How to Find the Vitamin D Sweet Spot

Vitamin D is the one nutrient we were designed to make ourselves, and somehow, most of us are still chronically low. Let’s learn about why that is and how to fix it.

‍

First Off, What Does Vitamin D Actually Do?

Vitamin D gets called a vitamin, but it behaves more like a hormone. Once it's made or absorbed, it travels through your bloodstream and influences hundreds of biochemical processes. It's involved in far more than just bone health, although that alone is reason enough to pay attention. 

Some researchers have even proposed that the reason humans evolved to find sunshine pleasurable may partly be an evolutionary drive to seek out vitamin D production. Your body rewards you for doing something it needs. And here's where it earns its keep:

Bones and teeth. Vitamin D regulates calcium and phosphorus absorption in the gut. Without it, calcium largely passes straight through you, regardless of how much dairy or leafy greens you eat. Over time, low vitamin D leads to reduced bone density and, in children, rickets, which is a condition we once thought was largely consigned to history.

Immune function. Vitamin D is directly involved in activating your immune cells, particularly T-cells. Low levels are consistently associated with increased susceptibility to infections and a higher risk of autoimmune conditions. There's a reason people get ill more in winter.

Mood and mental health. Vitamin D receptors are found throughout the brain, including in areas that regulate mood. Low levels are strongly linked to seasonal depression and have been associated with higher rates of anxiety, risk of neurodegeneration, and low mood more broadly.

Hormonal balance. For anyone with thyroid issues, PCOS, or concerns around fertility, vitamin D plays a meaningful supporting role. It influences insulin sensitivity and interacts with several key hormonal pathways.

Muscle function. It supports muscle contraction and repair, which matters whether you're a serious athlete or simply trying to stay strong as you age.

‍

So Where Does It Come From?

Let’s get into the science of it. Vitamin D3, the form your body uses most effectively, is made in your skin when UVB rays from the sun hit a cholesterol compound called 7-dehydrocholesterol. That reaction converts it into a precursor that your liver and kidneys then activate into the usable form.

However, UVB rays are only strong enough to trigger this process when the sun is sufficiently high in the sky, roughly between 10am and 3pm, and primarily during the spring and summer months in northern latitudes. In the UK and much of Northern Europe, from about October through March, the sun simply doesn't get high enough. You could (but would likely not want to!) sit outside all day in January and produce virtually no vitamin D.

A few other factors reduce synthesis even when the sun is out:

• Skin tone. Melanin is a natural UV filter, so those with darker skin need significantly more sun exposure to produce the same amount of vitamin D as someone with lighter skin. This is a natural protective mechanism as those with darker skin would have originally been located in places with high amounts of intense sunshine. This is obviously not true now and supplementation would likely be required all year round as a result. 
• Age. The skin's ability to synthesise vitamin D declines with age, meaning older adults need longer exposure for the same output. Which again can be compromised if someone finds they feel the cold more, or conversely struggles in the heat. Again year round supplementation would likely be indicated. 
• Sunscreen. SPF 30 blocks around 95% of UVB rays. Which is brilliant for preventing burning, but does meaningfully reduce vitamin D production if always applied before exposure.
• Glass. UVB rays don't pass through glass, so sitting by a sunny window doesn't count. Natural light exposure for mood enhancement yes, but Vitamin D, no. 
• Body coverage. Only exposed skin contributes. A face and two forearms out but whilst wearing a long-sleeved shirt and trousers isn't going to move the needle.

‍

What Happens When You Don't Get Enough?

Vitamin D deficiency is startlingly common. Estimates suggest that over 40% of adults in the UK are deficient or insufficient by the end of winter, and globally, it's one of the most widespread nutritional shortfalls.

The symptoms are easy to miss because they're so non-specific:

• Persistent fatigue that doesn't resolve with rest
• Low mood, brain fog, or a general flatness
• Frequent colds and infections
• Aching muscles or bones
• Slow wound healing
• Hair loss in some cases

Labs flag anything above 50 nmol/L as ‘sufficient’, which in functional medicine terms is really just ‘not actively deficient’. If you’d like to know your vitamin D levels, you can find it through this Omega-3 Complete with Vitamin D test.

Longer term, chronic deficiency is associated with increased risk of osteoporosis, cardiovascular disease, type 2 diabetes, multiple sclerosis, neurodegeneration and various cancers. I am never one to sensationalise anything so that isn’t to scare you, it’s just to highlight the critical importance of this nutrient. Also those risks are associated with ongoing insufficiency/deficiency, so if you test and are a little low then pop in a great supplement (code LIEBLING for discount) to bring you up you will be fine.

‍

On the Flip Side, Can You Get Too Much Sun?

Your skin has a built-in cut-off mechanism: once it's produced as much vitamin D as it can from a single session, it starts breaking the excess down. You literally cannot overdose on vitamin D from sunshine alone, only from supplements.

That being said, the risks of excessive, unprotected sun exposure are real:

Skin damage and skin cancer. UV radiation causes direct DNA damage to skin cells. Repeated sunburn, particularly in childhood and early adulthood, significantly increases the risk of melanoma, the most dangerous form of skin cancer. 

Premature ageing. UVA rays (which penetrate more deeply than UVB) break down collagen and elastin, accelerating wrinkling and pigmentation over time.

Photokeratitis. Eyes are vulnerable too. Prolonged, unprotected exposure can damage the cornea.

You do not need to choose between vitamin D and sun safety. Short, strategic, unprotected exposure during peak hours is the goal. We are talking about 15–30 minutes of sensible, mindful exposure before reaching for the SPF.

‍

4 Practical Tips for Finding the Vitamin D Sweet Spot

1. Time your exposure strategically. Aim for 15–30 minutes of unprotected sun exposure between 10am and 2pm, when UVB rays are strongest and vitamin D synthesis is most efficient. Expose as much skin as is practical, particularly skin that doesn’t usually see the sun - arms, legs, and décolletage all count. After that window, protect your skin. 

2. Get your levels tested. A simple blood test tells you exactly where you stand. Once you know your baseline, you can make informed decisions about supplementation and exposure rather than flying blind. Aim to test at the end of winter (when levels are typically lowest) and again in late summer.

3. Support absorption with the right co-factors. Magnesium is essential for converting vitamin D into its active form and many people are deficient in both simultaneously. Vitamin K2 works alongside D3 to direct calcium into bones rather than soft tissue. If you're supplementing, these partnerships matter. I use Bare Biology’s Beam & Balance Vit D3/K2.

4. Supplement smartly through winter and consider year-round if your levels are consistently low. The NHS recommends 400 IU daily for the general population, but in functional medicine practice, many adults need considerably more, typically 1,000–4,000 IU of D3 daily. Always work with a practitioner if you're considering higher doses, and retest to confirm you're hitting your target range. Ideally, we want to be getting as much vitamin D from the sun as possible, because it lasts roughly twice as long in the body compared to the same amount taken as a supplement. 

‍