Why I Still Check Every Patient's Vitamin D

One of the largest vitamin D trials ever run did not prove that everyone should take vitamin D, but it did uncover striking signals for cancer death, advanced cancer and autoimmune disease that every person deserves to understand.

Vitamin D was one of the first labs I learned to take seriously as a young physician, and it is still one of the first I check. Over the years I have watched patients with stubborn fatigue, recurrent infections, autoimmune flares and winter blues discover that their level was in the single digits. When we corrected it, as part of a whole-person plan, many of them felt like themselves again.

At the same time, I have watched vitamin D become a headline roller coaster: a miracle one year, a myth the next. So when patients ask me, “Does vitamin D actually work?”, I want to give them an honest answer grounded in the best science we have.

That is why I want to walk you through a key 2020 paper by Dr. JoAnn Manson and colleagues at Harvard, which summarizes the principal results of the VITamin D and OmegA-3 TriaL (VITAL) and places them alongside updated meta-analyses of other vitamin D trials.[1] VITAL followed nearly 26,000 adults for over five years, and its findings, together with the trials that followed, tell a more nuanced and more useful story than either the enthusiasts or the skeptics usually tell.

In this article you will learn how vitamin D works, what VITAL and other major trials found (including the disappointing ones), who is most likely to benefit, and how I test and treat vitamin D in my practice.

Vitamin D Is Really a Hormone

Vitamin D is less a vitamin than a steroid hormone, and nearly every tissue in your body has the machinery to respond to it.

Here is the journey from sunshine to signal:

1. Made in the skin. When ultraviolet B (UVB) light strikes the skin, it converts a cholesterol precursor (7-dehydrocholesterol) into vitamin D3. We also get smaller amounts from food (fatty fish, egg yolks, fortified foods) and from supplements.
2. Stored form in the liver. The liver converts vitamin D into 25-hydroxyvitamin D, or 25(OH)D. This is the circulating storage form, and it is what we measure in a blood test.
3. Activated in the kidney and in tissues. The kidney converts 25(OH)D into the active hormone, 1,25-dihydroxyvitamin D (calcitriol), mainly to regulate calcium and phosphorus. Importantly, many other cells, including immune cells, also carry the enzyme (CYP27B1) to make active vitamin D locally, right where it is needed.
4. Switches genes on and off. Active vitamin D binds the vitamin D receptor (VDR), found in immune cells, brain, muscle, gut lining, pancreas, heart and blood vessels. The VDR acts as a transcription factor, influencing the expression of hundreds of genes.

That last point is why vitamin D touches so many aspects of health. It is not just about bones. It is a master regulator that helps the immune system tell friend from foe, helps the gut lining hold together, and helps muscles and nerves work as they should.

What VITAL Found: Null on the Headline, Striking in the Details

VITAL found that 2,000 IU of vitamin D3 daily did not lower the overall rate of new cancers or heart attacks and strokes, but it was linked with fewer cancer deaths in some analyses, fewer advanced cancers, and 22% fewer autoimmune diagnoses.

How VITAL Was Designed

The Primary Results

In the main 2019 report, vitamin D did not significantly reduce invasive cancer (hazard ratio 0.96) or major cardiovascular events (hazard ratio 0.97), and all-cause mortality was unchanged (hazard ratio 0.99).[2] For a population of mostly well-nourished, generally healthy older adults, the headline was null.

The Signals Worth Paying Attention To

• Cancer death. Cancer mortality was 17% lower with vitamin D (hazard ratio 0.83), but this did not reach statistical significance. When the first two years of follow-up were excluded (cancers take years to develop and to kill), the reduction was 25% (hazard ratio 0.75) and statistically significant. Because this was a secondary analysis, it should be viewed as a signal rather than proof.[2]
• Advanced cancer. In a 2020 follow-up, vitamin D reduced the combined rate of metastatic or fatal cancer by 17% (hazard ratio 0.83, P = 0.04). The benefit appeared concentrated in people with a normal body weight (BMI under 25), with roughly a 38% reduction in that group, while people with higher BMI showed little benefit. The authors noted the BMI finding could be due to chance.[3]
• Autoimmune disease. In a 2022 analysis, people taking vitamin D had a 22% lower rate of confirmed new autoimmune disease, including rheumatoid arthritis, polymyalgia rheumatica, autoimmune thyroid disease and psoriasis (hazard ratio 0.78, P = 0.05). Compared with placebo alone, vitamin D alone was associated with 32% fewer cases.[4]

The Bigger Picture From Meta-analyses

The Manson review placed VITAL in the context of other randomized trials.[1] Two independent meta-analyses published in 2019 reinforce its cancer message: pooling randomized trials, vitamin D supplementation reduced cancer mortality by about 13% (relative risk 0.87) in one analysis[5] and about 16% (relative risk 0.84, across 52 trials and over 75,000 participants) in another, while not reducing new cancer diagnoses or all-cause mortality.[6]

Put simply, vitamin D may not stop cancer from starting, but it may change how aggressive cancer becomes. That is a hypothesis worth taking seriously.

The Benefits of Vitamin D, System by System

The strongest evidence for vitamin D lies in immune defense, autoimmunity, cancer mortality and diabetes prevention in high-risk people, while some popular claims (bones in replete adults, depression, heart disease) have not held up in large trials.

Immune Defense and Respiratory Infections

Vitamin D helps innate immune cells produce antimicrobial peptides such as cathelicidin and helps calm excessive inflammatory responses, a dual role of strengthening defense while preventing friendly fire.[26]

• An individual participant data meta-analysis of 25 trials (11,321 people) found vitamin D reduced acute respiratory infections by 12% overall. In people starting with very low levels (below 25 nmol/L, or 10 ng/mL) who took daily or weekly doses, the reduction was 70%.[9]
• An updated 2021 analysis of 46 trials (75,541 participants) confirmed a smaller overall benefit (8%), with the clearest protection from daily dosing, especially daily doses of 400 to 1,000 IU.[10] Trials using large, infrequent bolus doses showed less consistent protection.
• During the pandemic, an observational study of 191,779 US patients found SARS-CoV-2 positivity of 12.5% in people with levels below 20 ng/mL versus 5.9% at 55 ng/mL or higher. This association cannot prove cause and effect.[28]

Autoimmunity

The VITAL autoimmune finding (22% fewer new autoimmune diagnoses) is one of the most important results in the vitamin D literature for my patients.[4] It fits earlier observational work: in a study of US military personnel, higher blood levels of vitamin D were associated with a lower risk of later developing multiple sclerosis, with 41% lower risk for each 50 nmol/L (20 ng/mL) increase among white participants.[18]

Cancer

As discussed above, vitamin D does not appear to prevent cancer from occurring, but randomized evidence suggests it may reduce cancer deaths and advanced cancer, with daily dosing appearing more effective than intermittent bolus dosing.[3][5][6]

Blood Sugar and Type 2 Diabetes

In the D2d trial of 2,423 adults with prediabetes, 4,000 IU daily lowered diabetes risk by 12%, a result that did not reach statistical significance.[11] When D2d was combined with two similar trials in an individual participant data meta-analysis, vitamin D reduced progression to diabetes by 15% (hazard ratio 0.85), and the absolute risk dropped by 3.3% over three years. Participants who maintained blood levels of at least 50 ng/mL (125 nmol/L) had a 76% lower risk than those at 20 to 30 ng/mL, though that comparison was not randomized and should be read cautiously.[12]

Brain and Mood

• In a large US cohort of 12,388 older adults, people who took vitamin D supplements had a 40% lower rate of dementia diagnosis over follow-up. This was an observational study, so it shows association rather than proof.[16]
• In VITAL's depression study of 18,353 adults, vitamin D did not prevent depression or improve mood scores in the general population.[17] That does not rule out a benefit for people who are deficient, but it is an honest data point.

Heart and Blood Vessels

Large trials in mostly replete adults have not shown that vitamin D prevents heart attacks and strokes overall.[2][27] In the Australian D-Health trial, monthly vitamin D was associated with a 9% lower rate of major cardiovascular events, which did not reach statistical significance, and a 19% lower rate of heart attacks.[8] Genetic (Mendelian randomization) studies suggest the risk may be concentrated in people who are truly deficient, though these findings are debated (see the evidence check below).[23][24]

Bones and Muscles

Vitamin D is essential for absorbing calcium, and severe deficiency causes rickets in children and osteomalacia in adults.[19] But in people who are not deficient, more is not better: in VITAL, vitamin D did not reduce fractures,[13] and in a Swiss-led European trial of adults 70 and older (DO-HEALTH), 2,000 IU daily did not improve fractures, physical performance, blood pressure or cognition.[14]

The Honest Evidence Check: What Did Not Work

Most large vitamin D trials enrolled people who were not deficient, used doses or schedules that may not mirror good clinical practice, and largely found no benefit for their primary outcomes, and that context is essential.

Scientific credibility matters more to me than a satisfying narrative, so here is what cuts against the vitamin D story:

• No mortality benefit from monthly dosing. The D-Health trial gave 21,315 older Australians 60,000 IU once a month for about 5.7 years. All-cause mortality was not reduced (hazard ratio 1.04), and cancer mortality was numerically higher, though not significantly (hazard ratio 1.15).[7]
• No cardiovascular benefit from bolus dosing. The New Zealand ViDA trial (5,108 adults, an initial 200,000 IU then 100,000 IU monthly) found no reduction in cardiovascular disease, even in the deficient subgroup.[27]
• No fracture or functional benefit in replete adults. VITAL and DO-HEALTH found no reduction in fractures, and DO-HEALTH found no benefit for physical performance, blood pressure, infections or cognition.[13][14]
• No depression prevention. VITAL-DEP found no effect on depression risk in the general population.[17]
• Higher is not always better for bone. In a three-year Calgary trial, 10,000 IU daily caused a greater loss of bone density at the wrist (3.5%) than 400 IU daily (1.2%), although bone strength did not differ.[15]
• The genetic evidence is contested. Early nonlinear Mendelian randomization studies in UK Biobank suggested deficiency increases mortality and cardiovascular risk.[22][23] However, a major 2024 reanalysis, published after an earlier version of the paper was retracted and corrected, found no association between genetically predicted vitamin D and coronary disease, stroke or mortality, even at low levels.[24]
• Guideline bodies have become more conservative. The 2024 Endocrine Society guideline recommends empiric vitamin D for children and adolescents, adults 75 and older, pregnant people and adults with high-risk prediabetes, but suggests against supplementing above the recommended intake, and against routine testing, in healthy adults under 75.[20]

How I Read the Totality of Evidence

I respect these findings, and I also notice their limits. Most trials enrolled people with adequate baseline levels, and nutrient trials work best when they treat a deficiency, not when they add more to a full tank. Many used monthly or yearly bolus doses, which appear less effective than daily dosing for immune outcomes. And very few trials were designed around the people I see most: those with autoimmunity, chronic infections, gut malabsorption, or documented deficiency.

My approach is not to give everyone high-dose vitamin D. It is to test, find the people who are low, and correct them wisely, with daily dosing and appropriate cofactors.

Who Is Most at Risk of Low Vitamin D?

Vitamin D deficiency is common: in US national survey data from 2005 to 2006, 41.6% of adults had levels of 20 ng/mL or below, with far higher rates among Black (82.1%) and Hispanic (69.2%) adults.[29]

Risk factors include:

• Limited sun exposure. Northern latitudes, winter months, indoor work, full-coverage clothing and diligent sunscreen use all reduce skin production.
• Darker skin. Melanin is a natural sunscreen, so people with darker skin need more sun exposure to make the same amount of vitamin D.
• Older age. The skin's capacity to make vitamin D declines with age.
• Higher body weight. Vitamin D is fat-soluble and is sequestered in adipose tissue; people with obesity often need higher doses. (Recall that VITAL's advanced cancer benefit was concentrated in people with a normal BMI.[3])
• Gut malabsorption. Celiac disease, Crohn's disease, ulcerative colitis, pancreatic insufficiency, bariatric surgery and fat malabsorption all impair absorption.
• Liver and kidney disease, which impair activation.
• Certain medications, including some anticonvulsants, glucocorticoids, and certain HIV and antifungal medications, which can increase vitamin D breakdown.

In my own patient population, people with complex chronic illness, mold and mycotoxin exposure, chronic infections and mast cell activation often run low. That observation comes from clinical experience rather than trials, and I offer it as a reason to test, not as a proven cause. You can read more about these connections in my articles on mold and your brain and the gut-brain axis, and in my recent piece on trained immunity and beta glucans.

My Functional Medicine Approach: Test, Correct, Recheck

My approach is to measure 25(OH)D, correct deficiency with daily vitamin D3 and the right cofactors, and recheck to make sure the level lands in a healthy range without overshooting.

Step 1: Test

The 25-hydroxyvitamin D [25(OH)D] blood test is the standard measure of vitamin D status. Different expert bodies interpret it differently:

In my practice, for patients with autoimmunity, recurrent infections, or complex chronic illness, I generally aim for roughly 40 to 60 ng/mL. That is a clinical judgment, informed by the diabetes data suggesting extra benefit around 50 ng/mL[12] and by mortality data showing the highest risk at very low levels,[30] but it is not a target proven in randomized trials. When I test, I also often check calcium, magnesium, parathyroid hormone (PTH) and kidney function.

Step 2: Correct With Daily D3

• Choose vitamin D3 (cholecalciferol) and take it daily with a meal that contains fat. Daily dosing has the strongest evidence for immune protection.[10]
• Dose to your level. For maintenance, many adults do well on 1,000 to 2,000 IU daily. The 2011 Endocrine Society guideline noted that 1,500 to 2,000 IU daily may be needed to keep levels above 30 ng/mL, that the maintenance upper limit is 4,000 IU daily, and that up to 10,000 IU daily may be needed to correct deficiency.[21] Higher doses should be used for a limited time and under medical supervision.
• Recheck in about three months and adjust.

Step 3: Don't Forget the Cofactors

• Magnesium. Magnesium is required by the enzymes that activate and break down vitamin D. In a randomized trial, magnesium supplementation raised 25(OH)D in people starting near 30 ng/mL and lowered it in those starting higher, suggesting magnesium helps regulate vitamin D toward a balanced level.[25]
• Vitamin K2. Vitamin D increases calcium absorption, and vitamin K2 activates proteins that help direct calcium into bone and away from soft tissues. The clinical trial evidence for combining D and K2 is still limited, but I find the mechanistic rationale strong, particularly for patients on higher doses of vitamin D. People taking warfarin should not take vitamin K without their prescriber's guidance.

Step 4: Respect the Safety Limits

Vitamin D toxicity is uncommon but real, typically from very high doses taken for long periods, and it shows up as high calcium (nausea, confusion, kidney stones, kidney injury). Use extra caution, and only with your physician, if you have sarcoidosis or other granulomatous diseases, primary hyperparathyroidism, a history of kidney stones, or kidney disease. And remember the Calgary trial: more is not always better.[15]

Step 5: Get Smart Sun

Sensible midday sun on bare skin, for a short time and without burning, is the way our bodies were designed to make vitamin D. How much you need depends on your skin tone, latitude and season. Protect your skin from burning and keep up with skin checks.

Products I Use and Trust

• Dr. Jill Health® Vitamin D3/K2: 5,000 IU vitamin D3 with 45 mcg vitamin K2 as MK-7 (MenaQ7® PRO) per capsule, for patients who need correction under supervision.
• Dr. Jill Health® Daily D3: vitamin D3 for daily maintenance (available in 1,000 and 5,000 IU strengths).
• Thorne Vitamin D/K2 Liquid: a liquid option for flexible, drop-by-drop dosing.
• Dr. Jill Health® Activated Mag & Potassium, Magnesium Glycinate or Dr. Jill Health® Brain Mag Powder: magnesium support for vitamin D metabolism.
• Dr. Jill Health® Osteo Essentials: broader bone support for patients focused on skeletal health.

Please work with your own clinician on dosing, especially if you have any of the conditions listed above or take prescription medications.

Light, Healing and Hope

Vitamin D reminds us that we were made to live in relationship with light, and that healing often begins with simple things done faithfully.

A word of faith. There is something beautiful about the fact that our bodies were designed to make a life-giving hormone from sunlight. Malachi 4:2 speaks of the sun of righteousness rising “with healing in its wings.” I do not read that verse as a supplement recommendation, of course. But I do see in creation a pattern of light and restoration. When you step outside into the morning sun, take a moment to breathe, give thanks, and pray for the healing you need in body, mind and spirit.

Whether you are navigating autoimmunity, recurrent infections, low mood through the dark winter months, or simply want to build a resilient foundation for the years ahead, knowing and optimizing your vitamin D level is one of the most practical steps you can take. Test it. Treat it wisely. Recheck it. And walk forward with hope.

For more on how I integrate science and faith on the healing journey, I invite you to read my book, Unexpected: Finding Resilience Through Functional Medicine, Science and Faith.

Keep Learning

• Blog: Trained Immunity: How Beta Glucans Teach Your Innate Immune System to Remember
• Podcast: 331: Immune Resilience: The Future of Longevity with Dr. Isaac Melamed
• Shop: Dr. Jill's Immune Support collection

References

1. Manson JE, Bassuk SS, Buring JE; VITAL Research Group. Principal results of the VITamin D and OmegA-3 TriaL (VITAL) and updated meta-analyses of relevant vitamin D trials. J Steroid Biochem Mol Biol. 2020;198:105522. doi:10.1016/j.jsbmb.2019.105522
2. Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019;380(1):33-44. doi:10.1056/NEJMoa1809944
3. Chandler PD, Chen WY, Ajala ON, et al. Effect of vitamin D3 supplements on development of advanced cancer: a secondary analysis of the VITAL randomized clinical trial. JAMA Netw Open. 2020;3(11):e2025850. doi:10.1001/jamanetworkopen.2020.25850
4. Hahn J, Cook NR, Alexander EK, et al. Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial. BMJ. 2022;376:e066452. doi:10.1136/bmj-2021-066452
5. Keum N, Lee DH, Greenwood DC, Manson JE, Giovannucci E. Vitamin D supplementation and total cancer incidence and mortality: a meta-analysis of randomized controlled trials. Ann Oncol. 2019;30(5):733-743. doi:10.1093/annonc/mdz059
6. Zhang Y, Fang F, Tang J, et al. Association between vitamin D supplementation and mortality: systematic review and meta-analysis. BMJ. 2019;366:l4673. doi:10.1136/bmj.l4673
7. Neale RE, Baxter C, Romero BD, et al. The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality. Lancet Diabetes Endocrinol. 2022;10(2):120-128. doi:10.1016/S2213-8587(21)00345-4
8. Thompson B, Waterhouse M, English DR, et al. Vitamin D supplementation and major cardiovascular events: D-Health randomised controlled trial. BMJ. 2023;381:e075230. doi:10.1136/bmj-2023-075230
9. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. doi:10.1136/bmj.i6583
10. Jolliffe DA, Camargo CA Jr, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021;9(5):276-292. doi:10.1016/S2213-8587(21)00051-6
11. Pittas AG, Dawson-Hughes B, Sheehan P, et al. Vitamin D supplementation and prevention of type 2 diabetes. N Engl J Med. 2019;381(6):520-530. doi:10.1056/NEJMoa1900906
12. Pittas AG, Kawahara T, Jorde R, et al. Vitamin D and risk for type 2 diabetes in people with prediabetes: a systematic review and meta-analysis of individual participant data from 3 randomized clinical trials. Ann Intern Med. 2023;176(3):355-363. doi:10.7326/M22-3018
13. LeBoff MS, Chou SH, Ratliff KA, et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med. 2022;387(4):299-309. PMID 35939577
14. Bischoff-Ferrari HA, Vellas B, Rizzoli R, et al. Effect of vitamin D supplementation, omega-3 fatty acids, and a simple home strength exercise program on clinical outcomes in older adults: the DO-HEALTH randomized clinical trial. JAMA. 2020;324(18):1855-1868. doi:10.1001/jama.2020.16909
15. Burt LA, Billington EO, Rose MS, et al. Effect of high-dose vitamin D supplementation on volumetric bone density and bone strength: a randomized clinical trial. JAMA. 2019;322(8):736-745. doi:10.1001/jama.2019.11889
16. Ghahremani M, Smith EE, Chen HY, et al. Vitamin D supplementation and incident dementia: effects of sex, APOE, and baseline cognitive status. Alzheimers Dement (Amst). 2023;15(1):e12404. doi:10.1002/dad2.12404
17. Okereke OI, Reynolds CF 3rd, Mischoulon D, et al. Effect of long-term vitamin D3 supplementation vs placebo on risk of depression or clinically relevant depressive symptoms and on change in mood scores: a randomized clinical trial. JAMA. 2020;324(5):471-480. doi:10.1001/jama.2020.10224
18. Munger KL, Levin LI, Hollis BW, Howard NS, Ascherio A. Serum 25-hydroxyvitamin D levels and risk of multiple sclerosis. JAMA. 2006;296(23):2832-2838. PMID 17179460
19. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281. doi:10.1056/NEJMra070553
20. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947. PMID 38828931
21. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930. doi:10.1210/jc.2011-0385
22. Sutherland JP, Zhou A, Hyppönen E. Vitamin D deficiency increases mortality risk in the UK Biobank: a nonlinear Mendelian randomization study. Ann Intern Med. 2022. doi:10.7326/M21-3324
23. Zhou A, Selvanayagam JB, Hyppönen E. Non-linear Mendelian randomization analyses support a role for vitamin D deficiency in cardiovascular disease risk. Eur Heart J. 2022;43(18):1731-1739. doi:10.1093/eurheartj/ehab809
24. Sofianopoulou E, Kaptoge S, et al. Estimating dose-response relationships for vitamin D with coronary heart disease, stroke, and all-cause mortality: observational and Mendelian randomisation analyses (retraction and republication). Lancet Diabetes Endocrinol. 2024;12(1):e2-e11. doi:10.1016/S2213-8587(23)00287-5
25. Dai Q, Zhu X, Manson JE, et al. Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial. Am J Clin Nutr. 2018;108(6):1249-1258. doi:10.1093/ajcn/nqy274
26. Aranow C. Vitamin D and the immune system. J Investig Med. 2011;59(6):881-886. PMID 21527855
27. Scragg R, Stewart AW, Waayer D, et al. Effect of monthly high-dose vitamin D supplementation on cardiovascular disease in the Vitamin D Assessment Study: a randomized clinical trial. JAMA Cardiol. 2017. PMC5815022
28. Kaufman HW, Niles JK, Kroll MH, Bi C, Holick MF. SARS-CoV-2 positivity rates associated with circulating 25-hydroxyvitamin D levels. PLoS One. 2020;15(9):e0239252. doi:10.1371/journal.pone.0239252
29. Forrest KYZ, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res. 2011;31(1):48-54. PMID 21310306
30. Gaksch M, Jorde R, Grimnes G, et al. Vitamin D and mortality: individual participant data meta-analysis of standardized 25-hydroxyvitamin D in 26916 individuals from a European consortium. PLoS One. 2017. doi:10.1371/journal.pone.0170791
31. National Institutes of Health, Office of Dietary Supplements. Vitamin D: fact sheet for health professionals. ods.od.nih.gov

Disclaimer: This article is for educational purposes only and is not a substitute for individualized medical advice. Do not start, stop or change medications or supplements without consulting your healthcare provider. Statements about supplements have not been evaluated by the Food and Drug Administration. Dr. Jill Carnahan is the founder of Dr. Jill Health and may benefit from sales of products mentioned.

About Dr. Jill Carnahan, MD

Dr. Jill Carnahan, MD, ABIHM, ABoIM, IFMCP, is a triple board-certified functional medicine physician and Medical Director of Flatiron Functional Medicine in Louisville, Colorado. She is the author of the bestselling book Unexpected, host of Resiliency Radio, and is featured in the Doctor/Patient documentary. Learn more at jillcarnahan.com, shop her physician-formulated supplements at drjillhealth.com, and follow her on Instagram at @DrJillCarnahan.