By Tony Larkman | Personal Trainer

As autumn arrives, the clocks go back, daylight hours shorten and temperatures begin to fall. For those of us living in the UK, this also marks an important change in our body’s ability to produce one of its most essential nutrients: vitamin D.

Often referred to as the sunshine vitamin, vitamin D plays a vital role in maintaining healthy bones, muscles and normal immune function. Yet, despite its importance, many people are unaware that they may not be getting enough, particularly between October and March.

As a personal trainer, I spend a great deal of time discussing exercise, nutrition, recovery and healthy ageing with my clients. Vitamin D is one of those nutrients that deserves more attention, especially as we head into winter.

So, what exactly is vitamin D, why do we need it, how do we obtain it and should we all be taking a supplement?

What is vitamin D?

Vitamin D is a fat-soluble vitamin that behaves more like a hormone than a conventional vitamin.

Unlike most vitamins, which must be obtained primarily through food, our bodies can manufacture vitamin D naturally when our skin is exposed to ultraviolet B (UVB) radiation from sunlight.

Once produced in the skin or absorbed from food and supplements, vitamin D undergoes two important stages of activation:

1. The liver converts vitamin D into 25-hydroxyvitamin D, also known as calcidiol. This is the form measured in blood tests.
2. The kidneys convert it into its active hormonal form, calcitriol, which helps regulate calcium and phosphate levels throughout the body.

Vitamin D receptors are found in numerous tissues, including bone, muscle and cells of the immune system.

Its most established function is supporting calcium and phosphate regulation, but researchers have also investigated its potential influence on immune health, physical performance and several chronic diseases.

Importantly, vitamin D is essential for normal physiological function but taking more than you need does not necessarily provide additional health benefits.

Why is vitamin D particularly important during autumn and winter?

Here in Britain, our geographical location creates a seasonal problem.

The UK lies approximately between 50° and 59° north of the equator. During the autumn and winter months, the sun sits much lower in the sky.

Consequently, very little of the UVB radiation required for vitamin D production reaches our skin.

According to the NHS, between October and early March, sunlight in the UK is generally insufficient for our bodies to manufacture meaningful amounts of vitamin D.

This is an important distinction.

You can spend an hour walking outdoors on a beautiful sunny November afternoon, enjoying the fresh air and natural daylight but you will produce little or no vitamin D.

That doesn’t mean winter walks are pointless. Quite the opposite! Outdoor exercise remains beneficial for cardiovascular health, mental wellbeing, sleep and general fitness.

However, winter daylight and vitamin D production are two different things.

This seasonal limitation also affects many other northern European countries and regions at similar or higher latitudes although the precise months vary according to geographical location.

How much sunlight do we need to produce vitamin D?

Between approximately April and September, most people in the UK can produce vitamin D through exposure to sunlight.

However, there is no single scientifically reliable exposure time that applies to everyone.

Vitamin D production depends on several factors:

• Time of day and season.
• Geographical latitude.
• Skin pigmentation.
• Age.
• Cloud cover and atmospheric conditions.
• How much skin is exposed.
• Use of sunscreen.

During spring and summer, short periods outdoors with some skin exposed may be sufficient for many people.

For someone with lighter skin, this might mean several minutes to around 15 minutes of exposure on some summer days. People with darker skin may require longer exposure because melanin reduces the amount of UVB penetrating the skin.

These are broad illustrations, not recommended exposure prescriptions.

The important point is to avoid sunburn.

Although vitamin D production can occur during short periods of unprotected sun exposure, deliberately spending prolonged periods in the sun without sunscreen is not advisable.

There is no need to develop a tan, and sunburn increases the risk of skin cancer.

Sun protection should be used when needed and there is no justification for prolonged unprotected exposure simply to increase vitamin D.

Another misconception is that sunlight passing through a window provides the same benefit.

Unfortunately, ordinary window glass blocks most UVB radiation. Sitting in a sunny conservatory or beside a window will therefore not produce meaningful vitamin D.

And in Britain during October, November, December, January and February, spending longer outdoors will not compensate for the seasonal lack of effective UVB.

What does vitamin D actually do for the body?

1. Supports strong bones and teeth

One of vitamin D’s most important functions is helping the intestines absorb calcium and phosphate from the food we eat.

Both minerals are essential for maintaining healthy bones and teeth.

Without sufficient vitamin D, calcium absorption becomes less efficient, potentially compromising bone mineralisation.

Over time, severe deficiency can lead to osteomalacia in adults, a condition characterised by poorly mineralised bones, bone pain and muscle weakness.

In children, severe deficiency can cause rickets.

Vitamin D deficiency can also contribute to poor bone health and may increase fracture risk, particularly in vulnerable older adults.

However, supplementation alone is not a guaranteed way to prevent osteoporosis or fractures in otherwise healthy adults who already have adequate vitamin D levels.

Adequate dietary calcium, resistance training, weight-bearing activity and appropriate vitamin D status all contribute to maintaining skeletal health.

2. Supports normal muscle function

This is particularly relevant to anyone who exercises regularly.

Vitamin D is involved in normal muscle function and severe deficiency can cause muscle weakness, particularly affecting the muscles around the hips and thighs.

This may make everyday activities such as climbing stairs, standing from a chair or walking more difficult.

For older adults, significant weakness can contribute to reduced mobility and potentially increase the risk of falling.

Correcting a genuine vitamin D deficiency can improve deficiency-related muscle problems.

However, research does not consistently show that taking additional vitamin D improves strength, muscle mass or athletic performance in people whose vitamin D levels are already sufficient.

For athletes and regular exercisers, the objective should be to avoid deficiency rather than chase excessively high vitamin D levels.

3. Contributes to normal immune system function

Vitamin D plays a role in regulating the immune system.

Immune cells possess vitamin D receptors, and vitamin D is involved in several processes associated with immune responses.

Some research suggests supplementation may modestly reduce the risk of certain acute respiratory infections, particularly in specific populations or circumstances.

However, the findings across clinical trials have been mixed.

Vitamin D should not be regarded as a guaranteed way of preventing colds, influenza or other respiratory illnesses.

Maintaining adequate levels is sensible for normal immune function, but supplementation is not a substitute for vaccination, good nutrition, sufficient sleep and other established health measures.

4. Helps maintain physical function as we age

Ageing presents several challenges to our musculoskeletal system.

We naturally experience reductions in muscle mass, muscle strength and, in some individuals, bone density.

Resistance training and adequate protein intake remain fundamental to maintaining strength and independence.

Vitamin D also plays a supporting role by maintaining normal calcium metabolism, bone health and muscle function.

Older adults are particularly vulnerable to deficiency because their skin becomes less efficient at producing vitamin D and they may spend less time outdoors.

For someone already deficient, correcting vitamin D levels is an important part of maintaining musculoskeletal health.

But vitamin D supplementation should complement, rather than replace, a properly structured exercise programme.

5. What about mood, depression and fatigue?

Vitamin D has also attracted considerable interest in relation to mood, fatigue and seasonal wellbeing.

Some observational studies have found associations between low vitamin D levels and depression or fatigue.

However, an association does not prove that low vitamin D is the cause.

Large clinical trials have not consistently demonstrated that supplementation prevents depression or improves mood in people without deficiency.

Similarly, vitamin D is not an established treatment for seasonal affective disorder (SAD).

Nevertheless, significant vitamin D deficiency can contribute to tiredness and muscle symptoms, and correcting it is appropriate when clinically indicated.

If you experience persistent fatigue, low mood or unexplained weakness, it is important not to assume vitamin D is the sole explanation.

What happens if your vitamin D levels are too low?

Vitamin D deficiency can develop gradually and may initially cause no obvious symptoms.

When deficiency becomes more pronounced, possible consequences include:

• Muscle weakness, particularly around the hips and thighs.
• Bone pain or tenderness.
• Difficulty climbing stairs or rising from a seated position.
• Osteomalacia in adults.
• Rickets in children.
• Increased risk of falls or fractures in some vulnerable individuals.

Tiredness and general aches may also occur although these symptoms are nonspecific and can have many other causes.

Long-standing deficiency can have serious consequences for bone health.

The good news is that vitamin D deficiency is generally treatable although the appropriate dose depends on its severity and the individual’s medical circumstances.

Who is most at risk of vitamin D deficiency?

Although anyone living in Britain can experience low vitamin D levels, certain groups are particularly vulnerable.

These include people who:

• Have darker skin pigmentation.
• Spend most of their time indoors.
• Regularly cover most of their skin when outdoors.
• Are older, particularly those with limited mobility.
• Live in residential care or are housebound.
• Have medical conditions affecting nutrient absorption such as coeliac disease or Crohn’s disease.
• Have undergone certain forms of weight-loss surgery.
• Have significant liver or kidney disease.
• Take medications that interfere with vitamin D metabolism.

For people in these higher-risk groups, the NHS recommends considering vitamin D supplementation throughout the year rather than only during winter.

How much vitamin D should we take?

The NHS recommends that adults and children over four years old should consider taking 10 micrograms (400 international units – IU) of vitamin D daily during autumn and winter.

This is the standard UK public health recommendation.

Vitamin D supplements may be labelled in either micrograms (µg) or international units (IU).

The conversion is straightforward:

What do I personally take?

I take a daily vitamin D3 supplement containing 25 micrograms (1,000 IU) of cholecalciferol.

This is higher than the standard NHS recommendation of 400 IU but remains within the range generally considered safe for most healthy adults.

Indeed, UK public health guidance has previously identified 1,000 IU products as a suitable alternative where 400 IU supplements are unavailable.

However, that does not mean 1,000 IU is necessary or superior for everyone.

For most healthy adults, 400 IU daily is sufficient to follow the NHS winter supplementation advice.

Individual requirements may vary, especially where a deficiency has been diagnosed.

The general adult upper limit is 100 micrograms (4,000 IU) daily but this is a safety limit, not a recommended target.

Taking large doses over prolonged periods can cause excessive calcium accumulation in the blood, potentially damaging the kidneys and other organs.

More is not always better.

Vitamin D2 versus vitamin D3: what’s the difference?

When buying supplements, you may notice different forms of vitamin D.

The two main forms are vitamin D2 and vitamin D3.

Vitamin D2 – Ergocalciferol

Vitamin D2 is commonly derived from fungi including mushrooms exposed to ultraviolet light.

It can increase vitamin D levels and is suitable for vegans.

However, studies generally suggest it is less effective than vitamin D3 at raising and maintaining blood concentrations of 25-hydroxyvitamin D, particularly over time.

Vitamin D3 – Cholecalciferol

Vitamin D3 is the same form produced naturally in our skin following UVB exposure.

Most conventional D3 supplements are derived from lanolin, a substance obtained from sheep’s wool.

Vegan vitamin D3 supplements are also available, commonly derived from lichen.

Research generally indicates that vitamin D3 is more effective than vitamin D2 at increasing circulating vitamin D levels.

For this reason, D3 is often a practical first choice.

Tablets, capsules, sprays or drops?

Vitamin D is available in several formats including tablets, softgel capsules, oral sprays and liquid drops.

All can be effective when they provide an appropriate dose.

Because vitamin D is fat-soluble, taking it with a meal containing some fat may improve absorption.

However, there is no convincing evidence that an expensive spray or specialist preparation is automatically better than a conventional tablet or capsule.

Consistency and appropriate dosing matter more than marketing claims.

Can we obtain enough vitamin D from food?

Vitamin D occurs naturally in relatively few foods.

This is one reason supplementation is particularly useful during winter.

Some of the best dietary sources include:

Oily fish is particularly useful because it also provides omega-3 fatty acids and high-quality protein.

Eggs contribute smaller quantities while fortified foods can help increase daily intake.

An important point for UK consumers is that ordinary cow’s milk is generally not fortified with vitamin D unlike milk sold in some other countries.

Although a balanced diet is essential for good health, obtaining enough vitamin D from food alone can be difficult.

This is precisely why UK public health authorities recommend considering supplementation during the darker months.

Should you ask your GP to check your vitamin D levels?

This is a question I am frequently asked about nutritional supplements.

Vitamin D status can be assessed through a blood test measuring 25-hydroxyvitamin D, usually abbreviated to 25(OH)D.

Results are generally reported in nanomoles per litre (nmol/L) in the UK.

As a broad guide:

These are broad clinical interpretations and thresholds vary between guidelines.

The UK Scientific Advisory Committee on Nutrition recommends preventing levels from falling below 25 nmol/L to protect musculoskeletal health.

Does everyone need a blood test?

No.

Routine vitamin D testing is not generally recommended for otherwise healthy people simply because winter is approaching.

If you feel well and are following standard supplementation guidance, a blood test is usually unnecessary.

However, discussing testing with your GP may be appropriate if you have:

• Persistent unexplained bone pain or significant muscle weakness.
• Suspected osteomalacia or another metabolic bone condition.
• A medical condition affecting vitamin D absorption or metabolism.
• Recurrent low-trauma fractures or other relevant clinical concerns.
• A condition or treatment that requires medical monitoring of vitamin D.

A GP can determine whether testing is appropriate and whether other investigations are needed.

If a significant deficiency is identified, treatment may involve a medically supervised higher-dose replacement regimen rather than an ordinary daily supplement.

I would not recommend routinely paying for vitamin D blood tests without a clinical reason.

For most people, following established NHS guidance is a more practical approach.

Can you take too much vitamin D?

Yes.

Unlike some water-soluble vitamins, vitamin D is stored in body tissues and excessive supplementation can lead to toxicity.

Taking very high doses over time may cause hypercalcaemia, where calcium levels in the blood become abnormally elevated.

Possible symptoms include nausea, constipation, excessive thirst, frequent urination, weakness and confusion.

Severe cases can cause kidney damage and other complications.

Vitamin D toxicity is generally associated with excessive supplementation rather than ordinary dietary intake or sunlight exposure.

Adults should not exceed 4,000 IU (100 µg) daily unless specifically directed and monitored by a doctor.

People with certain medical conditions including some kidney disorders, high calcium levels or sarcoidosis should seek medical advice before supplementing.

It is also worth checking multivitamins and other supplements to avoid unintentionally taking overlapping doses.

Does vitamin D improve sporting performance?

As a personal trainer and competitive rower, I find this area particularly interesting.

Vitamin D has been investigated in relation to muscular strength, endurance performance, recovery, inflammation and injury risk.

There are plausible physiological reasons why adequate vitamin D status is important for athletes.

However, the clinical evidence needs to be interpreted carefully.

For someone who is deficient, correcting that deficiency is important for normal muscle and bone function.

But for athletes who already have adequate levels, research has not consistently demonstrated improvements in VO₂ max, maximal strength, endurance performance or recovery simply by taking additional vitamin D.

It would therefore be misleading to describe vitamin D as a performance-enhancing supplement.

My approach is to regard vitamin D as part of the foundations of good health rather than a shortcut to improved athletic performance.

The priorities remain consistent training, appropriate recovery, sufficient energy intake, good nutrition and adequate sleep.

Vitamin D helps ensure the body has the nutrients it needs to function normally.

My recommendations for the autumn and winter months

For most healthy adults living in the UK, the practical advice is relatively straightforward.

1. Consider taking 400 IU (10 µg) of vitamin D daily between October and March in accordance with NHS guidance.
2. If you are at higher risk of deficiency, consider supplementation throughout the year.
3. Vitamin D3 is a sensible supplement choice although vitamin D2 is also effective.
4. Include vitamin D-rich foods such as oily fish, eggs and fortified products in your diet.
5. Continue exercising outdoors throughout winter for the numerous benefits of natural daylight, fresh air and physical activity.
6. Don’t assume winter sunshine provides sufficient UVB to manufacture vitamin D.
7. Speak to your GP if you have symptoms or medical conditions that might warrant testing.
8. Avoid unnecessarily high supplement doses.

Final thoughts

Vitamin D is one of those nutrients that is easy to overlook until levels become too low.

Its importance for calcium absorption, bone health and normal muscle function is well established.  Plus it also contributes to the normal functioning of the immune system.

For those of us living in Britain, the seasonal reduction in UVB sunlight makes autumn and winter supplementation a sensible, inexpensive and evidence-based precaution.

Personally, I take 1,000 IU of vitamin D3 daily, although the standard NHS recommendation of 400 IU is sufficient for most healthy adults following general winter guidance.

As with so many aspects of health and fitness, the objective is not to take as much as possible but to ensure we are getting enough.

Whether you’re an athlete, someone who enjoys regular exercise or simply looking to maintain strength, mobility and good health as you get older, adequate vitamin D should form part of your overall approach to wellbeing.

Good health is built on consistent habits, not miracle supplements. Vitamin D is one small but important part of that picture.

Tony Larkman
Personal Trainer

Disclaimer: This article provides general health and nutritional information and is not a substitute for individual medical advice. Anyone with an existing medical condition, suspected deficiency or concerns about supplementation should consult an appropriate healthcare professional.

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