The sunshine hormone and its sidekick

Don’t let the name fool you. Vitamin D works like a hormone, and I treat it like one. 1 I test it in every client, I recheck it, and I aim for optimal, not normal. Its sidekick, magnesium, gets the same attention.

This piece is for anyone who was told their labs were fine and was left to figure out the rest alone. In range is not the same as thriving.

Think about testosterone. The standard range for young men is wide: its top is more than three times its bottom. 2 A 30-year-old man can land near the floor of that range, hear that he is normal, and still feel flat and not like himself. Normal is a wide net. I wrote about that gap in the metabolic health article. Vitamin D works the same way in my book.

The sunshine hormone

Your skin makes vitamin D from sunlight, or you eat it, and your body activates it in two steps. The liver makes 25-hydroxyvitamin D, or 25(OH)D, the form a blood test measures. The kidneys then make calcitriol, the active hormone. 1 Its best-known job is calcium and bone, and researchers are still mapping its roles in immunity, metabolism, and more. 1 3

Running low is common, and how common depends on where you draw the line. In national data from 2011 to 2014, about 1 in 4 Americans was under 20 ng/mL (nanograms per milliliter). 4 In pooled data from 2001 to 2018, about 2 in 3 were under 30 ng/mL, the line the Endocrine Society used until 2024. 5 Extra weight (fat holds on to vitamin D), little sun, darker skin, trouble absorbing food, and a northern winter all raise the odds. 1 6 7

Its sidekick

Magnesium is a cofactor, a mineral that more than 300 enzyme systems need to do their work. Most of it lives in your bones and cells. Less than 1% is in your blood. 8 Magnesium also helps run the enzymes that make and break down vitamin D. 9 In the one trial that tested the pairing, magnesium nudged vitamin D up in people starting near 30 ng/mL and down in people starting higher, as if it helps keep vitamin D in balance. 10 That is one more reason I look at the two together.

It is also easy to come up short. About half of Americans take in less magnesium from food and drink than the average requirement. 11 That is intake, not a diagnosis, but it is a gap worth knowing about.

Test, don’t guess

Vitamin D and magnesium testing are necessary in my book. I check both in every client, I recheck after three to six months, and then I keep checking periodically. That part does not change much from person to person.

Here is why. The same intake lands differently in different bodies. People carrying more weight reach lower levels on the same amount, and the rise is not a straight line: it depends on where you start. 6 1 In one analysis, people with overweight or obesity were still climbing 6 to 12 months in. 12 The only way to know where you landed is to look.

I also keep you on the same lab. Results vary from lab to lab: in one study, blood from the same people, sent to six labs, averaged anywhere from about 17 to 36 ng/mL. 13 Same lab, same method, a trend you can read.

For magnesium, I check RBC (red blood cell) magnesium. The standard serum test sees less than 1% of your body’s magnesium, and its normal range came from a 1970s survey, not from health outcomes, so a normal result does not prove your stores are full. 8 14 No single magnesium test tells the whole story, the red-cell test included, which is why I read the red-cell number next to your diet, symptoms, medicines, and kidney function. 8

Conventional guidance leans the other way. In 2024, the Endocrine Society stopped endorsing its old “sufficient” level and suggested against routine testing in healthy adults, because trials have not pinned down which level is best. It also suggested that healthy adults under 75 not take more than the recommended daily amount. 15 16 That advice is written for healthy adults with no medical reason to test, and I understand the caution. But not knowing the perfect number is a reason to look closer, not to look away. I would rather know your number than wait for a problem.

Optimal, not normal

Once I have your number, I do not stop at “in range.” I like to see my clients in the upper part of the range, for vitamin D and for magnesium. That is my clinical judgment, built from years of practice and from watching what helps people who were told they were fine. It is not a target any guideline sets, 15 and I do not put it above everything else in your plan. We treat YOU, not the number.

Aiming higher is exactly why I monitor. Too much vitamin D shows up as high blood calcium, 1 so I recheck calcium along with vitamin D. A higher target belongs under a clinician’s monitoring, not in a do-it-yourself plan. How you get there matters as well. For adults 50 and older, the Endocrine Society prefers steady daily amounts over large, spaced-out doses, 16 and the big spaced doses have gone badly in trials: a yearly megadose raised falls and fractures in older women, and larger monthly doses raised falls compared with a smaller monthly dose. 17 18 And more is not automatically better: in a three-year trial, the two higher daily amounts lowered forearm bone density compared with a low one. 19 Higher is a judgment call with trade-offs, which is why I keep it a conversation, not a rule.

What the research shows

Vitamin D has real wins and promising signals, and they deserve to be said warmly.

Prediabetes. Across three trials, vitamin D lowered the chance of developing type 2 diabetes by about 15%, or about 3 fewer cases per 100 people over three years. 20 The Endocrine Society now suggests vitamin D for adults with high-risk prediabetes, on top of lifestyle changes. 16
Cancer deaths, with daily use. Pooled trials found about 13% fewer cancer deaths with daily vitamin D, though not fewer cancers. The largest monthly-dose trial leaned the other way, which may be one more point for steady daily intake. 21 22 23
Autoimmune disease. In VITAL, a large US trial, new autoimmune diagnoses (conditions where the immune system attacks the body, such as rheumatoid arthritis) were 22% lower over about five years while people took vitamin D. A later recount trimmed that to 15%, short of statistical certainty, and it faded after people stopped, so this one is still being worked out. 24 25

And magnesium:

Blood pressure. About 2 points lower across 34 trials. Small and real, and not a replacement for blood pressure treatment. 26
Insulin resistance. Better scores in trials lasting four months or more, a big piece of the metabolic health article. 27
Migraine. Canada’s headache guideline gives magnesium a strong recommendation for prevention, and in the best-known trial, attacks fell 41.6% against 15.8% on placebo. 28 29 Those trials used amounts above the usual supplement limit, so this one belongs with a clinician. 8
Sleep. Small, early evidence, and some people notice a difference. 30 31

What about the big prevention trials? In large trials of people who were mostly not low to begin with, adding vitamin D did not lower rates of cancer, heart events, death, fractures, falls, or depression. 32 23 33 34 35 36 That is worth knowing. Most of those trials gave everyone the same fixed amount, whatever their starting level. My approach is different: I test first, adjust to the person, and recheck. Trials have not yet shown that aiming for a level beats a fixed amount, so that part is my clinical judgment. 15

For men: fix it for its own sake

Men tend to come to vitamin D through testosterone. Men with more vitamin D tend to have more testosterone, and the two rise and fall with the seasons. 37 In randomized trials, though, vitamin D did not raise testosterone, including a 2026 analysis that pooled 11 trial comparisons. 38 39 40 41 The link most likely runs through shared roots such as body weight and sunshine: extra weight lowers vitamin D, and in men with obesity, losing weight raises testosterone. 42 43

So I fix a low vitamin D because it matters on its own, not as a stand-in for treating testosterone. The same goes for zinc and magnesium stacks: in small trials, they did not raise testosterone, and in tiny studies, zinc helped only men who were short on zinc. 44 45 46 When energy, drive, or recovery have changed, a real testosterone workup belongs in the same plan, with these basics checked right alongside it.

For women: cycles, nights, and bones

For women, these two show up everywhere: in the premenstrual stack, on the nightstand in perimenopause, and in the bone conversation.

Cycles. Calcium is the quiet standout for premenstrual symptoms. In a trial of 466 women, symptoms fell 48% by the third cycle, against 30% on placebo, and the American College of Obstetricians and Gynecologists gives it a conditional recommendation. 47 48 Vitamin D looks promising for PMS and period pain in small pooled trials, though the evidence is still early. 49 50 Magnesium is the one many women swear by, and its trials are small so far. 51 Premenstrual dysphoric disorder (PMDD), the severe form, deserves real treatment; magnesium is not established for it. 52

Perimenopause. Magnesium may help sleep a little, as I covered in the sleep article. 30 31 It does not treat hot flashes: in a trial in breast cancer survivors, it did no better than placebo, and The Menopause Society calls hormone therapy the most effective treatment for them. 53 54 So magnesium sits beside a hormone conversation. It does not replace one. I wrote about that conversation in my first article.

Bone. Calcium and vitamin D are the floor, not the whole plan. The foundation is enough protein, calcium, and vitamin D plus regular activity, and for women at higher risk, estrogen or other medicines can prevent bone loss. 55 56

What is known about the trends

What	The claim	What is known
D3 with K2	K2 steers calcium to bone	A common pairing. In a trial of 365 men with calcified heart valves, K2 taken with vitamin D did not slow calcification compared with placebo, and vitamin K’s bone benefit is still unproven. 57 58
“Magnesium activates vitamin D”	Vitamin D needs magnesium	True at the level of chemistry, and in the one trial, magnesium seemed to help keep vitamin D in balance. 9 10
Glycinate	The calming form	Gentle on the stomach for some people; one small trial found a small sleep benefit. 59 31
Threonate	The brain form	Interesting early science: a rat study and two small human trials, funded by the companies behind it. 60 61 62
Oxide	Just filler	The least absorbed form (about 4% in one study), which is why it works as a laxative. 63 8
Sprays and bath flakes	Absorbed through the skin	Relaxing, but absorption through the skin has not been shown. 64 65
Sunscreen	Blocks your vitamin D	As people actually use it, sunscreen does not seem to cause low vitamin D, and in the only randomized trial found, people assigned to daily sunscreen later had fewer invasive melanomas. Protect your skin. Not all sunscreens are created equal, and I am picky about what goes on skin, so if you do not trust the one in your hand, cover up instead: shade, a hat, and long sleeves count. 66 67
Mood	Lifts the winter blues	Worth checking when mood is low. In people already diagnosed with depression, small trials suggest it may help as an add-on to care; in adults 50 and older without depression, it did not prevent it. 68 36
Immunity	Fewer colds	A small edge in pooled trials, larger with daily use. 69

What the viral numbers measured. Some of these findings are genuinely interesting. The headlines just stretch them.

“3 years younger.” Media coverage of a VITAL substudy that found about 140 fewer base pairs of telomere shortening over four years (telomeres cap the ends of chromosomes). A promising signal in a blood-cell marker, not a measure of aging itself. 70
“7.5 years younger.” The paper’s own conversion of a 2.24-point gap on a cognition score after six weeks of magnesium threonate, in a trial its supplier funded. 61
“Heart attacks halved.” TARGET-D aimed for higher vitamin D levels after a heart attack. Its main goal, fewer major heart events overall, was not met. Fewer repeat heart attacks (3.8% versus 7.9%) was a secondary finding, the kind that needs its own trial to confirm. Worth watching. 71
“25% more testosterone.” One 2011 study of 54 overweight men losing weight. The rise was within the vitamin D group, against the men’s own starting point, and their final levels barely differed from placebo. 72 41

The tell is always the same: ask what was measured, in whom, and compared with what.

Holistic Hacks & Functional Facts

Food first. For magnesium, think pumpkin and chia seeds, almonds, leafy greens, beans, and whole grains; refining grains strips much of it out. 8 Few foods carry much vitamin D: fatty fish, cod liver oil, and UV-exposed mushrooms do, egg yolks have a little, and fortified foods supply most of it in American diets. 1

Sun, with sense. Skin makes vitamin D only when the sun’s UVB rays are strong enough. In a classic lab study, Boston sun made none from November through February. UVB does not pass through window glass, and darker or older skin makes less. 1 7 In trials, supplements raised blood levels more than advice to get sun. 73 74 Protect your skin in a way you trust, and let the test tell you where you stand.

Gut health matters always. What you swallow is half the story; what you absorb is the other half. Gut conditions that disrupt absorption can lower vitamin D and drain magnesium, 1 8 and in one small study, vitamin D absorbed about a third better with a meal containing fat. 75 Gut health deserves its own article, and it is coming.

Quality counts. I do recommend supplements as part of care. Not all supplements are created equal, so I look for third-party testing and real potency, not a famous name on the label. And when your levels are low, something is better than nothing.

Check with a clinician first if you have kidney disease, which raises the risk of magnesium building up to toxic levels, 8 76 or high blood calcium. 1 Medicines matter too: thiazide water pills plus vitamin D can raise calcium, steroids and orlistat (a weight-loss medicine) lower vitamin D, magnesium binds some antibiotics and bone medicines, some water pills and long-term acid reducers lower magnesium, and calcium can keep thyroid medicine from being absorbed. 1 8 77 Loose stools are magnesium’s common side effect. 8 If a supplement makes you feel worse, stop and tell us, or whoever suggested it.

Bring your numbers (and your bottles) to the conversation

A few things make any vitamin D or magnesium conversation more useful:

Your recent labs, even the “normal” ones, with dates and the lab’s name.
Every supplement you take, or a photo of each label.
Your medicines and health history, especially kidney problems, gut issues, water pills, acid reducers, thyroid medicine, steroids, and bone medicines.
Changes you have noticed: sleep, mood, energy, your cycle, sex drive, or strength.

Test, don’t guess. Optimal, not normal.

Every fact above is free, and I hope you use it. But a list cannot read your number against your story, notice that your vitamin D sits beside a shifting testosterone, cycle, thyroid, or sleep, or tell you whether your supplements are doing their job. That is where my expertise comes in. I test vitamin D and magnesium in every client, read them beside your hormones and the rest of your labs, and recheck as we go. Then we build a plan that fits you and monitor it closely. You do not have to figure it out alone.

Feel like yourself again. That is the goal, and knowing your numbers is a good place to start.

Book your comprehensive consultation, or explore care for men and women at Rez.

About Kellie

Kellie Marsoobian, FNP-C, is the founder and lead provider of Rez. She is Advanced BHRT certified through WorldLink Medical, having completed the Advanced Bioidentical Hormone Replacement Therapy course series taught by Neal Rouzier, MD.

This article is general education, not personal medical advice. A qualified clinician can help assess individual symptoms and discuss suitable care.

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