Vitamin D has long been known for helping build strong bones and teeth, but scientists now understand that its role in the body goes much further.
Increasing research shows that low vitamin D levels can influence several hormone systems, affecting everything from mood and energy levels to reproductive health, immunity and metabolism.
Once it is produced in the skin through sunlight and activated by the liver and kidneys, it travels throughout the body, binding to vitamin D receptors found in almost every organ.
Health experts say vitamin D deficiency has become increasingly common worldwide due to modern indoor lifestyles, prolonged screen time, sunscreen use, obesity, poor diets and limited exposure to direct sunlight.
According to the World Health Organisation (WHO), vitamin D deficiency remains a significant global public health concern, particularly among populations with limited sun exposure or inadequate dietary intake.
Studies estimate that more than one billion people worldwide may have insufficient vitamin D levels.
Yet, vitamin D is vital in the body since it regulates hundreds of genes involved in cell growth, immune function and hormone production.
"When vitamin D levels fall, the body doesn't simply lose support for healthy bones. Several hormonal systems can also be affected," researchers say.
Thyroid hormones
The thyroid gland controls metabolism, body temperature and energy production.
Studies have shown that people living with autoimmune thyroid diseases, including Hashimoto's thyroiditis and Graves' disease, are more likely to have low vitamin D levels.
Scientists believe vitamin D helps regulate immune responses that may protect the thyroid from autoimmune damage, although vitamin D alone is not considered a treatment for thyroid disorders.
Insulin
Vitamin D also influences insulin, the hormone responsible for regulating blood sugar.
Researchers have found that deficiency is associated with reduced insulin sensitivity, increasing the risk of insulin resistance, prediabetes and Type 2 diabetes.
Scientists believe vitamin D supports healthy pancreatic beta cells, which produce insulin, while also helping reduce inflammation that contributes to insulin resistance.
Reproductive hormones
Vitamin D receptors are found in the ovaries, uterus and placenta.
Research suggests healthy vitamin D levels may support normal ovarian function, menstrual health and fertility.
Low vitamin D has also been linked with polycystic ovary syndrome (PCOS), one of the leading causes of infertility among women, although researchers caution that deficiency alone does not cause the condition.
Several fertility studies have found that women with adequate vitamin D levels may experience improved outcomes during assisted reproductive treatments.
Men's reproductive health
Vitamin D also appears to influence male reproductive health.
Vitamin D receptors exist within the testes, and several observational studies have associated healthy vitamin D levels with higher testosterone concentrations, improved sperm quality and better reproductive function.
However, scientists caution that more clinical trials are needed before vitamin D supplements can be recommended specifically for boosting testosterone.
Stress hormones
Vitamin D may also influence cortisol, the body's primary stress hormone.
Researchers say deficiency has been associated with fatigue, anxiety and depressive symptoms, although mental health is influenced by many biological, psychological and environmental factors.
Scientists continue investigating whether correcting vitamin D deficiency improves emotional well-being.
Mood and brain health
The brain contains vitamin D receptors in areas involved in mood regulation.
Vitamin D is believed to help regulate neurotransmitters, including serotonin and dopamine, chemicals involved in happiness, motivation, learning and emotional well-being.
While several studies have found an association between low vitamin D and depression, experts stress that supplementation is most effective for genuinely deficient people rather than as a universal treatment for depression.
Immunity and inflammation
Vitamin D also plays an important role in regulating the immune system. It helps control inflammatory responses while supporting the body's ability to fight infections.
This immune-balancing effect is one reason researchers continue studying vitamin D in relation to autoimmune diseases and chronic inflammatory conditions.
Signs you may have low vitamin D
Vitamin D deficiency often develops silently.
Some people experience no symptoms at all, while others may notice:
Persistent fatigue
Muscle weakness
Bone or joint pain
Frequent infections
Low mood
Difficulty concentrating
Slow wound healing
Hair thinning in some people
Because these symptoms overlap with many other medical conditions, doctors recommend blood testing rather than self-diagnosis.
Who needs vitamin D the most?
Although everyone requires vitamin D, some groups are at a much greater risk of deficiency.
Babies and infants: Vitamin D is essential for healthy bone growth during infancy.
Babies who do not receive enough vitamin D risk developing rickets, a disease that softens bones, delays growth and may lead to skeletal deformities.
Because breast milk naturally contains only small amounts of vitamin D, many paediatric organisations, including the American Academy of Pediatrics, recommend that exclusively or partially breastfed babies receive 400 IU (10 micrograms) of vitamin D daily from shortly after birth unless advised otherwise by a healthcare provider.
Toddlers and children: Growing children require vitamin D to absorb calcium and phosphorus, helping build strong bones, healthy teeth and muscles.
Children who spend little time outdoors or consume few vitamin D-rich foods may be more likely to become deficient.
Teenagers: Rapid growth during adolescence increases the body's demand for vitamin D.
Combined with increased screen time and more hours spent indoors, many teenagers may not receive enough sunlight.
Pregnant women: Vitamin D supports the baby's developing skeleton, teeth and immune system during pregnancy.
Research has linked low vitamin D levels during pregnancy with an increased risk of pre-eclampsia, gestational diabetes, premature birth and low birth weight, although scientists continue studying these relationships.
Breastfeeding mothers: After childbirth, women continue to require adequate vitamin D to protect their own bone health and muscle function.
Maintaining healthy vitamin D levels also supports maternal well-being, although breastfed infants generally still require their own vitamin D supplementation.
Older adults: As people age, their skin becomes less efficient at producing vitamin D from sunlight.
Combined with spending less time outdoors, this increases the risk of osteoporosis, fractures, falls and muscle weakness.
People with darker skin: People with darker skin naturally produce vitamin D more slowly because melanin reduces the skin's ability to absorb ultraviolet B (UVB) rays needed for vitamin D production.
This does not mean they require more vitamin D than other people. Rather, they often need longer exposure to sunlight to produce the same amount.
Those who stay indoors: Modern lifestyles mean many people spend most of the day inside offices, schools or homes.
Even in sunny countries like here, limited outdoor exposure can significantly reduce vitamin D production.
People with obesity or digestive disorders: Obesity, kidney disease, liver disease, Crohn's disease and coeliac disease may reduce the body's ability to absorb, store or activate vitamin D.
For most healthy people, sunlight remains the body's best natural source of vitamin D.
When ultraviolet B (UVB) rays strike the skin, they trigger vitamin D production within minutes.
Exactly how much vitamin D the body produces depends on several factors, including skin tone, time of day, season, cloud cover, clothing, sunscreen use, age and air pollution.
For people living near the Equator, including Kenya, experts generally recommend:
People with lighter skin: Around 10–20 minutes of direct midday sunshine on the face, arms and legs several times each week.
People with darker skin: Around 20–40 minutes of direct midday sunshine on exposed skin several times each week, as higher melanin levels reduce the rate of vitamin D production.
According to the NIH Office of Dietary Supplements, there is no exact conversion between minutes spent in the sun and International Units (IU) of vitamin D because production varies significantly between individuals.
However, research led by vitamin D expert Dr Michael Holick of Boston University estimates that exposing most of the body to enough sunlight to produce slight skin pinkness, well before burning, can generate the equivalent of approximately 10,000 to 25,000 IU of vitamin D in people with lighter skin.
People with darker skin can produce similar amounts but generally require longer exposure.
Unlike supplements, the body naturally limits vitamin D production from sunlight, making toxicity from sun exposure alone extremely unlikely.
Most healthy people who receive adequate sunlight and eat a balanced diet do not require high-dose vitamin D supplements.
However, supplementation may be recommended if:
A blood test confirms vitamin D deficiency.
You are pregnant or breastfeeding, and your healthcare provider recommends it.
You have osteoporosis or another bone disorder.
You spend very little time outdoors.
You have darker skin and limited sun exposure.
You have kidney, liver or digestive diseases that interfere with vitamin D absorption.
You are an older adult living mainly indoors.
Doctors caution against taking high-dose vitamin D without medical advice because excessive intake can cause vitamin D toxicity, resulting in dangerously high calcium levels, kidney stones, nausea, dehydration and heart rhythm problems.
Foods rich in vitamin D are also advised. Like Egg yolks, beef liver or fortified milk. Including these foods alongside safe sun exposure can help maintain healthy vitamin D levels.
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