Vitamin D is a fat-soluble vitamin that acts more like a hormone than a typical nutrient. Its primary role in bone health is to regulate calcium absorption in the gut. It increases the efficiency with which dietary calcium enters the bloodstream. When vitamin D levels are adequate, the gut absorbs roughly 30–40 percent of dietary calcium. When vitamin D is deficient, that absorption can fall to as little as 10–15 percent. This means that even a calcium-rich diet delivers far less to the skeleton than intended.

CALLOUT: Vitamin D deficiency is remarkably common, including in sunny countries. Factors like sun avoidance, indoor lifestyles, darker skin, obesity, age-related changes in skin efficiency, and the use of sunscreen all reduce the body's ability to produce vitamin D from sunlight. Many women with osteoporosis are deficient without knowing it.

How vitamin D is made and obtained

The body produces vitamin D when ultraviolet B (UVB) rays from sunlight hit the skin. This is the primary source for most people. Dietary vitamin D alone is rarely sufficient to maintain adequate levels. The process is affected by:

  • Latitude and season: UVB rays are weaker at higher latitudes and in winter, and in some regions are insufficient to produce vitamin D for months of the year
  • Time of day: UVB is most available between approximately 10am and 3pm
  • Skin coverage and sunscreen: both significantly reduce vitamin D production
  • Skin pigmentation: darker skin requires longer sun exposure to produce the same amount of vitamin D
  • Age: the skin's ability to synthesise vitamin D declines significantly after age 50
  • Body weight: vitamin D is fat-soluble, meaning it is sequestered in fat tissue and less available in the bloodstream in people with obesity

QUOTE: "Vitamin D is sometimes called the sunshine vitamin. The reality is that modern life, sun protection habits, and the natural changes of ageing mean that many women simply do not produce enough. Blood testing is the only way to know for certain."

Dietary sources of vitamin D

Very few foods contain meaningful amounts of vitamin D naturally. The best dietary sources are:

  • Oily fish: salmon, mackerel, sardines, herring (100g serving): 200–600 IU depending on species and preparation
  • Egg yolks: approximately 40 IU per egg
  • Beef liver: approximately 50 IU per 100g
  • UV-exposed mushrooms: mushrooms exposed to sunlight or UV light during growing: variable but can be a meaningful source
  • Fortified foods: some milks, plant milks, cereals, and margarines are fortified with vitamin D, though levels vary by country and brand

CALLOUT: Oily fish is by far the best dietary source of vitamin D. Two to three servings per week makes a meaningful contribution particularly for women who have limited sun exposure. For most women with osteoporosis, however, supplementation is also necessary.

How much vitamin D do you need?

Vitamin D status is measured by the blood test serum 25-hydroxyvitamin D (25-OHD). General international evidence supports:

  • Sufficient levels: 50nmol/L or above (some guidelines use 75nmol/L as the optimal threshold for bone health)
  • Deficiency: below 30nmol/L
  • Insufficiency: 30–50nmol/L

In terms of daily intake, most guidelines recommend:

  • Adults under 70: 600–800 IU (15–20 micrograms) per day
  • Adults over 70: 800–1,000 IU (20–25 micrograms) per day
  • Women with diagnosed osteoporosis or confirmed deficiency: often 1,000–2,000 IU per day under medical guidance

CALLOUT: The only reliable way to know your vitamin D status is a blood test. If you have been diagnosed with osteoporosis and have not had your vitamin D level checked, ask your doctor at your next appointment. It is a simple, inexpensive test that directly informs how much you need to supplement.

Supplementation

For most women with osteoporosis, vitamin D supplementation is part of the treatment plan. This is because achieving adequate levels through sun and diet alone is difficult. Vitamin D3 (cholecalciferol) is the preferred form. It is more effective at raising blood levels than vitamin D2 (ergocalciferol).

Supplements are available in a wide range of doses. Many calcium supplements now contain vitamin D, which can simplify the daily routine. However, it is worth checking whether the combined dose is sufficient for your needs. Many combination supplements contain 400 IU of vitamin D, which may not be enough if your levels are already low.

Very high supplementation such as consistently above 4,000 IU per day over extended periods can cause toxicity. So supplementation should be guided by your blood levels rather than self-directed escalation.