you are not very hungry

Healthdirect reminds people on weight loss medicines that it is important to eat enough food and nutrients each day, even when you do not feel hungry. When there is less room on the plate, the aim is for each mouthful to carry more:

• Include a protein food and a vegetable or fruit in most of what you eat, even when the portion is small.
• Vary the foods through the week. A mix of colours, wholegrains, dairy or fortified alternatives, and different protein foods covers more nutrients than the same few safe foods.
• Put nutrient-dense foods first, before drinks and snacks that fill you up but offer little else.
• Keep drinking. Dehydration makes tiredness and nausea worse.

Our guide to what to eat on GLP-1 medication covers eating when you are barely hungry, and nausea and constipation covers the food side of the common gut effects.

Be wary of supplements bought on impulse. A multivitamin or a single nutrient may help some people, but others do not need one, and some, iron in particular, can do harm if you do not need them. Talk to your doctor, pharmacist or dietitian before starting.

Whether your intake is covering what you need, and where to put your effort first, is hard to judge alone. A consult looks at your actual eating and your side effects. About the GLP-1 consult →

Tests and monitoring: ask your doctor

There is no agreed Australian schedule for checking nutrient levels in people on these medicines. A 2025 review argues that monitoring along the lines used after bariatric surgery could help. A 2026 consensus statement takes a more measured view: it says testing may be considered when there are signs of a deficiency, and that routine screening is rarely necessary. Because opinions differ, the sensible step is to ask your doctor whether any tests make sense for you, particularly if you have symptoms, a history of low iron or vitamin D, a restricted way of eating, or take other medicines that affect nutrients.

When to talk to your doctor