Vitamin D for GLP-1 Users and IBD: When Sublingual Delivery Actually Matters

Why GLP-1 Medications and IBD Create a Vitamin D Problem

Vitamin D deficiency affects roughly one in seven GLP-1 medication users and more than one in three people with inflammatory bowel disease — and most of them do not know it. A study of approximately 461,000 GLP-1 users found that 13.6% developed vitamin D deficiency within 12 months of starting medication, while a 2025 tertiary center study of 184 IBD patients recorded a 36.4% deficiency rate (defined as serum 25-hydroxyvitamin D below 20 ng/mL). These are not rare edge cases. They are predictable consequences of how these conditions and medications alter nutrient processing in the body.

GLP-1 receptor agonists like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) work in part by slowing gastric emptying and reducing appetite. The result: people eat less, and the food they do eat moves through the gastrointestinal tract differently. Researchers have called nutrient deficiencies “a common consequence” of GLP-1 use, with vitamin D, iron (3.2%), and B12 (2.6%) leading the list.

For people with Crohn’s disease, ulcerative colitis, and other forms of IBD, the problem runs deeper. Chronic inflammation in the gut wall impairs fat absorption — and vitamin D is a fat-soluble nutrient that depends on healthy intestinal mucosa to cross into the bloodstream. When that lining is inflamed, ulcerated, or surgically shortened, standard oral vitamin D supplements may pass through without being fully utilized.

This article examines the research on sublingual (under-the-tongue) and buccal (inside-the-cheek) vitamin D delivery for these populations — what the evidence actually shows, where it falls short, and when a standard capsule works just as well.

How Common Is Vitamin D Deficiency in These Populations?

More common than most clinicians screen for, according to the data. In GLP-1 users, deficiency rates are significant and under-monitored. A 2026 study in Childhood Obesity tracking 2,031 pediatric GLP-1 users (ages 10–17) found a 12.4% vitamin D deficiency rate within the first year — and only 5% received nutritional counseling within 30 days of their prescription. If screening gaps exist in monitored pediatric populations, adult primary care is likely doing no better.

In IBD, the numbers are starker. The 2025 tertiary center study (n=184) found that 36.4% of IBD patients had vitamin D levels below 20 ng/mL. Patients with deficiency were more likely to be on biologic therapy (86.9% vs. 73.2%), which suggests that those with the most active disease — and therefore the most compromised intestinal absorption — are the ones most likely to be deficient. Vitamin D deficiency in IBD also correlated with higher systemic inflammation markers, creating a cycle where inflammation impairs absorption and low vitamin D may worsen inflammatory outcomes.

These numbers matter because vitamin D is not just a bone nutrient. It plays documented roles in immune regulation, muscle function, and mood — all areas that GLP-1 users and IBD patients report struggling with.

Why Standard Oral Supplements Sometimes Fall Short

For most healthy adults, a vitamin D3 capsule or softgel taken with a fat-containing meal works well. The nutrient dissolves in the stomach, gets absorbed in the small intestine along with dietary fat, and enters circulation through the lymphatic system. This is the standard pathway, and it works.

But this pathway has a prerequisite: a functional small intestine with intact mucosa and adequate fat absorption. In people with IBD — particularly Crohn’s disease affecting the ileum — chronic inflammation disrupts this process. Villous atrophy, mucosal ulceration, and surgical resections (including ileostomy) reduce the absorptive surface area. Fat malabsorption, a documented feature of active Crohn’s disease, compounds the problem for fat-soluble vitamins specifically.

GLP-1 users face a different but related challenge. Slowed gastric emptying changes how long nutrients sit in the stomach’s acidic environment before reaching the small intestine. Reduced caloric intake means fewer fat-containing meals to facilitate absorption. And nausea — reported by 20–44% of GLP-1 users depending on the medication and dose — may lead people to avoid taking supplements altogether or to take them on an empty stomach, further reducing uptake of fat-soluble nutrients.

None of this means oral supplements are useless for these populations. It means they may be less reliable, and it explains why some researchers have explored alternative delivery routes.

What the Research Says About Sublingual and Buccal Vitamin D

The most rigorous evidence for buccal vitamin D delivery in a malabsorption population comes from Kojecky et al., published in Frontiers in Medicine in 2025. This randomized controlled trial enrolled 120 patients with inflammatory bowel disease and compared a buccal D3 nanoemulsion against a standard capsule. The primary finding: the buccal formulation delivered a 36% higher area under the curve (AUC) for serum 25-hydroxyvitamin D compared to the capsule form (Kojecky et al., Frontiers in Medicine, 2025, PMC12414931).

This is a meaningful finding for people with compromised digestion. The buccal route allows vitamin D to be absorbed through the highly vascularized oral mucosa, which does not depend on intestinal fat absorption. For someone with active ileal Crohn’s disease or a shortened bowel, this is a delivery pathway that sidesteps the specific anatomical problem.

A case report published in 2016 documented a patient with Crohn’s disease and an ileostomy who had been unable to correct vitamin D deficiency with oral supplementation. After switching to sublingual vitamin D2, the patient’s serum levels normalized — providing a clinical anecdote consistent with the mechanism Kojecky’s trial later confirmed in a larger population.

For GLP-1 users specifically, no published RCTs have tested sublingual vitamin D head-to-head against capsules. The theoretical rationale is that mucosal delivery avoids the slowed gastric transit caused by these medications, which could improve consistency of uptake — but this remains an assumption that needs clinical validation. We would never overstate what the research has not yet tested.

The Honest Caveat: When a Capsule Works Just as Well

Here is where we include the part most supplement brands skip: the evidence that sublingual delivery does NOT always outperform a standard capsule.

Williams et al. published a randomized controlled trial in the European Journal of Clinical Nutrition in 2019, enrolling 75 healthy adult volunteers. The study compared an oral vitamin D3 spray (3,000 IU daily) against a matched capsule over six weeks. The result: both formats raised serum 25-hydroxyvitamin D by approximately 2 nmol/L per day. There was no statistically significant difference in outcomes between the spray and capsule groups (Williams et al., European Journal of Clinical Nutrition, 2019).

This is an important finding because it establishes a boundary condition. If your digestive system is working normally — no IBD, no surgical resections, no GLP-1-related malabsorption — a sublingual or buccal vitamin D format does not appear to offer a measurable advantage over a well-formulated capsule taken with food. The delivery route matters most when the standard route is compromised.

We think this distinction matters. A supplement format should be chosen based on your specific situation, not on a blanket claim that one route is always superior. If you are a healthy adult who can swallow a capsule with a meal, a capsule is a perfectly effective way to supplement vitamin D. If you have IBD, an ileostomy, active Crohn’s disease, or are experiencing GI side effects from GLP-1 medications that make oral supplementation unreliable, the sublingual route is worth discussing with your healthcare provider.

What to Look for in a Sublingual Vitamin D Format

Not all sublingual or dissolvable formats are created equally. If you and your provider decide this route makes sense for your situation, here are the factors worth evaluating — regardless of which brand you choose.

Vitamin D3 (cholecalciferol) vs. D2 (ergocalciferol): Most clinical guidelines and the Endocrine Society recommend D3 over D2 for supplementation. D3 raises and maintains serum 25-hydroxyvitamin D levels more effectively per unit dose. The Kojecky trial used a D3 nanoemulsion, consistent with this preference.

Dose transparency: The label should state the exact amount of vitamin D per unit in both IU and micrograms. Look for products that list every ingredient and its amount — not just “proprietary blend” language that obscures what you are actually taking.

Format integrity: A sublingual or buccal product only works through the oral mucosa if it is actually held in the mouth long enough for mucosal absorption. A strip or tablet that dissolves quickly and is designed to be held under the tongue or against the cheek is functionally different from a chewable gummy that is swallowed. The mechanism depends on contact time with the oral lining.

XYNE’s Bone Support Strips include vitamin D3 alongside calcium and other bone-supporting nutrients. The strips dissolve in about 30 seconds and are designed for buccal contact. Every ingredient and amount is printed on the label, and the product is tested by the manufacturer’s laboratory.

When to Talk to Your Healthcare Provider

Vitamin D supplementation at standard doses (600–2,000 IU daily for most adults) is generally considered safe. But if you fall into any of these categories, a conversation with your healthcare provider is not optional — it is the responsible next step:

You are on a GLP-1 medication and have not had your vitamin D levels checked since starting. The 13.6% deficiency rate in the research suggests this should be part of routine monitoring for GLP-1 users, but many providers have not yet incorporated it into their standard protocol.

You have IBD, celiac disease, or any condition affecting intestinal absorption. With a 36.4% deficiency rate in IBD patients, screening should be routine. Your provider can order a serum 25-hydroxyvitamin D test and determine whether your current supplementation strategy (format, dose, timing) is actually working.

You are considering high-dose vitamin D supplementation (above 4,000 IU daily). Vitamin D is fat-soluble and can accumulate. Hypercalcemia from vitamin D toxicity, while uncommon, is a real clinical concern. The Endocrine Society notes that malabsorption patients may need higher doses, but “higher” should be defined by blood work, not by guesswork.

You are taking medications that interact with vitamin D metabolism, including certain anticonvulsants, glucocorticoids, and cholestyramine. Your provider can adjust dosing and monitor levels accordingly.

We include this section because we believe a supplement company that does not tell you when to see a doctor instead of buying their product is not acting in your interest.

Frequently Asked Questions

Can GLP-1 medications like Ozempic cause vitamin D deficiency?

Research suggests yes. A study of approximately 461,000 participants found that 13.6% of GLP-1 users developed vitamin D deficiency within 12 months of starting medication. Researchers have described nutrient deficiencies as “a common consequence” of GLP-1 receptor agonist use, likely related to reduced food intake and altered gut motility.

Is sublingual vitamin D more effective than capsules?

It depends on your digestive health. In healthy adults, a 2019 RCT by Williams et al. found no difference between oral spray and capsule vitamin D3 over six weeks — both raised serum levels by about 2 nmol/L per day. However, in IBD patients, a 2025 RCT by Kojecky et al. found buccal vitamin D3 delivered 36% higher AUC compared to capsules. The advantage appears to matter most when digestion is compromised.

How much vitamin D should I take if I have IBD?

Dosing should be individualized with your healthcare provider based on your blood levels. The Endocrine Society suggests that people with malabsorption conditions may need 2–3 times the standard recommended dose, but your provider should test your 25-hydroxyvitamin D levels and adjust accordingly. Do not self-prescribe high doses without medical guidance.

What are the signs of vitamin D deficiency?

Common signs include persistent fatigue, bone pain or tenderness, muscle weakness, mood changes, and frequent illness. However, many people with vitamin D deficiency have no obvious symptoms, which is why blood testing (serum 25-hydroxyvitamin D) is the only reliable way to know. If you are on a GLP-1 medication or have IBD, ask your provider about routine screening.

Do XYNE strips contain vitamin D?

XYNE’s Bone Support Strips include vitamin D3 alongside calcium and other bone-supporting nutrients. Every ingredient and its exact amount are printed on the label. The strips dissolve in about 30 seconds and require no water.

Related Reading

• On a GLP-1? The Nutrient Gaps Ozempic and Wegovy Leave Behind
• Supplement Format Guide for Gut Issues
• Supplement Bioavailability Explained
• Why Supplements Don't Work: The Absorption Problem
• Sublingual vs. Oral Supplements: What the Science Says
• First-Pass Metabolism Explained

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.