Why Sunlight Might Not Be Enough: New Research on Vitamin D Absorption

You've heard you can get all your vitamin D from the sun. Maybe you've even spent more time outdoors this summer, assuming that would take care of it. Here's what's actually going on.

New research from the United Kingdom suggests that for certain groups—particularly older adults and people with darker skin—routine sun exposure during summer months may not be enough to maintain healthy vitamin D levels. A study tracking nearly 300 people across northern Britain found that vitamin D status often remained low year-round in populations most at risk, even when sunshine was plentiful.

The finding challenges a widely held assumption: that a few months of summer sun can reliably replenish vitamin D stores for everyone. In practice, the relationship between sunlight and vitamin D is more variable than many of us realize.

What vitamin D does—and why absorption matters

Vitamin D plays a foundational role in calcium absorption, which supports normal bone health. It also contributes to immune function and muscle health. Your body produces vitamin D when ultraviolet B (UVB) rays from sunlight interact with a cholesterol compound in your skin, converting it into a precursor form that your liver and kidneys then activate.

That process sounds straightforward. But the efficiency of it depends on several factors: skin pigmentation, age, geography, time of year, how much skin is exposed, and how much time you spend outdoors. The UK study observed that older adults and individuals from minoritized ethnic backgrounds—groups already known to face higher risk of vitamin D inadequacy—did not show the expected seasonal rise in vitamin D levels during summer months.

The honest answer is that "getting sun" isn't a one-size-fits-all solution.

Who struggles to maintain vitamin D from sunlight alone?

Several groups face structural challenges when it comes to producing vitamin D from sun exposure:

• Older adults: Skin's capacity to synthesize vitamin D declines with age. Someone over 65 may produce as little as 25% of the vitamin D a younger person would from the same amount of sun exposure.
• People with darker skin tones: Higher levels of melanin—the pigment that protects skin from UV damage—also reduce the skin's ability to produce vitamin D from sunlight. This doesn't mean production stops, but it does mean significantly more sun exposure is required to achieve the same effect.
• People living at higher latitudes: In regions farther from the equator (including much of the northern U.S., Canada, and northern Europe), UVB radiation is too weak during fall and winter months for the skin to produce meaningful amounts of vitamin D, regardless of time spent outdoors.
• People who spend most of their time indoors: Work schedules, mobility limitations, and lifestyle factors all reduce sun exposure. Glass windows block UVB rays, so sitting near a sunny window doesn't count.

What matters is that these aren't edge cases. They describe a substantial portion of the population.

Why summer sun didn't help in the UK study

The UK research is genuinely preliminary, but the pattern it identified is consistent with what we already know about vitamin D synthesis. Even during summer, participants in high-risk groups showed little to no improvement in vitamin D levels. This suggests that either they weren't getting enough UVB exposure—due to weather, clothing, indoor time, or sunscreen use—or their bodies weren't efficiently converting that exposure into usable vitamin D.

In practice, this means that relying on seasonal sun exposure to "top off" vitamin D stores may leave certain populations under-supported for most or all of the year.

What supplementation offers

Vitamin D supplementation bypasses the variables of sun exposure entirely. A supplement delivers a consistent, measurable dose that doesn't depend on latitude, skin tone, weather, or how much time you spent outside last week.

The two main supplemental forms are:

• Vitamin D2 (ergocalciferol): Derived from yeast or mushrooms.
• Vitamin D3 (cholecalciferol): Derived from lanolin (sheep's wool) or lichen (a plant-based source). D3 is generally considered more effective at raising and maintaining blood levels of vitamin D.

Bioavailability—the proportion of a nutrient your body can actually absorb and use—varies by formulation. Vitamin D is fat-soluble, which means it's absorbed more efficiently when taken with a meal that contains some fat. Some supplements pair vitamin D with healthy fats or emulsifiers to improve absorption. Others use liposomal delivery or micellization, both of which enhance how readily the nutrient enters your bloodstream.

Not all vitamin D supplements are formulated the same way. The specificity is the point.

How to think about your own vitamin D status

If you're in one of the groups described above—older, with darker skin, living at a higher latitude, or spending most of your time indoors—it's worth asking your healthcare provider to check your vitamin D levels with a simple blood test. The test measures 25-hydroxyvitamin D, the form your body stores and uses as a reserve.

Optimal levels are still debated, but many clinicians aim for a blood level of at least 30 ng/mL (75 nmol/L) to support normal bone and immune function. Levels below 20 ng/mL are generally considered inadequate.

Your doctor can help you determine whether supplementation makes sense and, if so, what dose is appropriate. Ask questions. Vitamin D toxicity is rare but possible at very high doses sustained over time, so more isn't always better.

What this means for supplement quality

If you do supplement, formulation matters. Look for products that:

• Use vitamin D3 rather than D2
• Pair the nutrient with fat or use an enhanced-absorption technology
• Are third-party tested for purity and potency
• Clearly disclose the source and dose on the label

Manna's vitamin D3 is formulated with bioavailability in mind—sourced from lichen (making it vegan-friendly), paired with healthy fats, and manufactured to meet rigorous quality standards. We're not here to tell you what your vitamin D level should be or whether you need to supplement. We're here to make sure that if you do, the product you're taking is doing what it's supposed to.

You can learn more about our approach to formulation and ingredient sourcing on our product pages.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.