Our complete guide to optimal Vitamin D Levels

Let’s talk about a hot topic – Vitamin D, where most people still think of Vitamin D as a simple nutrient – something you “top up” during summer or with a supplement.
But Vitamin D isn’t actually a vitamin at all – shock! It’s a prohormone, meaning your body converts it into a hormone that influences immunity, bone density, metabolism, brain function and even mood regulation.

It’s one of the most powerful, and most commonly deficient, nutrients in the UK so let’s spend the next 3-minutes getting you up-to-date with the facts.

Why Vitamin D Matters (Key Benefits)

Vitamin D plays a pivitoal role, impacting 2,700 genes that help to keep our system running and functioning at optimal health. A few key areas Vitamin D helps with is:

• Immune function: supporting antiviral defence and lowering inflammation
• Bone and muscle health:  regulating calcium absorption, strengthening bones, and supporting muscle contraction
• Mood and mental wellbeing: low levels are associated with seasonal depression, low motivation and fatigue
• Metabolic health: helpping to maintain insulin sensitivity and energy production
• Hormone regulation: influencing thyroid function, sex hormones, and cellular repair

And its importance becomes even more striking during menopause.

Vitamin D, Menopause & Bone Health: Why Women Need More Support

Post-menopause, oestrogen levels naturally drop – and estrogen is protective for bone strength and calcium metabolism.
This hormonal shift accelerates bone loss, increasing the risk of:

• Osteopenia
• Osteoporosis
• Fractures
• Loss of muscle mass (sarcopenia)

Vitamin D is essential here because it improves calcium absorption, reducing bone turnover helping with bone formation and strength. Supports osteoblast activity, which is the building cells of bone, and helps to maintain muscle function, which reduces fall risk. In fact, evidence tells us Vitamin D supplementation (≥800 IU/day) plus calcium reduces fall risk by up to 22% in older adults.

Women post-menopause often require higher Vitamin D levels to maintain bone integrity compared to younger adults. Ensuring levels are in the optimal range (as I explain below) is one of the simplest, evidence-based ways to protect future mobility and longevity.

Vitamin D Deficiency in the UK: The Winter Problem

It’s estimated that approximately 1 in 6 people in the UK are clinically deficient (< 25 nmol/L) in Vitamin D, while 50% of Brits have suboptimal Vitamin D levels; and in winter this number spikes sharply, as between October and March, the sun is too weak for your skin to synthesise Vitamin D; even on a clear day.

How Much Sunlight Do We Actually Need?

Your skin only makes Vitamin D when exposed to UVB rays from sunlight. When UVB rays hit the skin, they convert 7-dehydrocholesterol (a cholesterol derivative) into pre-vitamin D3.
This molecule then undergoes a heat-dependent rearrangement into Vitamin D3 (cholecalciferol), which enters the bloodstream where the liver and the kidneys then convert it to 1,25(OH)₂D aka. calcitriol – the active hormone we all know.
But key variables affect how much we can produce as the sun must be at least 45° above the horizon for UVB to reach you and have a UV index of 7 or more.

To produce an effective amount of Vitamin D naturally, we typically need:

• 10-15 minutes of full-body exposure on a UV index 7 day
• 20-30 minutes when the UV index is around 3.5
• 5-7.5 minutes when the index reaches 14

Cloud cover, clothing, time indoors and shorter days all compound the deficit. None of these conditions exist in the UK winter.
If you’re uncertain you can track your area’s UV index here: https://d.vitamin.today/

How to Improve Vitamin D When Sunlight Is Poor

If we’re uable to make enough Vitamin D from sunlight how else can we obtain it? Well, here are the three most effective approaches you can use over the winter months:

1. Diet Support
Food alone typically isn’t enough to reach optimal levels, but it helps maintain a baseline.

Top dietary sources include:

• Fatty fish such as salmon, sardines, trout, mackerel
• Egg yolks
• Mushrooms exposed to sunlight or UV (portobello, shiitake, maitake; one of the best plant-based sources)
• Offal and liver
• Fortified foods such as cereals, spreads, oat/soy milks
• Cheese

Mushrooms are particularly interesting because UV-exposed mushrooms can contain very high levels of Vitamin D2, supporting intake even in plant-based diets.

2. Cold Exposure – A Forgotten Evolutionary Mechanism
This is where things gets fascinating.

New research, including a 2025 clinical paper, suggests that cold exposure can increase circulating Vitamin D levels independent of sunlight.

How Cold Changes Vitamin D Physiology

When you immerse yourself in cold water several mechanisms come into action:

1. Brown and beige fat activate to generate heat
2. These tissues undergo metabolic uncoupling to produce heat instead of ATP – this is how cold plunging helps to increase your metabloism and increases calorie exoenditure
3. During this process they emit a very faint near-infrared (NIR) glow
4. NIR wavelengths can mobilise Vitamin D from fat stores and
5. Activates the enzymes that convert Vitamin D into its usable form (25(OH)D → 1,25(OH)₂D)

Cold also compresses the skin and stiffens the hydration layers around cells – a mechanical effect that “wakes up” dormant enzyme pockets involved in hormone conversion.

Why This Matters Evolutionarily

For populations living in low-light, Arctic environments – such as the Inuit – winter sun wasn’t strong enough for Vitamin D synthesis, so their biology had to adapt through:

• Cold-induced thermogenic activation
• High-fat, Vitamin D rich diets
• Near-infrared heat release from brown fat
• Increased Vitamin D turnover to maintain levels without sunlight

The cold acted as an internal signal for the body to keep Vitamin D moving – amazing when you think about it!

Cold Exposure at Avanto°

Our cold plunge at Avanto° is intentionally designed to activate brown fat safely and powerfully, making it a winter-friendly tool to stimulate vitamin mobilisation, improve energy and support metabolic health through inceased metabolisim.

We would recommend 3-6 minutes of cold exposure to help stimulate these (magical) pathways.

3. Smart Supplementation
Finally, we come to supplementing VitaminD. This is the most reliable winter strategy and we’ve made it easy for you.

Based on the literature, the general recommended daily intake for:

1. General maintenance: 1,000–2,000 IU per day (this is reserved for people who already have optimal levels)
2. Those with low levels or higher needs: 2,000–4,000 IU per day (this is recommended for most peple – women going through menopause, low sun exposure, darker skin tone, indoor lifestyle)
3. People who are chronically sufficient: 20,000 IU 2x per week (this higher dose should ideally be guided by testing)

Absorption Trick: Take Vitamin D With Fat
Vitamin D is a fat-soluble substance, meaning it requires fat to be absorbed. When supplementing with Vitamin D it’s important to take alongside food and not on an empty stomach or fasted state.

One study shows you get a 30-32% higher absorption rate when taken with a high-fat meal (e.g. eggs, avocado, nuts, oily fish) compared with a low-fat meal.

This dramatically improves effectiveness.

What Should Your Vitamin D Levels Be?

Vitamin D is measured as 25(OH)D in nmol/L in the UK.

NHS / Standard Reference Ranges

• Deficient: < 25 nmol/L
• Insufficient: 25–50 nmol/L
• Adequate for basic health: 50–75 nmol/L

However, I would argue 75 nmol/L is still too low and if we’re wanting to reach optimal health then we need to be striving for the optimal ranges (supported by endocrine, bone-health & sports medicine research)

• Optimal for immunity & bone health: 75–120 nmol/L
• Optimal for performance, mood & menopause support: 100–150 nmol/L

Most people feel their best – metabolically, mentally and hormonally – in the 100–150 nmol/L range.

So how do you know where you’re at? It’s best to test your levels.

Testing Your Vitamin D Levels

Most tests cost around £40 for an easy at-home blood test. Here’s two easy options for you to take:

1. MedChecks Vitamin D Test (£39) – this is the gold-standard finger-prick test that will provide you with your exact Vitamin D level
2. Tescos Vitamin D Blood Spot Test (£8) – this is a more affordable snapshot of deficiency, but will only reveal whether you’re deficinet or not, it will not provide you with an exact level.

Both of these tests will help you understand whether you need dietary support, supplementation, or more targeted dosing.

Summary: Your Winter Vitamin D Plan

Because sunlight is too weak to synthesise Vitamin D between October and April, most of us in the UK will not be getting enough vitamin D naturally.

To stay strong through the darker months I strongly recommend to:

1. Increase dietary Vitamin D (especially oily fish & mushrooms)
2. Experiment with cold exposure to stimulate vitamin mobilisation (Avanto° is ideal)
3. Supplement smartly – and always with a higher-fat meal
4. Track levels every 3-6 months to ensure you stay in the optimal range

Vitamin D affects everything from immunity to bone health to mood, and winter is wehn we’re at our most vunerable aand exactly when we need it most.