GLP-1 medications can be powerful tools for weight loss—partly because they alter your eating habits. Since GLP-1 medications reduce appetite, many people taking them naturally eat smaller portions, skip meals, or find certain foods harder to tolerate. While this change to your eating can bring down your weight, it can also make it more challenging to get enough essential nutrients. Research shows that people eat an average of 16-39 percent less food while on a GLP-1 medication. That’s a major drop-off! Over time, eating this much less food may increase the risk of falling short on key vitamins and minerals.

That’s where a multivitamin can come in. While not everyone taking a GLP-1 needs a multivitamin, it is recommended for most people, says Melissa Mitri, M.S., R.D. A multi can be beneficial any time you’re eating less than normal—but especially when you’ve just started a GLP-1 or have increased your dose. During these times, Mitri says side effects like nausea and low appetite mean nutrition support is particularly helpful. 

A vitamin supplement can’t replace a balanced eating pattern, but it can help fill nutritional gaps from reduced food intake. Here’s what GLP-1 users should look for in a multi.

• ABOUT OUR EXPERTS: Melissa Mitri, M.S., R.D., is a Connecticut-based weight loss expert and nutrition communicator. Brittany Brown, R.D., I.B.C.L.C., C.D.E., is a registered dietitian based in Nova Scotia, Canada. Briana Reamer, M.S., R.D.N., L.D.N., is a registered dietitian and owner of More Than a Dietitian. 

Vitamin D

Many people on a GLP-1 medication limit their fat intake to support weight loss and help manage medication side effects. This affects their absorption of fat-soluble vitamins, says Briana Reamer, M.S., R.D.N., L.D.N. One such micronutrient: vitamin D. This important vitamin supports muscle and heart health, insulin sensitivity, and more.  

Vitamin D is the most common nutritional deficiency among people taking a GLP-1. One retrospective study found that, of nearly half a million GLP-1 users, 7.5% had a vitamin D deficiency within 6 months, and 13.6% had one by 12 months. Another report found that GLP-1 users got only an average of 20% of the necessary vitamin D.  

Read More: How To Choose The Right Vitamin D Supplement For You

If you’re too low on D, you may experience fatigue, muscle weakness and cramps, and bone and joint pain. Luckily, a quick blood test can show you your nutrient status. If you suspect your levels are low, consult a healthcare provider. They can help you determine your levels (and the optimal supplement dose to restore them). Most multivitamins contain 800-1000 International Units (IU)—great for a maintenance dose, but not necessarily enough if you’re deficient. If you’re running low, your doctor may recommend you bump your dose even higher.

Calcium

Adequate calcium intake is essential during weight loss to help protect bone and lean mass, yet most GLP-1 users don’t consume enough of this mineral. That’s not ideal, since weight loss increases bone turnover and may reduce bone mineral density over time—particularly when calorie intake is low or when weight loss happens rapidly. GLP-1 medications may slightly reduce bone mineral density, too. Long story short: Consuming enough calcium is key to counteract the effects of weight loss on bone health.

Most multivitamins contain some calcium, but often not enough for your daily needs. (Calcium is a bulky mineral, so fitting the full recommended amount into a standard multivitamin tablet or capsule can be difficult.) Most people need 1,000 milligrams (mg) calcium daily. Since it’s best to get some of your calcium from food, look for multis with 300-600 mg of calcium to help supplement what you’re getting from your diet. If your diet lacks calcium (from sources like milk, yogurt, cottage cheese, tofu, cheese, and fortified plant-based milks), talk with a healthcare professional to see whether a separate calcium supplement makes more sense.

Magnesium

Another ingredient to look for in your multivitamin while on a GLP: magnesium. “Magnesium is another common deficiency in GLP-1 users,” says Mitri. But you definitely don’t want to skimp on magnesium when taking a GLP-1. It offers the one-two punch of metabolic support and side effect management. “Magnesium helps in blood sugar management and can also help manage GLP-1 side effects like constipation,” Mitri says. 

As you look for a multi with magnesium, just note that the form of this mineral matters for its health benefits. Magnesium citrate or oxide can help ease constipation and restore magnesium levels, whereas magnesium glycinate is gentler on the stomach and may help with relaxation. Look for a multivitamin that contains magnesium citrate or oxide if you experience constipation. On the other hand, choose a multivitamin with magnesium glycinate if you’re experiencing diarrhea or loose stools. Look for supplements with 200-300 mg of magnesium. Supplements with more than 350 mg of magnesium may cause uncomfortable side effects like diarrhea or abdominal cramping.

Iron

When shopping for a multi to take while on GLP-1, keep your eye on iron. Food sources of iron include red meat, lentils, spinach, and fortified breakfast cereals—but if your appetite is low, you might not be eating enough of these to get your fill. “In addition to reducing intake of iron, GLP-1s may reduce iron absorption in the body,” explains Mitri. Iron is essential for red blood cell production and transporting oxygen throughout the body. Having too little in your system could give you symptoms like fatigue, pale skin, headaches or dizziness, or cold hands or feet.

Supplemental iron can cause gastrointestinal side effects, which may exacerbate what you’re already feeling with a GLP-1 medication. For this reason, Mitri recommends choosing a multivitamin containing iron bisglycinate. This form is more easily absorbed and may cause less constipation than other forms.

Read More: 6 Signs You’re Not Getting Enough Iron

One important sidebar: Many multivitamins that contain iron may not contain calcium (and vice versa), since these two nutrients inhibit each other’s absorption. Routine blood work and conversations with your healthcare provider can help determine which nutrient—calcium or iron—to prioritize in a multivitamin. They may also suggest you take iron or calcium separately from a multivitamin. 

Other Things GLP-1 Users Should Look for in a Multivitamin

• Ingredient list. Check the ingredient list of your GLP-1 multivitamin, especially if it’s a gummy. “Be aware that some gummies have added sugars, and they overall have limited key minerals, such as iron and calcium,” notes Reamer. 
• Third-party testing. Third-party testing lets you know a supplement has gone through rigorous quality assurance processes to help maintain purity and ensure no contamination. (All supplements from The Vitamin Shoppe brand undergo third-party testing.) Look for NSF, ConsumerLab, or USP labeling on supplements to verify high-quality third-party testing.
• Age-specific indicators. Brittany Brown, R.D., I.B.C.L.C., C.D.E., recommends choosing a multivitamin designed for your age and sex. Certain nutrient requirements—such as iron and calcium—vary with these demographics. Choosing one specific to you can offer assurance that you’re getting the supplement amount you need.

The Bottom Line

A multivitamin can help fill nutrition gaps while taking a GLP-1, especially if your appetite is low or you’re eating less. Look for key nutrients like vitamin D, calcium, magnesium, and iron, and consider options that are third-party tested and designed for your age and sex. And if you’ve been taking the same multi for years, it may be time for a check-in with your doctor. “Multivitamin needs can change over time, depending on the person, their individual dose, how suppressed their appetite is, and more,” says Mitri. Bloodwork and a thorough medical history can provide even more specific guidance on what to look for.