Vitamin D Explained: The Nutrient You Cannot Afford to Ignore

Vitamin D is having a moment - and for good reason. Research continues to reveal just how many bodily functions depend on adequate vitamin D, from immune defence to mood regulation to metabolic health. Yet despite its importance, deficiency remains remarkably common.

If you live somewhere with real winters, spend most of your time indoors, have darker skin, or are over 50, there is a good chance your vitamin D levels are lower than optimal. Understanding this essential nutrient - what it does, why deficiency happens, and how to address it - can make a real difference in how you feel.

More Than a Vitamin

Vitamin D is technically a hormone precursor. When sunlight hits your skin, it triggers the production of vitamin D, which then gets converted in your liver and kidneys into its active hormonal form. This active form influences cells throughout your body - vitamin D receptors exist in nearly every tissue.

This is why vitamin D affects so much more than just bones.

What Vitamin D Does

Bone Health

This is vitamin D's most well-known role. Vitamin D is essential for calcium absorption - without it, you absorb only about 10-15% of dietary calcium instead of the 30-40% you need. Chronic deficiency leads to weak, soft bones (osteomalacia in adults, rickets in children) and contributes to osteoporosis.

Immune Function

Vitamin D is a powerful immune modulator. It helps activate immune cells that fight pathogens while also regulating inflammation to prevent overreaction. Low vitamin D is associated with increased susceptibility to respiratory infections and may play a role in autoimmune conditions.

Research has shown that people with adequate vitamin D levels have lower rates of respiratory infections, including colds and flu. During the COVID-19 pandemic, vitamin D status emerged as one factor influencing outcomes.

Mood and Mental Health

Vitamin D receptors are found throughout the brain, and the vitamin plays a role in neurotransmitter production. Low levels are consistently associated with depression, and seasonal affective disorder (SAD) is linked to reduced sun exposure and vitamin D production during winter months.

While vitamin D supplementation is not a cure for depression, optimising levels is often part of a comprehensive approach to mood support.

Muscle Function

Vitamin D is necessary for muscle contraction and strength. Deficiency is associated with muscle weakness, increased fall risk (particularly in older adults), and impaired physical performance. Athletes with optimal vitamin D levels tend to perform better than those who are deficient.

Metabolic Health

Vitamin D influences insulin sensitivity and glucose metabolism. Low levels are associated with increased risk of type 2 diabetes and metabolic syndrome.

Hormonal Balance

Vitamin D plays a role in hormone production, including thyroid hormones and sex hormones. Deficiency can contribute to hormonal imbalances.

Cardiovascular Health

Low vitamin D is associated with increased cardiovascular disease risk, though the relationship is complex and research is ongoing.

Why Deficiency Is So Common

Limited sun exposure. Modern life keeps most people indoors. Even when outside, sunscreen (necessary for skin cancer prevention) blocks vitamin D production.

Geographic location. If you live north of about 37 degrees latitude (roughly the line from San Francisco to Philadelphia in the US), the sun is not strong enough for vitamin D production during winter months, regardless of how much time you spend outside.

Skin pigmentation. Melanin reduces vitamin D production. People with darker skin need significantly more sun exposure to produce the same amount of vitamin D as those with lighter skin.

Age. The skin's ability to produce vitamin D decreases with age. Older adults produce about 25% as much vitamin D from the same sun exposure as younger people.

Body weight. Vitamin D is fat-soluble and can become sequestered in fat tissue, reducing circulating levels. People with higher body fat often have lower vitamin D levels.

Gut health. As a fat-soluble vitamin, D requires adequate fat absorption. Gut conditions affecting fat absorption (celiac disease, inflammatory bowel disease, etc.) impair vitamin D uptake.

Limited dietary sources. Few foods naturally contain significant vitamin D. Fatty fish, egg yolks, and mushrooms exposed to UV light provide some, but it is difficult to get adequate vitamin D from food alone.

Signs of Deficiency

Vitamin D deficiency can be subtle, with symptoms that develop gradually:

• Fatigue and tiredness
• Muscle weakness or pain
• Bone pain or tenderness
• Frequent illness or infections
• Depression or low mood, especially in winter
• Slow wound healing
• Hair loss
• Brain fog

Many people with deficiency have no obvious symptoms until levels are quite low - which is why testing matters.

Testing and Optimal Levels

A simple blood test measures 25-hydroxyvitamin D (25(OH)D), your circulating vitamin D level.

Conventional reference ranges often consider 30 ng/mL (75 nmol/L) as sufficient. However, many functional medicine practitioners and researchers suggest optimal levels are higher:

• Deficient: Below 20 ng/mL (50 nmol/L)
• Insufficient: 20-30 ng/mL (50-75 nmol/L)
• Adequate: 30-50 ng/mL (75-125 nmol/L)
• Optimal: 50-80 ng/mL (125-200 nmol/L)

If you have never had your vitamin D tested or have risk factors for deficiency, it is worth checking.

Raising Your Levels

Sunlight

Sun exposure is the most natural way to get vitamin D. For fair-skinned individuals, 10-20 minutes of midday sun on exposed arms and legs (without sunscreen) several times per week can produce adequate vitamin D during summer months. People with darker skin need longer exposure.

However, sun exposure is not practical year-round for many people, and the skin cancer risk of excessive sun must be balanced against vitamin D needs.

Food Sources

• Fatty fish (salmon, mackerel, sardines)
• Cod liver oil
• Egg yolks
• Mushrooms exposed to UV light
• Fortified foods (milk, orange juice, cereals)

Food alone rarely provides optimal vitamin D levels, but these sources contribute.

Supplementation

For most people, supplementation is the most reliable way to achieve and maintain optimal levels.

Vitamin D3 (cholecalciferol) is the preferred form - it is the same form your body produces from sunlight and raises blood levels more effectively than D2.

Dosing depends on your current level, body weight, and individual factors. Common maintenance doses range from 1,000-5,000 IU daily for adults. Higher doses may be needed initially to correct deficiency. Testing helps determine appropriate dosing.

Take with fat for better absorption - vitamin D is fat-soluble.

Consider vitamin K2. Vitamin D increases calcium absorption, and K2 helps direct that calcium to bones rather than soft tissues. Many practitioners recommend taking D3 with K2.

Monitor levels. If supplementing, retest after 3-6 months to ensure you are reaching optimal levels without going too high. While toxicity is rare, very high levels (above 100 ng/mL) can be problematic.

The Bottom Line

Vitamin D deficiency is common, consequential, and correctable. Given how many functions depend on adequate vitamin D - and how many people are unknowingly deficient - testing and optimising your levels is one of the simplest, highest-impact things you can do for your health.

If you are tired, getting sick frequently, struggling with mood, or just not feeling your best, vitamin D is worth investigating. It is not a cure-all, but for the many people who are deficient, it can make a meaningful difference.

Want to learn about other nutrients? Explore magnesium deficiency or understand iron deficiency.