In short
When intake falls sharply, nutrients fall with it, and appetite suppression means nothing prompts you to correct it. Iron, B12, vitamin D and folate are the.
By Alsen Walker · Updated September 19, 2026
When intake falls sharply, nutrients fall with it, and appetite suppression means nothing prompts you to correct it. Iron, B12, vitamin D and folate are the ones most worth checking. The right response is a blood test rather than a supplement aisle, because several of these are harmful in excess.
Do not start iron without testing. Iron is not benign in excess, and some people carry conditions that cause iron to accumulate. Symptoms of deficiency and overload overlap, so guessing can send you in the wrong direction. A ferritin test is straightforward and settles it.
For how this fits alongside the rest of eating and training on a GLP-1, see Eating and Training on a GLP-1: The Complete Guide.
Why deficiencies develop here
Eating half of what you used to means roughly half the vitamins and minerals, and the reduction is rarely even. Foods that go first tend to be the ones people find heavy, which often includes meat, and meat carries iron, B12 and zinc.
Aversion narrows things further. If dairy has gone, calcium and B12 drop; if meat has gone, iron drops; if fat has been cut hard, the fat-soluble vitamins are affected.
None of this is dramatic and none of it announces itself early, which is the problem.
The ones worth checking
Iron and ferritin. Deficiency causes fatigue, breathlessness on exertion, pallor and hair shedding. Common in menstruating women and in anyone who has stopped eating meat.
Vitamin B12. Fatigue, tingling in hands or feet, poor concentration. Comes mainly from animal foods, so it falls when those do. Metformin can also affect B12 over time, which matters for people with type 2 diabetes.
Vitamin D. Widely low regardless of diet. Matters more here because bone density is already under pressure during weight loss.
Folate. Relevant for anyone who might become pregnant, and worth raising before conception rather than after.
Calcium. Not routinely tested as a nutritional marker, and intake is worth reviewing alongside vitamin D for bone health.
Ask for testing, not a multivitamin
The reason is that treatment depends on which one is low, and supplementing blind can be useless or harmful.
A reasonable request at a review: given how much my intake has dropped, should we check iron, ferritin, B12, vitamin D and folate. Most prescribers will agree readily.
If you have symptoms that overlap with several deficiencies, which is common, testing is the only way to separate them.
Where a general multivitamin fits
A standard multivitamin is reasonable insurance for many people with genuinely low intake, and it is not a substitute for testing when there are symptoms.
Two cautions. Fat-soluble vitamins, A, D, E and K, accumulate and can reach harmful levels, so stacking multiple products containing them is a real risk. And biotin, common in hair and nail supplements, interferes with several laboratory tests including some thyroid and cardiac assays, so tell your clinician if you take it before any blood work.
If you have had bariatric surgery
Your existing supplementation and monitoring schedule takes precedence over anything general. Adding a medication that further reduces intake makes that monitoring more important rather than less.
Tell your bariatric team you have started a GLP-1.
The better fix
Supplements patch a gap; they do not close it. If intake has fallen far enough to cause deficiencies, that is the thing to address, and it is a conversation with your prescriber about pace and tolerability as much as with a dietitian about food.
Iron is the one not to guess at, covered further in iron deficiency on GLP-1s. If intake has fallen far enough to cause deficiencies, the intake itself is the thing to address.
Frequently asked questions
Should I take a multivitamin?
Often reasonable if intake is low. Not a replacement for testing if you have symptoms.
Can I take iron just in case?
No. Test first. Excess iron causes harm.
How often should I be tested?
Depends on your intake, symptoms and history. Ask your prescriber what is sensible for you.
Do gummy vitamins count?
They vary and often contain less than expected alongside added sugar. Read the label.
Why am I tired if my bloods are normal?
Fatigue here more often traces to total intake, hydration or blood sugar than to a specific deficiency.
Should I tell my prescriber about supplements?
Yes, all of them. Several interact with medications or with laboratory tests.
Sources and further reading
- National Institute of Diabetes and Digestive and Kidney Diseases (NIH). Treatment for overweight and obesity. www.niddk.nih.gov
- MedlinePlus (US National Library of Medicine). Semaglutide injection: drug information. medlineplus.gov
- MedlinePlus (US National Library of Medicine). Tirzepatide injection: drug information. medlineplus.gov
- National Institute of Diabetes and Digestive and Kidney Diseases (NIH). Prescription medications to treat overweight and obesity. www.niddk.nih.gov
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