Iron announces itself. You feel heavy, pale, breathless. B12 announces itself with tingling and fog. Vitamin D and zinc are quieter. You feel a little more down than you'd expect. A little more prone to catching things. Hair that seems thinner. A cut that takes too long to heal. Easy to explain away — easy to miss entirely.

Both play direct roles in muscle function, immune response, hair follicle health, and mood. Running low on either for months, while also dealing with rapid weight loss and low food intake, creates a cumulative drag that's hard to see clearly from the inside.

Vitamin D needs dietary fat to be absorbed — and fat intake falls sharply on a GLP-1. That affects absorption from supplements too, not just food. Many women also start out low, because vitamin D gets sequestered in fat tissue and becomes less available at higher body weight.

There's a delayed effect worth knowing about: as fat tissue decreases, stored vitamin D releases for a while before levels fall again. Some women feel fine for months before their numbers show a meaningful decline.

Taking D3 so it actually works

1Take it with your fattiest meal of the day. Even a small amount helps — nut butter, avocado, olive oil on vegetables, a handful of nuts.
2Pair D3 with K2 (MK-7 form). K2 helps direct calcium toward bone rather than arteries — an important pairing at meaningful D3 doses.
3Get a baseline lab if you can. A 25-hydroxyvitamin D test tells you where you're starting. Below 20 ng/mL may warrant a loading dose before maintenance.
4Retest at three months. D3 takes time to move levels. Give it a quarter before deciding whether your dose is working.

Shellfish, red meat, and seeds are the main zinc sources — and shellfish and red meat are among the hardest foods to eat consistently on a GLP-1. Unlike iron or vitamin D, your body keeps almost no zinc in reserve. A few weeks of low intake is enough to start depleting it.

Zinc also affects eating itself. Deficiency alters taste perception, making food taste blander or slightly metallic — which compounds an already difficult eating situation.

Vitamin D and zinc usually get discussed in terms of immunity and hair, which is fair. But both have a less-discussed role that matters more in the context of a GLP-1: muscle protein synthesis.

Low vitamin D is associated with muscle weakness independent of exercise — your muscles respond less effectively to resistance training when it's low. Zinc deficiency impairs the cellular machinery that turns dietary protein into muscle tissue. Which means you can be hitting your protein targets and doing your sessions and still losing more muscle than you should, because the micronutrient environment isn't supporting the process.

Symptoms to watch for vitamin D: afternoon fatigue, low mood or emotional flatness, leg weakness, slower wound healing, more frequent illness. For zinc: diffuse hair thinning, slow healing, illness that drags on, taste changes, white spots on nails, dry skin.

Continue to Part 4: What happens when you stop Ozempic or Wegovy →