What nutrients are people actually running low on while on a GLP-1?

A 2026 narrative review pooling six studies and over 480,000 adults on GLP-1 receptor agonists found vitamin D deficiency was the most common finding, present in 7.5 percent of users at 6 months and rising to 13.6 percent at 12 months. Iron depletion was also frequent, with GLP-1 users showing 26 to 30 percent lower ferritin levels than comparison groups, and more than 60 percent of users consumed below the estimated daily requirement for both calcium and iron. Thiamine and B12 shortfalls also increased the longer people stayed on treatment.

Why does a GLP-1 cause nutrient deficiencies in the first place?

The mechanism is the same one that drives the weight loss: appetite suppression and delayed gastric emptying mean less food, and less variety of food, passes through the body. That is not automatically a problem when someone is eating well. It becomes one when a smaller appetite defaults to whatever is easiest to get down (crackers, toast, a protein shake on repeat) rather than a rotation of foods that covers vitamin D, iron, calcium, and B vitamins.

Which nutrient deficiency on a GLP-1 is the most urgent to catch early?

Thiamine, in rare but serious cases. Case reports have linked rapid weight loss and prolonged poor intake on a GLP-1 to thiamine deficiency severe enough to cause neurological symptoms, including numbness, weakness, and vision changes. That combination of symptoms after significant, fast weight loss is a same-day call to your prescriber, not a wait-and-see one. This is uncommon, but it is the reason routine bloodwork during GLP-1 treatment matters more than it would for weight loss without medication.

How do I prevent nutrient deficiencies on a GLP-1 without derailing weight loss?

Build the smaller amount of food you can manage around variety, not just protein. Vitamin D and calcium from fortified dairy or a fortified alternative, iron from meat, poultry, or legumes paired with a vitamin C source to help absorption, and B12 from animal products or a fortified source if you eat mostly plant-based. Ask your prescriber to check vitamin D, B12, iron studies, and calcium at your regular visits rather than waiting for symptoms, and let your doctor decide if a supplement is warranted rather than adding one on your own.

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Nutrient Deficiencies on a GLP-1: What a New 2026 Review Found, and How to Prevent Them

A 2026 review of over 480,000 GLP-1 users found real gaps in vitamin D, iron, calcium, thiamine, and B12. Here is what it found and how to eat ahead of it.

Heather Bauer, RD · September 11, 2026

If you are on a GLP-1, a 2026 review of the actual data is worth knowing about: pooling six studies and over 480,000 adults, it found real, measurable gaps in vitamin D, iron, calcium, thiamine, and B12, not a hypothetical risk. The fix is not another supplement aisle. It is building the smaller amount of food a suppressed appetite allows around variety, and getting routine bloodwork instead of waiting for symptoms.

What did the new 2026 study actually find?

A narrative review published in Clinical Obesity pooled six studies covering more than 480,000 adults on GLP-1 receptor agonists and found vitamin D deficiency was the single most common finding: present in 7.5 percent of users at 6 months and rising to 13.6 percent by 12 months (Urbina et al., Clinical Obesity, 2026). Iron told a similar story: GLP-1 users showed ferritin levels 26 to 30 percent lower than comparison groups, and more than 60 percent of users were consuming below the estimated daily requirement for both calcium and iron. Average vitamin D intake from food came in at only about 20 percent of the recommended amount. Thiamine and B12 shortfalls also grew more common the longer people stayed on treatment.

None of this means a GLP-1 is unsafe. It means the appetite suppression that makes the medication effective for weight loss also makes it easy to under-eat the nutrients that do not show up on a scale.

Why does a GLP-1 cause this in the first place?

The same mechanism that drives the weight loss drives the nutrient gap. GLP-1 receptor agonists suppress appetite and slow gastric emptying, so less food, and less variety of food, moves through the body in a given week. That is not automatically a problem. Someone eating a smaller amount of varied, nutrient-dense food can still cover their bases.

It becomes a problem when a shrunken appetite defaults to whatever is easiest to get down: crackers, toast, the same protein shake on repeat, because trying anything else feels like too much volume. That pattern quietly narrows the range of vitamins and minerals coming in, even while total calories and even protein can look reasonable on paper.

Which deficiency is the most urgent to watch for?

Most of what the review found (vitamin D, iron, calcium) develops slowly and is caught by routine bloodwork long before it becomes serious. Thiamine is the exception worth knowing about specifically. Case reports have documented rare instances of severe thiamine deficiency following rapid weight loss and prolonged poor intake on a GLP-1, in one case progressing to a neurological complication with numbness, weakness, and vision changes that required hospital treatment (Zahir et al., BMC Neurology, 2025).

This is uncommon. It is not a reason to stop a medication that is working. It is the reason that new or worsening numbness, weakness, confusion, or vision changes after significant, fast weight loss on a GLP-1 is a same-day call to your prescriber, not a symptom to track and wait out. It is also the reason routine labs during GLP-1 treatment carry more weight than they would during weight loss without medication.

How do you prevent this without derailing weight loss?

Build the smaller amount of food you can manage around variety, not just protein. A few concrete swaps:

Vitamin D and calcium: fortified dairy or a fortified plant alternative, rather than skipping the food group entirely because it feels heavy.
Iron: meat, poultry, or legumes, ideally paired with a vitamin C source (citrus, peppers, tomato) at the same meal, since that combination meaningfully improves how much iron your body actually absorbs.
B12: animal products by default, or a fortified source if you eat mostly plant-based, since B12 is one of the few nutrients that plant foods do not reliably provide.
Thiamine: whole grains, pork, and legumes, which matters more the longer someone has had reduced intake.

None of this requires guessing. Ask your prescriber to add vitamin D, B12, iron studies, and calcium to your regular labs during treatment, rather than waiting for a symptom to prompt the test, and let your doctor decide whether a supplement is actually warranted rather than adding one on your own.

This is the same food-first logic behind protecting muscle on a GLP-1: a suppressed appetite does not have to mean a narrower diet, it means the food that does fit has to work harder. A coaching layer that tracks what you are actually eating, not just the scale, is what catches a narrowing pattern before a lab test has to.