Vitamin D

Vitamin D is a fat-soluble secosteroid hormone that functions both as a nutritional requirement and a true steroid hormone. The Holick 2007 NEJM review (PMID 17634462) established the modern framework for understanding vitamin D deficiency as a global health issue. It's synthesized in skin from UVB radiation or obtained from food/supplements, then activated through liver and kidney hydroxylation to 1,25-dihydroxyvitamin D which binds the vitamin D receptor expressed in most tissues. Classical functions include calcium homeostasis and bone mineralization — deficiency causes rickets in children and osteomalacia in adults. Extra-skeletal functions include immune regulation (Jolliffe 2021 PMID 33798465 showed respiratory infection reduction), cardiovascular function, and muscle function (Bischoff-Ferrari 2009 PMID 19523980 showed falls reduction). Approximately 40-75% of adults globally have suboptimal vitamin D due to indoor lifestyle, higher latitudes, skin pigmentation, and sun avoidance. Correction of deficiency is clearly beneficial.

Most guidelines suggest serum 25(OH)D above 30 ng/mL (75 nmol/L) as minimum sufficient, with 40-60 ng/mL often cited as optimal. The Endocrine Society guidelines (Holick 2011 PMID 21646368) recommend at least 30 ng/mL. For most adults, 2000-4000 IU cholecalciferol (D3) daily achieves adequate levels, though individual response varies substantially. Start with 2000 IU daily; recheck 25(OH)D at 3-4 months; adjust based on result. Users with baseline deficiency (<20 ng/mL) need higher doses (5000-10,000 IU daily) for 2-3 months to correct, then maintenance. Higher doses may be needed for: obesity (2-3x standard dose); darker skin; limited sun exposure; older adults; certain medications; genetic polymorphisms. Dose is individual — monitor 25(OH)D rather than following fixed recommendations. Take with fat-containing meal for absorption.

The evidence is nuanced. The VITAL trial (Manson 2019 PMID 30415629) in 25,871 mostly replete U.S. adults found no significant reduction in cardiovascular events or cancer with 2000 IU/day over 5.3 years. D-Health (Neale 2022 PMID 35341732) found no all-cause mortality reduction with monthly dosing in older Australians. D2d (Pittas 2019 PMID 31173679) showed no diabetes prevention. These major trials tempered enthusiasm for universal supplementation. However, Chowdhury 2014 BMJ meta-analysis (PMID 24690623) of observational data showed low 25(OH)D associated with higher mortality. Jolliffe 2021 Lancet meta-analysis (PMID 33798465) showed respiratory infection prevention, particularly in deficient populations. Bischoff-Ferrari 2009 (PMID 19523980) showed falls prevention in older adults. The practical picture: correction of deficiency provides clear benefits; supplementation in already-replete populations shows smaller effects. Vitamin D supplementation is strongly supported for correcting deficiency and specific applications (bone health, falls prevention, respiratory infection prevention in deficient), but not a universal longevity drug.

Vitamin D3 (cholecalciferol) is strongly preferred over D2 (ergocalciferol). D3 is more effective at raising and maintaining serum 25(OH)D and is the form produced by human skin. Most quality supplements use D3. D2 works but less effectively — may need higher doses for equivalent effect. For vegan users, lichen-derived D3 provides vegan-appropriate option with equal efficacy to animal-derived D3. Regarding vitamin K2 — particularly MK-7 form — co-supplementation is strongly recommended in longevity-focused protocols. While vitamin D enhances calcium absorption, /compound/vitamin-k2 activates matrix Gla protein and osteocalcin which direct calcium to bone rather than soft tissues (arteries, kidneys). The combination addresses theoretical concerns about vitamin D-related vascular calcification and supports both bone and cardiovascular health. Standard dose: vitamin K2 (MK-7) 100-200 mcg daily alongside vitamin D. Many quality products provide D3+K2 combination for convenience. Magnesium 300-400 mg daily also complements as cofactor for vitamin D activation.

Possibly, but often difficult in modern life. Adequate cutaneous synthesis requires UVB exposure (wavelength 290-315 nm) to skin. Summer midday sun (UV index 3+) with 25-50% of skin exposed for 10-30 minutes can provide substantial vitamin D. However, limitations include: latitude (minimal UVB reaches earth above 37 degrees during winter); skin pigmentation (darker skin requires 3-6x longer exposure); age (reduced synthesis in aging skin); sunscreen use (blocks UVB); clothing and indoor lifestyle; time of day (midday most effective); season. Most residents of temperate climates have inadequate winter sun exposure regardless of summer habits. Many darker-skinned individuals have persistent deficiency even with regular outdoor time. Sun exposure does not cause vitamin D toxicity (cutaneous synthesis self-regulates). Reasonable approach: deliberate moderate summer sun (balance with skin cancer prevention) complements but typically does not replace supplementation at higher latitudes or in winter. Monitor 25(OH)D to assess whether your lifestyle provides adequate vitamin D.

Vitamin D toxicity (hypercalcemia) is rare at typical supplementation doses but possible with very high chronic intake. The primary concern is hypercalcemia — elevated serum calcium causing nausea, fatigue, frequent urination, confusion, and in severe cases kidney stones or cardiac issues. Toxicity typically requires chronic doses >10,000 IU/day sustained for months, specific predisposing conditions (sarcoidosis, granulomatous disease), severe renal impairment, or certain genetic factors. Serum 25(OH)D >150 ng/mL generally considered toxic range. At typical supplementation doses (1000-5000 IU/day), toxicity is extremely rare in individuals without predisposing conditions. The Institute of Medicine upper intake level is 4000 IU/day; Endocrine Society suggests 10,000 IU/day as upper safe limit. Monitoring 25(OH)D levels detects excessive supplementation early — keep levels under 80-100 ng/mL for routine safety. Users with sarcoidosis, kidney stones, hyperparathyroidism, or on digoxin should use vitamin D only under medical supervision. For healthy adults using quality products at 2000-5000 IU/day, safety profile is excellent.

Yes, evidence supports vitamin D roles in immune function, particularly respiratory infection prevention in deficient populations. The Jolliffe 2021 Lancet meta-analysis (PMID 33798465) of 46 trials with 75,541 participants found vitamin D supplementation reduced acute respiratory infection risk by 8% overall, with larger effects (18-23%) in those with baseline 25(OH)D <25 ng/mL, and larger effects with daily rather than bolus dosing. The prior Martineau 2017 BMJ meta-analysis (PMID 28202713) established similar findings. Mechanistically, vitamin D induces antimicrobial peptide expression (cathelicidin LL-37, beta-defensins) in immune and respiratory epithelial cells, enhances monocyte-macrophage function, and modulates T-cell responses. For COVID-19, Murai 2021 (PMID 33646582) showed mixed results; benefits most apparent in severely deficient patients. The practical picture: correcting vitamin D deficiency provides meaningful immune benefits, particularly for respiratory infection prevention. Universal high-dose supplementation provides smaller benefits. Maintain 30-50 ng/mL 25(OH)D as reasonable immune support, particularly during winter months.

Yes, vitamin D is foundational for bone health. Classical functions include enhancing intestinal calcium absorption, supporting osteoblast activity, and maintaining bone mineralization. Severe deficiency causes osteomalacia (adult rickets) with bone pain and fractures. Less severe deficiency contributes to osteoporosis and fracture risk. For older adults specifically, Bischoff-Ferrari 2009 meta-analysis (PMID 19523980) showed 800-1000 IU/day reduced falls by 19% — likely through improved muscle function combined with bone effects. For osteoporosis prevention and treatment, vitamin D combined with adequate calcium (1000-1200 mg/day from diet), /compound/vitamin-k2, magnesium, and resistance training represents comprehensive approach. Typical doses for bone health: 2000-4000 IU vitamin D daily adjusted to achieve 30-60 ng/mL 25(OH)D. Calcium supplementation only if dietary intake inadequate. Regular DEXA bone density monitoring for at-risk individuals. Combined with medications like bisphosphonates when indicated. Vitamin D deficiency correction clearly beneficial for bone; modest benefits in already-replete individuals.

Vitamin D has several relevant drug interactions. Anticonvulsants (phenytoin, phenobarbital, carbamazepine, primidone) accelerate vitamin D catabolism — users on these medications typically need 3-5x standard doses. Chronic corticosteroids (prednisone) accelerate vitamin D metabolism and increase osteoporosis risk — higher vitamin D doses typically appropriate. Thiazide diuretics reduce urinary calcium excretion; combined with vitamin D may increase hypercalcemia risk — monitor calcium. Orlistat (fat absorption inhibitor) reduces vitamin D absorption — separate timing. Bile acid sequestrants (cholestyramine) reduce absorption — separate by 2+ hours. Digoxin toxicity increased by hypercalcemia — avoid hypercalcemia on digoxin. For users on these medications or on multiple medications, physician consultation appropriate. No significant negative interactions with: statins, most blood pressure medications, most antibiotics, most antidepressants, thyroid medications, /compound/metformin, /compound/rapamycin, /compound/omega-3-fatty-acids, /compound/creatine. The interaction profile is navigable; most users can safely take vitamin D with typical medications.

Vitamin D integrates as foundational element across longevity stacks. Core pairing with /compound/vitamin-k2 (MK-7 100-200 mcg daily) directs calcium to bone rather than soft tissues — essentially standard combination. Magnesium 300-400 mg daily is required cofactor for vitamin D activation — co-supplementation important. /compound/omega-3-fatty-acids 1-3 g EPA+DHA daily, /compound/creatine 3-5 g daily, and quality protein intake form the broader foundational longevity stack. Vitamin D complements /compound/zinc and vitamin C for immune support. For mitochondrial support, vitamin D pairs with /compound/urolithin-a, /compound/coq10, /compound/nmn, /compound/nr, and peptide interventions /compound/ss-31 and /compound/mots-c. For senolytic protocols, vitamin D continues during /compound/fisetin, /compound/dasatinib + /compound/quercetin cycles without interference. For users on /compound/rapamycin or /compound/metformin, vitamin D remains appropriate. The broad compatibility makes vitamin D easy to incorporate into virtually any longevity protocol. Monitor 25(OH)D to optimize dose; maintain target 40-60 ng/mL for most longevity-focused users.