Expert Q&A

When and how to take Vitamin D when on an extended water fast — what does the research actually say if you're on a GLP-1 like semaglutide or tirzepatide?

Vitamin D Absorption Basics During Water Fasts

Vitamin D is fat-soluble, meaning it normally requires dietary fat for optimal absorption in the small intestine. During an extended water fast—typically 48-72 hours or longer with only water, black coffee, or plain tea—there are no fats present. Research from the Journal of Clinical Endocrinology & Metabolism shows absorption drops by 30-50% without accompanying lipids. In The 30-Week Tirzepatide Reset, we address this by recommending patients take their Vitamin D with a small amount of MCT oil or avocado oil on non-fasting days, but during true water-only phases, we shift to sublingual or micellized forms that bypass some fat dependency.

Research on GLP-1 Medications Like Tirzepatide and Semaglutide

Studies in Obesity Reviews and Diabetes Care indicate that GLP-1 receptor agonists such as tirzepatide and semaglutide slow gastric emptying by up to 50%, which can further impair fat-soluble nutrient uptake. A 2022 trial in the American Journal of Clinical Nutrition found that patients on these medications had 18% lower serum Vitamin D levels after three months unless supplemented strategically. Our protocol in The 30-Week Tirzepatide Reset uses low-dose tirzepatide cycling precisely to avoid prolonged gut slowdown during fasting windows. We recommend 4,000-5,000 IU daily of Vitamin D3, taken at the end of a fast with the first meal containing healthy fats like olive oil or fatty fish to restore absorption efficiency immediately.

Practical Timing and Dosing Guidelines

For extended water fasts within our 30-week program, break the fast gently before taking standard Vitamin D capsules. If fasting exceeds 72 hours, use a water-miscible Vitamin D drop placed under the tongue mid-fast; studies show sublingual delivery maintains blood levels within 10% of fat-accompanied doses. Always pair with magnesium (300-400 mg) and Vitamin K2 (100 mcg) to prevent calcium buildup—critical when managing joint pain or diabetes alongside weight loss. Track levels every 8-12 weeks; aim for 50-70 ng/mL serum 25(OH)D. Our patients following the lectin-free low-carb diet with Japanese-style walking see faster normalization because reduced inflammation improves receptor sensitivity.

Integrating With the Full Reset Protocol

The biggest mistake is treating Vitamin D in isolation. In The 30-Week Tirzepatide Reset, we combine smart cycling of tirzepatide with Detox Drops, red light therapy, and chaotic intermittent fasting in maintenance. This prevents the rebound weight gain common when relying on medication alone. One patient following our 69 Transformation Steps lost 42 pounds in 22 weeks, normalized A1C from 7.4 to 5.6, and raised Vitamin D from 18 ng/mL to 62 ng/mL without continuous high-dose shots. Focus on metabolic freedom: use the medication as a tool while rebuilding natural hormone signaling through real foods and recovery habits. This approach delivers the non-scale victories—better energy, less joint pain, stable blood pressure—that make results last.

💬 What the Community Says

The community shows strong interest in safely combining extended water fasting with GLP-1 medications like tirzepatide, but opinions split on Vitamin D timing. Many in their 40s and 50s report taking 5,000 IU mid-fast using sublingual drops or with the first refeed meal containing fats, claiming stable energy and no major deficiencies. Others debate absorption science, with some citing personal bloodwork showing drops after 5-day fasts and preferring to pause supplementation entirely during water-only phases. Insurance barriers and past diet failures fuel caution; users frequently share success stories of improved labs when pairing Vitamin D with magnesium and K2. A vocal minority warns against long fasts on semaglutide due to slowed digestion, while most appreciate protocols that cycle medication and emphasize real-food resets over dependency. Overall, lived experiences highlight trial-and-error around joint pain relief and blood sugar stability during these combined approaches.
Cite this answer
Clark, R. (2026). When and how to take Vitamin D when on an extended water fast — what does the re. *CFP Weight Loss*. https://ask.cfpweightloss.com/ask/when-and-how-to-take-vitamin-d-when-on-an-extended-water-fast-what-does-the-research-actually-say-if-you-re-on-a-glp-1-like-semaglutide-or-tirzepatide
Russell Clark, FNP-C, APRN, FNP-C, APRN
About the Author

Russell Clark, FNP-C, APRN, is the founder of CFP Weight Loss in Nashville and CFP Fit Now telehealth. Over 35 years in healthcare — Army Nurse Reserves, Level 1 trauma ER, hospitalist — he developed a 30-week protocol integrating real foods, detox, and low-dose tirzepatide cycling that has helped hundreds of patients lose 30–90 pounds. He and his wife Anne-Marie lost a combined 275 pounds using the same protocol.

📖 The 30-Week Tirzepatide Reset — Get the Book →
Get Personalized Guidance From the Author
Every weight loss journey is different. Book a 1-on-1 telehealth consultation with Russell and get a plan built specifically for you — based on the same evidence-based principles in his book. Available to patients in all 50 states.
Book Your Consultation →
Have a follow-up question?
More Questions 200 answered
▸ 78kg to 58kg for an 18 y/o: How likely am I to have loose skin after weight loss specifically for women over 40 — what certified coaches recommend? ▸ How does chasing skinny ever get old for long-term results while doing intermittent fasting? ▸ What can I ask for if estrodiol cream remains in shortage? Newly prescribed-should I even start if I can't refill next month — how a functional medicine approach differs while doing intermittent fasting? ▸ Weight loss without GLP-1 — what does the research actually say while doing intermittent fasting? 📖 Insulin Resistance — A condition where your cells stop responding to insulin, locking fat in storage; our protocol is designed to unlock thes… ▸ My weight loss progress so far as a 23 year male who stared eating less unhealthy and does little to moderate exercise. Am I doing well — evidence-based answer for CFP patients for people with insulin resistance? ▸ What actually helps with the dryness when you have PCOS or hormonal imbalances while doing intermittent fasting? ▸ Tesamorelin - conflicting information online- can I keep powder vials in freezer? Can I keep reconstituted vials in fridge — what do certified weight loss coaches recommend for those with hypothyroidism or Hashimoto’s? ▸ What are the wegovy doses in order on a low-carb or ketogenic diet for people with insulin resistance? 📖 Tirzepatide Micro-Dosing — The practice of using the lowest effective dose to achieve results, minimizing side effects while maximizing the longevi… ▸ My weight loss progress so far as a 23 year male who stared eating less unhealthy and does little to moderate exercise. Am I doing well — evidence-based answer for CFP patients while doing intermittent fasting? ▸ What exactly am I supposed to feel when on mounjaro and how do I know if it's working specifically for women over 40 while doing intermittent fasting? 📗 The 30-Week Tirzepatide Reset — by Russell Clark, FNP-C, APRN · The complete clinical protocol — 30 weeks of real foods, detox, and low-dose tirzepatide cycling that has helped hundreds of patients lose 3 ▸ Sterilized soybean sprout — how a functional medicine approach differs: best practices and common mistakes to avoid? ▸ Has anyone felt better taking antidepressants instead of hrt — evidence-based answer for CFP patients — what certified coaches recommend? ▸ Will eating meintenance calories make me lose weight for those with hypothyroidism or Hashimoto's when you have PCOS or hormonal imbalances? ▸ Omnipod. Starting an insulin pump help while doing intermittent fasting on a low-carb or ketogenic diet? ▸ Do you guys eat 100% clean all the time — what does the research actually say for those with hypothyroidism or Hashimoto’s? 📖 Metabolic Stall — A plateau in weight loss often caused by hidden inflammation or hormonal adaptation, which we break using our specific 7… ▸ What to do if you have to eat late dinners?? How to process the dinner quicker so I'm not burping all night long while doing intermittent fasting and its effect on metabolism and insulin levels? ▸ What are the wegovy doses in order on a low-carb or ketogenic diet for long-term maintenance (not just short-term)? ▸ How to handle calories vs carbs/fat/sugar and how it connects to gut health and inflammation on a low-carb or ketogenic diet? 📖 Autophagy — The body's natural 'housecleaning' process that occurs during fasting or low-insulin states, promoted during our mainten… ▸ Been on this diet for 11 months and its effect on metabolism and insulin levels and its effect on metabolism and insulin levels? 📗 The 30-Week Tirzepatide Reset — by Russell Clark, FNP-C, APRN · The complete clinical protocol — 30 weeks of real foods, detox, and low-dose tirzepatide cycling that has helped hundreds of patients lose 3 ▸ Is it normal to lose 6 kg in one week during the weight loss plateau phase for long-term maintenance (not just short-term)? ▸ If a random dance partner app existed .. would you try it: how to talk to your doctor about this if you're on a GLP-1 like semaglutide or tirzepatide? ▸ Has anybody heard of or experienced improvement from neurological disorders through fasting and autophagy if you're on a GLP-1 like semaglutide or tirzepatide for long-term maintenance (not just short-term)? ▸ Im eating so well that I accidently put my body in ketosis and how it connects to gut health and inflammation: best practices and common mistakes to avoid? ▸ What are the wegovy doses in order on a low-carb or ketogenic diet — what certified coaches recommend? ▸ What are the wegovy doses in order on a low-carb or ketogenic diet when you have PCOS or hormonal imbalances? 📖 High-Dose Dependency — The pharmaceutical trap of staying on maximum medication doses indefinitely, which we avoid through our smart cycling me… ▸ Can you take vitamin D with a tbsp of oil on an extended water fast for people with insulin resistance during the weight loss plateau phase? ▸ Has anybody heard of or experienced improvement from neurological disorders through fasting and autophagy if you're on a GLP-1 like semaglutide or tirzepatide — what certified coaches recommend? 📗 The 30-Week Tirzepatide Reset — by Russell Clark, FNP-C, APRN · The complete clinical protocol — 30 weeks of real foods, detox, and low-dose tirzepatide cycling that has helped hundreds of patients lose 3 ▸ Best Curcumin Supplements: how to talk to your doctor about this if you're on a GLP-1 like semaglutide or tirzepatide? 📖 Strategic Fat Loading — A critical 48-hour phase at the start of the reset where we prime the body with healthy fats to trigger the transition f… ▸ Anyone remember the metal lid you had to use a butter knife to pop off: what to track and how to measure progress: best practices and common mistakes to avoid? ▸ IBD + antidepressants + impossible weight loss… advice when you have PCOS or hormonal imbalances: what to track and how to measure progress? ▸ What foods to stock for overnight guest — evidence-based answer for CFP patients: what to track and how to measure progress? ▸ Do yall ever daydream of who you were before your diagnosis when you have PCOS or hormonal imbalances when you have PCOS or hormonal imbalances? ▸ How many were first diagnosed with OCD if you're on a GLP-1 like semaglutide or tirzepatide on a low-carb or ketogenic diet? 📖 One Box Protocol — A revolutionary approach where we stretch one 60 mg box of tirzepatide across three metabolic phases, prioritizing long-… ▸ What foods to stock for overnight guest — evidence-based answer for CFP patients — what certified coaches recommend? ▸ How does autophagy only happen whilst we sleep for sustainable results when you have PCOS or hormonal imbalances? ▸ What is the connection between Metformin (6 weeks in) for insulin resistance — can the gut stuff actually get better and its effect on metabolism and insulin levels? 📗 The 30-Week Tirzepatide Reset — by Russell Clark, FNP-C, APRN · The complete clinical protocol — 30 weeks of real foods, detox, and low-dose tirzepatide cycling that has helped hundreds of patients lose 3 ▸ Starting a 7 day water fast, any tips to keep hydrated on a low-carb or ketogenic diet: what to track and how to measure progress? ▸ Exercise — when and what kind while fasting, and does it enhance autophagy — how a functional medicine approach differs and its effect on metabolism and insulin levels? ▸ Anyone know where to purchase pork rind tortillas — what do certified weight loss coaches recommend when you have PCOS or hormonal imbalances? 📖 Lectin-Free Living — Eliminating inflammatory plant proteins that damage the gut lining and trigger the autoimmune responses that often stall… ▸ Can you really Metformin (6 weeks in) for insulin resistance — can the gut stuff actually get better on a low-carb or ketogenic diet? ▸ Low TSH, Low T4, and Low T3 simultaneously on Levothyroxine — anyone experienced this and the role of cortisol and stress hormones — what certified coaches recommend? ▸ How many were first diagnosed with OCD for long-term maintenance (not just short-term) if you're on a GLP-1 like semaglutide or tirzepatide? 📖 Gut Microbiome Repair — The restoration of healthy intestinal bacteria through the removal of lectins and grains, essential for long-term weight… ▸ Is the carnivore diet the best WOE to lose weight — what does the research actually say specifically for women over 40? ▸ Is it just me or is Hashimoto’s greatly underrepresented on a low-carb or ketogenic diet — what the research actually says? ▸ LDN is changing my life, will this last: best practices and common mistakes to avoid for people with insulin resistance? ▸ Should I be worried yet, been sitting for 5 days no movement while doing intermittent fasting: best practices and common mistakes to avoid? 📗 The 30-Week Tirzepatide Reset — by Russell Clark, FNP-C, APRN · The complete clinical protocol — 30 weeks of real foods, detox, and low-dose tirzepatide cycling that has helped hundreds of patients lose 3 ▸ To what extent do Mendelian Randomization Studies provide causal evidence for a particular claim — what does the research actually say specifically for women over 40? 📖 Anti-Inflammatory Protocol — A way of eating that prioritizes whole foods and eliminates triggers to quiet the internal 'fire' that prevents fat cell… ▸ How effective is autophagy really helping me lose fat — what certified coaches recommend? ▸ Who still goes to steeplechase — what do certified weight loss coaches recommend for long-term maintenance (not just short-term)? ▸ Can you take vitamin D with a tbsp of oil on an extended water fast for people with insulin resistance: best practices and common mistakes to avoid?