Best Supplements to Take on GLP-1 Medications UK 2026

Most GLP-1 users focus on what they're losing — weight, appetite, cravings. Few think about what else they're losing: micronutrients, electrolytes, and protein that previously came from food they no longer eat.

Semaglutide and tirzepatide work by suppressing appetite significantly. In the STEP 1 trial, participants on semaglutide 2.4mg lost 14.9% of body weight over 68 weeks. Tirzepatide 15mg produced 20.9% weight loss in SURMOUNT-1 over 72 weeks. That kind of caloric restriction, sustained over months, creates real nutritional gaps if you don't address them.

This article breaks down which supplements are genuinely worth taking, which are optional, and which are a waste of money on GLP-1 therapy.

Why GLP-1 Creates Nutrient Deficiencies

GLP-1 receptor agonists slow gastric emptying and reduce appetite through central mechanisms. The result: most users eat 500-900 fewer calories per day than before. That deficit is intentional — it's how the weight loss works. But food isn't just calories. When intake drops, so does intake of vitamin B12, iron, vitamin D, magnesium, electrolytes, and dietary protein.

Three factors compound this:

Selective food avoidance. Nausea affects 44% of users (vs 16% on placebo). Many find dense, nutrient-rich foods — red meat, eggs, leafy greens — less appealing or harder to tolerate. Processed carbohydrates often feel easier. The result is a diet lower in quality, not just quantity.

Reduced absorption of B12. Semaglutide reduces gastric acid secretion, which impairs B12 absorption from food. This is the same mechanism seen with long-term metformin and proton pump inhibitor use, both of which cause documented B12 depletion.

Muscle loss accelerated by deficit. Without adequate protein and resistance training, roughly 40% of weight lost on GLP-1 comes from lean mass rather than fat (presented at ENDO 2025). Supplementing strategically helps preserve what you've built.

The Evidence: Essential, Optional, and Skip

Vitamin B12: The Most Important One

B12 deficiency develops slowly and is often missed. Symptoms — fatigue, brain fog, tingling in hands and feet — can take 12-24 months to appear, and by the time blood tests show frank deficiency, some neurological damage may already have occurred.

On GLP-1 therapy, reduced gastric acid secretion means less intrinsic factor is available to bind dietary B12. Methylcobalamin (the active form) bypasses this problem when taken as a sublingual tablet or spray — it's absorbed directly through the oral mucosa.

Dose: 1,000mcg methylcobalamin daily, sublingual where possible. If you're getting quarterly blood tests (which you should be — see the GLP-1 monitoring bloodwork guide), include B12 and folate in the panel.

Vitamin D3 + K2

Approximately 70% of UK adults are deficient in vitamin D during winter months. On a calorie-restricted diet, fat-soluble vitamin intake drops further. Vitamin D is essential for calcium absorption, immune function, and — relevant here — insulin sensitivity. Low vitamin D correlates with poorer metabolic outcomes.

K2 is included because D3 mobilises calcium, and K2 directs that calcium into bones rather than arteries. The combination is clinically meaningful.

Dose: 2,000-4,000 IU D3 daily with K2 100-200mcg (MK-7 form). Take with a meal containing dietary fat for absorption.

Magnesium

Dietary magnesium comes primarily from leafy greens, nuts, seeds, and wholegrains. When food intake drops, magnesium drops with it. Magnesium is involved in over 300 enzymatic processes, including ATP production, insulin signalling, and muscle contraction.

Low magnesium impairs sleep quality, reduces insulin sensitivity, and increases muscle cramp frequency — all things GLP-1 users regularly report.

Magnesium glycinate and magnesium threonate are better tolerated than cheaper oxide forms (which cause loose stools). Glycinate is good for general use; threonate has evidence for cognitive benefit and sleep specifically.

Dose: 300-400mg elemental magnesium daily in the evening. See the full breakdown in the electrolytes guide.

Iron: Critical for Women

Iron deficiency is the most common nutritional deficiency in the UK generally, and the risk increases significantly on GLP-1 therapy. Menstruating women who were previously meeting their iron needs through dietary red meat intake are at particular risk when GLP-1-induced nausea makes beef and lamb difficult to eat.

Symptoms of iron deficiency — fatigue, breathlessness, poor concentration — overlap heavily with GLP-1 adaptation symptoms, making it easy to attribute them to the medication rather than deficiency.

A ferritin level below 30 mcg/L warrants supplementation even if haemoglobin is normal. Iron bisglycinate is the best-tolerated form — significantly less constipating than ferrous sulphate.

Dose: 15-25mg elemental iron daily (bisglycinate form), taken on an empty stomach with vitamin C to enhance absorption. Separate from calcium and magnesium by at least 2 hours.

Protein: The Non-Negotiable

Protein isn't a micronutrient, but it deserves a place here because inadequate protein intake is the most common and consequential nutritional error on GLP-1 therapy.

The target is 1.2-1.5g per kilogram of body weight per day. At reduced calorie intake, hitting that from food alone is genuinely difficult. A protein supplement — whey concentrate, isolate, or plant-based blend — fills the gap.

This gets its own full article: Protein on GLP-1: How Much You Need to Prevent Muscle Loss.

Omega-3 (EPA/DHA)

GLP-1 medications have impressive cardiovascular benefits independent of weight loss — SELECT trial data showed 20% reduction in major cardiovascular events on semaglutide. Omega-3 supplementation works through overlapping mechanisms: reducing triglycerides, lowering inflammation markers, and supporting endothelial function.

The combination is additive rather than redundant.

Dose: 1-2g combined EPA+DHA daily from high-quality fish oil or algal oil (for vegans).

Electrolytes

If you're experiencing nausea and reduced fluid intake, electrolyte depletion happens faster than most people expect. Sodium, potassium, and magnesium all drop when food intake falls sharply.

Symptoms: headaches, fatigue, muscle cramps, dizziness when standing. These are often misattributed to GLP-1 adaptation when they're actually straightforward electrolyte deficiency.

The full breakdown is in the electrolytes on semaglutide guide.

What to Actually Buy

If budget is a constraint, prioritise in this order:

1. Methylcobalamin B12 (sublingual, 1,000mcg)
2. Vitamin D3 + K2
3. Magnesium glycinate
4. Protein supplement (if not hitting 1.2g/kg from food)
5. Iron (women only, or if ferritin confirmed low)
6. Omega-3

A comprehensive multivitamin can cover bases 1-5 partially, but most multivitamins use inferior forms (cyanocobalamin B12, magnesium oxide, calcium carbonate) and at doses too low to correct deficiency. Think of a multi as insurance, not treatment.

Timing and Practical Notes

Morning stack: D3+K2 (with breakfast), omega-3 (with food), B12 (sublingual, any time)

Evening stack: Magnesium glycinate (30 minutes before bed)

Separate from GLP-1 injection: No known interactions between GLP-1 medications and the supplements listed here. Take them as normal.

Iron timing: On an empty stomach if tolerated, otherwise with a light meal — but away from other minerals for at least 2 hours.

For men specifically looking to optimise training and hormones alongside GLP-1 treatment, Male Optimal covers testosterone, training protocols, and bloodwork in detail.

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Always consult your GP or prescribing clinician before starting a new supplement regimen, particularly if you have existing medical conditions or take other medications.