What Are the Benefits of Vitamin D? (And Why Most of Us Are Low)
Written by Gordon Maudsley
Key Takeaways
- Vitamin D contributes to the normal function of the immune system, an EFSA-approved health claim backed by decades of research.
- Low Vitamin D is linked to fatigue, muscle weakness and low mood. All symptoms I know from personal experience.
- Around half of UK adults have lower than optimal Vitamin D levels, and in winter the body simply cannot make it from sunlight.
- Most Vitamin D3 supplements are derived from sheep’s wool. Ours comes from algae, and is just as bioavailable.
- Vitamin D is found in both our Immunity and Total Wellness blends.
When I was rebuilding my health after glandular fever and post-viral fatigue, one of the first things my coach flagged was Vitamin D. I was exhausted, picking up every infection going, struggling to get through the day and low Vitamin D fitted that picture exactly.
I’d always assumed I probably had enough. Turns out that for a significant chunk of the year in the UK, it doesn’t matter how much time you spend outdoors. Your body physically cannot make Vitamin D from sunlight. That was news to me.
It’s also news to a lot of people. Which is one reason I made sure Vitamin D was in both our blends. Here’s what the research actually says about why it matters.
What actually is Vitamin D?
Vitamin D is sometimes called the “sunshine vitamin” and unlike most nutrients, the main source isn’t food. It’s your skin.
When UVB rays from sunlight hit the skin, they trigger a chemical reaction that produces Vitamin D3 (cholecalciferol). The liver and kidneys then convert it into its active form, which the body puts to work, from calcium absorption and bone maintenance to immune regulation and muscle function.
The problem in the UK is simple. Between October and March, the sun’s angle is too low for effective UVB penetration. It doesn’t matter whether the sun is out. The wavelength of light that triggers Vitamin D synthesis isn’t reaching us. For roughly five months of the year, your body produces essentially none. The NHS itself recommends that everyone in the UK consider supplementing during these months for exactly this reason.
And even outside of winter, a 2025 analysis of UK adult health data found that around half of UK adults have lower than optimal Vitamin D levels.
That’s not a niche problem. That’s most of us.
Vitamin D and UK sunlight: How big is the gap?
Here’s a comparison that puts it in context:
| Period | UVB availability in UK | Can your body make Vitamin D? |
| Late March – September | Good during midday hours | Yes, short regular outdoor exposure is sufficient |
| October – March | Effectively zero | No, regardless of time spent outside |
| Year-round for darker skin | Lower synthesis efficiency | Often not enough; year-round supplementation often advised |
| Year-round for older adults | Reduced skin synthesis | Often not enough; year-round supplementation often advised |
The gap is real, and for many people it’s year-round. Supplementation is the most practical way to fill it.
What does Vitamin D do for your body?
1. It supports your immune system
This is the one I come back to most, given my own history. When my immune system was at its lowest (picking up every cold, taking weeks to recover) Vitamin D was one piece of the puzzle my coach put back in place.
The European Food Safety Authority (EFSA) has formally approved the claim that “Vitamin D contributes to the normal function of the immune system” and the science behind that approval is solid.
A 2024 systematic review and meta-analysis published in ScienceDirect covering 16 randomised controlled trials found that Vitamin D supports the immune system and helps regulate inflammation.
A review published in JBMR Plus described Vitamin D’s role in both the innate immune response (the rapid, non-specific defence) and the adaptive response (the targeted, learned defence). Both matter. Both are affected by deficiency.
I won’t claim that topping up your Vitamin D will stop you ever getting ill. That’s not how it works. But supporting normal immune function is a meaningful thing, especially through a British winter.
2. It supports muscle function and energy
This one surprised me when I first came across it.
Research from Newcastle University found a direct link between Vitamin D levels and mitochondrial efficiency, the “power stations” inside muscle cells. Patients with severe Vitamin D deficiency who were treated showed significant improvements in muscle function and fatigue.
A 2025 study published in the European Journal of Cardiovascular Medicine confirmed the picture: participants with deficient Vitamin D levels had significantly higher fatigue scores and lower muscle strength.
EFSA has also formally approved the claim that Vitamin D contributes to the maintenance of normal muscle function.
3. It contributes to bone health
This is the most well-established benefit of all, and worth including because it’s genuinely important.
Vitamin D is essential for the body to absorb calcium properly. Without adequate Vitamin D, calcium from food passes through largely unused. Over time, low Vitamin D is one of the key contributors to weakened bones and increased fracture risk. EFSA has approved multiple claims in this area, including that Vitamin D contributes to maintenance of normal bones and supports normal absorption of calcium and phosphorus.
This matters at every age, not just in older adulthood when it tends to get discussed.
4. It may support mood
This is the most nuanced of the four, so I’ll be honest about what the evidence actually shows.
A 2025 meta-analysis published in Frontiers in Psychiatry, covering 20 randomised controlled trials, found that Vitamin D supplementation significantly reduced depressive symptoms. The effect was most obvious in people who were deficient to begin with.
A review published in PMC explored that Vitamin D appears to play a role in the regulation of neurotransmitters and various brain functions, including mood.
I won’t overstate this. Mood is complex, and Vitamin D isn’t a treatment for depression. But there’s a reasonable body of evidence suggesting that when levels are low, mood can suffer and that correcting deficiency may help. Given how many UK adults are low, particularly through the darker months, it’s worth knowing about.
Why does Just Good Stuff use algae-derived Vitamin D?
Here’s something most people don’t think to ask: where does the Vitamin D3 in supplements actually come from?
The honest answer, for the vast majority of products on the market, is lanolin, the fatty secretion from sheep’s wool. It’s cheap, it’s abundant and it works. But it’s animal-derived, which means it’s not suitable for vegans or vegetarians, and it carries a very different environmental footprint to plant-based alternatives.
Just Good Stuff uses Vita-algae D™, a Vitamin D3 derived entirely from algae. It’s plant-based, it’s certified organic and it’s chemically identical to the D3 the body produces itself. Research confirms it is equally bioavailable. Your body absorbs and uses it the same way.
Same nutritional result. Cleaner source. No animal products. That matters to me, both because Just Good Stuff is 100% vegan, and because I’d rather our ingredients came from somewhere I’m comfortable with.
Are there any side effects of Vitamin D?
At normal supplementation doses, Vitamin D is very well tolerated and side effects are rare.
The NHS and EFSA set the safe upper limit at 100 micrograms (4,000 IU) per day for healthy adults. Toxicity, which can cause elevated calcium levels, only occurs with very high doses taken over a prolonged period, well above what you’d find in a food supplement.
A few things worth knowing:
- Certain medications can interact with Vitamin D, including orlistat, some anticonvulsants and thiazide diuretics. If you take any regular medication, it’s worth checking with your GP or pharmacist.
- If you have a kidney condition, granulomatous disease, or are prone to hypercalcaemia, speak to your doctor before supplementing.
- As with all supplements, if you’re pregnant, breastfeeding, or managing a medical condition, please consult your GP before use.
Where to find Vitamin D in Just Good Stuff
Vitamin D, as Vita-algae D™, is included in both our blends:
Immunity is built around supporting your body’s ability to ward off infection and recover more quickly. Alongside Vitamin D, it contains Organic Baobab, Organic Camu-Camu, Organic Ginger, Organic Turmeric, Organic Ginseng, Organic Acerola and more. All organic, all additive-free.
Total Wellness is designed for broader, everyday wellbeing: energy, balance, gut health and general nutritional support. Vitamin D sits alongside Barley Grass, Spirulina, Kale, Chlorella, Matcha, Inulin and a range of organic berries and seeds.
Both blends are 100% certified organic, 100% vegan, manufactured in the UK by a BRC/ISO accredited facility, and contain no fillers, additives or hidden nasties.
Because health shouldn’t be complicated.
Written by Gordon Maudsley
