The Wellness Brew - Issue 05

Most people treat Vitamin D like a checkbox — take a supplement, move on. What the research actually shows is more complicated: up to one billion people worldwide are estimated to be deficient, most supplements are taken in a way that limits how much the body can actually absorb, and without adequate magnesium, Vitamin D may not work at all for a large portion of the people taking it. This week, we dig into what Vitamin D is, why sunlight is central to it, what deficiency looks like over time, and the practical steps worth taking. But first, we filtered the noise — here's what's worth knowing this week.

The Supplement You Buy on Amazon May Contain Almost None of What the Label Claims

Independent lab testing by SuppCo — a consumer health testing platform — purchased 44 of Amazon's best-selling supplements and sent them through ISO-accredited labs. The results were striking: 22 of the 44 supplements — 50% — failed to meet their label claims. Of those 22 failures, 20 contained between 0% and 3% of the main ingredient claimed on the label. That is not a rounding error — it is effectively counterfeit. The problem is rooted in Amazon's commingling practices, where authentic products from verified sellers can be mixed in warehouses with unverified inventory from third-party sellers, making it nearly impossible to guarantee what ends up in a buyer's hands. Amazon announced in early 2026 it would end commingling and introduce new third-party testing requirements — a tacit acknowledgment of how serious the problem has become. Until those changes are fully enforced, the safest options are to purchase supplements directly from a brand's own website, or from verified retailers. SuppCo, November 2025

Only 1 in 4 Sunscreens Meets Basic Safety Standards

The Environmental Working Group's 2025 Guide to Sunscreens analyzed more than 2,200 SPF products and found that only 498 — roughly one in four — met their criteria for safety and efficacy. The concern is not that sunscreen causes harm — protecting skin from UV radiation is important — it is what many sunscreens are made of. An FDA study found that six common chemical UV filters, including oxybenzone, homosalate, avobenzone, and octinoxate, are absorbed through the skin at levels that exceed the FDA's own safety threshold after a single application. Some were detectable in the blood weeks after last use and have also been found in breast milk and urine. Several of these ingredients are classified as potential endocrine disruptors. Only two active sunscreen ingredients — zinc oxide and titanium dioxide — are currently recognized as safe and effective by the FDA. Both are mineral-based. The simple action: look for mineral sunscreens with zinc oxide as the active ingredient and avoid products listing oxybenzone, homosalate, or octinoxate. EWG 2025 Sunscreen Guide

Your Vitamin D Supplement May Not Be Working — And Magnesium Is Likely Why

A randomized clinical trial from Vanderbilt-Ingram Cancer Center, published in The American Journal of Clinical Nutrition in December 2025, found that magnesium acts as a biological regulator of Vitamin D — raising levels in people who are deficient and reducing them in those who already have too much. The finding may explain why Vitamin D supplementation produces inconsistent results across people and why many large Vitamin D studies have produced mixed outcomes. Approximately 40% of U.S. adults are deficient in Vitamin D — but up to 80% do not consume enough magnesium to meet the recommended daily allowance, meaning many people are supplementing with Vitamin D against a backdrop of magnesium deficiency that undermines its effectiveness. Not all magnesium supplements work the same way. Magnesium glycinate is highly bioavailable and well-tolerated, making it the best general option for correcting deficiency — it also supports sleep and anxiety reduction. Magnesium L-threonate is a newer form that crosses the blood-brain barrier more effectively than other forms and has early evidence supporting cognitive performance and memory. For most people, magnesium glycinate taken daily is the right starting point. Vanderbilt University Medical Center / American Journal of Clinical Nutrition, December 2025

Sunlight and Vitamin D: The Nutrient a Billion People Are Missing

Vitamin D is one of the most discussed nutrients in medicine and one of the most misunderstood. Most people know they are supposed to have enough of it. Far fewer understand what it actually does, why deficiency is so widespread, or why getting it from a supplement alone may be less straightforward than it sounds. Up to one billion people worldwide are estimated to be Vitamin D deficient — and a December 2025 randomized clinical trial from Vanderbilt University found that without adequate magnesium, Vitamin D supplementation may not work at all for a large portion of that group. This week we dig into the science: what Vitamin D is, what sunlight has to do with it, what deficiency looks like over time, and what you can actually do about it.

What Vitamin D Is and Why Sunlight Is Central to It

Vitamin D is technically a hormone, not a vitamin in the traditional sense. It is fat-soluble, stored in body fat and liver tissue, and its active form functions as a signaling molecule that binds to receptors found in nearly every tissue in the body — including the immune system, the cardiovascular system, the brain, the gut, muscle tissue, and bone.

Unlike almost every other essential nutrient, the body is designed to produce Vitamin D on its own — but only under a very specific condition: exposure to UVB radiation from sunlight. When UVB rays — wavelengths between 280 and 315 nanometers — penetrate the outer layers of the skin, they trigger a photochemical reaction that converts a cholesterol-based compound called 7-dehydrocholesterol into previtamin D3. Body heat then converts previtamin D3 into Vitamin D3, which enters the bloodstream, travels to the liver, and is converted into 25-hydroxyvitamin D — the form measured in blood tests and the primary marker of Vitamin D status. From there, the kidneys convert it into the active hormonal form that the body uses throughout its tissues. [1]

This is a system that worked well for most of human evolutionary history, when people spent significant time outdoors in daylight. It is a system that is failing in the modern world, where indoor work, sunscreen use, clothing coverage, and life at northern latitudes all suppress or eliminate UVB exposure for large portions of the year.

The body cannot produce Vitamin D through a window — glass blocks UVB radiation entirely. It cannot produce meaningful amounts during winter months at latitudes above approximately 35 degrees north, which includes most of the continental United States, all of Canada, and essentially all of Europe. In Boston, effective Vitamin D synthesis from sunlight is limited to roughly April through October. In Edmonton, it is barely five months of the year. For people living and working indoors in these regions, sunlight is effectively not a reliable source of Vitamin D at all — and diet alone cannot compensate, because very few foods contain it in meaningful quantities. [2]

What Vitamin D Is Actually Doing

The receptors for Vitamin D are found in more than 35 different tissues throughout the body — a distribution that reflects how broadly it is involved in maintaining normal function. Several of its roles are well-established and clinically recognized.

Bone health is the most documented. Vitamin D is essential for calcium absorption in the gut — without adequate levels, the body cannot absorb calcium effectively regardless of how much is consumed in the diet. Deficiency causes the bones to progressively weaken — in adults this manifests as osteomalacia, and over time as osteoporosis and elevated fracture risk. [3]

Immune regulation is increasingly recognized as one of Vitamin D's most important roles. Vitamin D receptors are present on virtually all immune cells, and the active form of Vitamin D modulates both innate and adaptive immune responses — helping calibrate the immune system's reaction to pathogens while preventing it from overreacting. A 2025 study published in The American Journal of Clinical Nutrition, analyzing data from 36,258 participants in the UK Biobank, found that severe Vitamin D deficiency was associated with a 33% higher rate of hospitalization for respiratory tract infections compared to those with sufficient levels. For each 10 nmol/L increase in Vitamin D, the hospitalization rate decreased by 4%. [4]

Muscle function is also affected. Vitamin D receptors are expressed in muscle tissue, and deficiency is associated with muscle weakness, reduced grip strength, and increased fall risk in older adults — all of which have significant downstream consequences for mobility and independence.

The research on Vitamin D's relationship to cardiovascular disease, cancer, and metabolic conditions is more complex. Observational studies consistently show associations between low Vitamin D levels and increased risk across a wide range of chronic diseases. However, several large randomized controlled trials — including the VITAL trial, which enrolled more than 25,000 participants — found that Vitamin D supplementation did not reduce the risk of cardiovascular events or cancer in adults who were not severely deficient at baseline. Association does not equal causation, and low Vitamin D may in many cases be a marker of poor health rather than a driver of it. [5]

Why Deficiency Is So Widespread

Approximately 40% of adults in the United States have Vitamin D levels considered deficient or insufficient. Globally, estimates suggest up to one billion people are affected. Several factors explain why this is so prevalent despite decades of awareness about the problem.

Sunlight is the most efficient source by far, but modern life systematically limits exposure. Most working adults spend the bulk of their daylight hours indoors. And for anyone living above the 35th parallel — which covers roughly the upper two-thirds of the United States — the sun's angle during winter months means that UVB rays travel through too much atmosphere to reach the earth's surface at effective levels. [2]

Skin pigmentation is also a significant factor. Melanin, the pigment that gives skin its color, acts as a natural UV filter — which is biologically protective against skin cancer but also reduces Vitamin D synthesis. Individuals with darker skin require substantially more sun exposure to produce equivalent amounts of Vitamin D, which is one reason Vitamin D deficiency disproportionately affects people of color, particularly in northern climates.

Age reduces production efficiency. As the skin ages, it contains less 7-dehydrocholesterol, the precursor compound needed for Vitamin D synthesis, and the conversion process becomes less efficient. Older adults living in northern latitudes who spend limited time outdoors face compounded deficiency risk.

Body weight is a factor. Vitamin D is fat-soluble, meaning it is sequestered in fat tissue and becomes less bioavailable in individuals with higher body fat. People with obesity often test deficient even with adequate sun exposure or supplementation, because the Vitamin D is absorbed into fat stores rather than circulating in the bloodstream. [3]

And as covered in The Filter this week — magnesium status may be the most underappreciated factor of all. The December 2025 Vanderbilt trial found that magnesium acts as a biological regulator of Vitamin D, raising levels in those who are deficient while reducing them in those with excess. Up to 80% of adults do not consume enough magnesium to meet the recommended daily allowance, which means a large portion of people supplementing with Vitamin D may be doing so against a backdrop of magnesium deficiency that limits how effectively the body can activate and use it.

What the Evidence Says You Can Do

The science points clearly in a few directions. Sunlight exposure — not to the point of burning, but enough for meaningful UVB penetration of unprotected skin — remains the most efficient way for the body to produce Vitamin D. Supplementation is the right tool for people who cannot reliably get sun exposure due to geography, season, skin type, or lifestyle. And knowing your actual levels before supplementing is more useful than guessing. The practical steps are in the Actionable Takeaways below.

Four things you can do this week:

1. Have your Vitamin D levels tested. Most people who suspect they are deficient simply start taking a supplement without knowing where they actually stand. If your Vitamin D levels are already high, excessive supplementation over time can cause toxicity. More practically, knowing your baseline gives you something concrete to optimize against. Ask your doctor to add a 25-hydroxyvitamin D test to your next blood panel — it is a standard, inexpensive test. The optimal functional range is generally considered to be 40–60 ng/mL, though the conventional clinical threshold for deficiency is below 20 ng/mL.

2. If you live above the 35th parallel, supplement through fall and winter. Above a line running roughly through Los Angeles, Atlanta, and Charlotte, the sun's angle from October through March is too low to produce meaningful UVB radiation at the skin — regardless of how much time you spend outdoors. For anyone in the northern United States, Canada, or Europe, sunlight is not a reliable Vitamin D source for five to six months of the year. A daily supplement of 1,000–2,000 IU of Vitamin D3 — the form most readily used by the body — is a reasonable baseline for most adults during these months, though your ideal dose depends on your baseline levels from your bloodwork. Vitamin D3 is preferred over D2, which is less bioavailable and less effective at raising blood levels.

3. Take your Vitamin D with a meal that contains fat. Vitamin D is fat-soluble, meaning it requires dietary fat to be absorbed properly. Taking it on an empty stomach or with a low-fat meal significantly reduces how much the body actually absorbs. Take it with your largest meal of the day — typically one that contains eggs, olive oil, avocado, nuts, fish, or any other fat-containing food. This is a small habit change that meaningfully improves the return on every supplement you take.

4. Consider supplementing magnesium. As with Vitamin D, getting your magnesium levels tested before supplementing is recommended. Magnesium regulates Vitamin D levels in the body — and up to 80% of adults do not consume enough magnesium daily to meet the recommended allowance. If you are supplementing with Vitamin D but your levels are not improving, magnesium deficiency may be the reason. Magnesium glycinate is the most practical form for most people — it is highly bioavailable, gentle on the digestive system, and well-tolerated daily. Food sources of magnesium include dark leafy greens, pumpkin seeds, almonds, black beans, dark chocolate, and whole grains — building these into your diet is the best long-term approach.

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Wellness, filtered.

The Wellness Brew

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Disclaimer: The Wellness Brew is for informational purposes only and does not constitute medical advice. The content published here is not intended to diagnose, treat, cure, or prevent any disease or health condition. Always consult a qualified healthcare professional before making any changes to your diet, supplement routine, or lifestyle.