Mounjaro and Vitamin D Deficiency: What the Research Shows

You're losing weight. Your bloods look great. But you're more tired than ever, and your mood is dipping. Sound familiar?

If you're on Mounjaro (or Wegovy, or Ozempic) and you're noticing energy, mood or hair changes, vitamin D is the first thing to check. Here's what the research shows — and what to actually do about it.

The numbers that should be in your Mounjaro starter pack (but aren't)

Per a 2024 analysis flagged by Harvard Health:

• 49% higher risk of vitamin D deficiency in GLP-1 users vs non-users
• 13.6% of users clinically deficient at 12 months
• Risk grows the longer you're on the drug

This isn't a Mounjaro-specific failure. It's the natural consequence of eating dramatically less of the foods that contain vitamin D (oily fish, eggs, fortified dairy) while your body's daily requirement stays the same.

What vitamin D actually does (and why running low hurts)

Vitamin D is involved in:

• Mood — strong evidence linking low D to low mood and seasonal patterns
• Energy — fatigue is the most common deficiency symptom
• Bone density — long-term low D combined with weight loss can quietly weaken bones
• Immunity — winter colds hit harder
• Hair quality — D deficiency is implicated in shedding
• Sleep — deficiency disrupts melatonin pathways

Most of these are exactly the symptoms people blame on Mounjaro itself. Sometimes it's just the drug. Often, it's the deficiency the drug created.

The UK winter compound problem

Before you even started Mounjaro, you were already at risk. NHS guidance is for all UK adults to supplement vitamin D from October to March — because UK sun between October and March doesn't deliver the UVB wavelengths your skin needs to make D3.

That's the baseline UK problem. Now add:

• 30% less food intake
• Reduced consumption of D-rich foods (oily fish, eggs)
• Possible slower nutrient absorption from delayed gastric emptying

The result is a deficiency picture that the NHS recommended dose of 400 IU per day was never designed to cover.

What dose do you actually need?

The NHS minimum is 400 IU (10 mcg). That figure was calibrated for the average UK adult eating a normal diet.

For most adults already at risk — and especially anyone on a GLP-1 — clinicians typically recommend 1,000 IU daily as a baseline. The upper safe limit established by the European Food Safety Authority is 4,000 IU/day, so 1,000 IU sits well inside the safe range.

Solmate provides 1,000 IU of D3 daily, paired with K2 (MK-7). The K2 matters because vitamin D increases calcium absorption, and you want that calcium going to bones — not lining your arteries.

When to test

Ask your GP for a 25-hydroxyvitamin D (25(OH)D) blood test. Interpretation in the UK:

• Below 30 nmol/L — clinically deficient. Needs treatment dose.
• 30–50 nmol/L — insufficient. Should supplement.
• 50–75 nmol/L — adequate but not optimal.
• Above 75 nmol/L — optimal.

If you're more than 3 months into Mounjaro, get the test.

What to do this week

1. Book a 25(OH)D blood test with your GP.
2. Start a daily D3 + K2 supplement at 1,000 IU. The benefit usually shows up at 4–8 weeks.
3. If you're noticing low mood, fatigue or sleep issues, talk to your GLP-1 prescriber too. Some of it is the drug. Some isn't.

See Solmate's full stack →

This isn't medical advice. Solmate is a food supplement, not a treatment for vitamin D deficiency. If you're on prescribed medication, talk to your GP or prescriber before adding supplements.