Vitamin D

What is vitamin D?

What is Vitamin D?Vitamin D refers to a group of related fat-soluble vitamins and is vital for good health throughout our lives. It plays many roles in our bodies including aiding in bone mineralization, muscle contraction, nerve conduction and helps regulate cell growth.

There are two significant forms of Vitamin D which are important for humans. One is vitamin D3 (cholecalciferol) which is formed in our bodies when sunlight penetrates our skin. A small amount of vitamin D3 is also available from a few animal foods. The other is Vitamin D2 (ergocalciferol) and is found in a small range of foods derived from plants.

Vitamin D2 and D3 are generally assumed to have equal levels of efficacy in humans, although recent data suggest that vitamin D3 may be more efficient at increasing serum 25-hydroxyvitamin D levels – this is the measure used in a blood test to determine vitamin D levels.

Sources of vitamin D

Sources of Vit DThere are three sources of vitamin D, the main one for Australians is from exposure to sunlight. It’s important to realise that although we live in a generally sunny country, many things can affect the amount of exposure to UV radiation and vitamin D synthesis in the body.

These include – season, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen. Also, UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D.1

The factors that affect vitamin D levels make it difficult to provide general guidelines on adequate amounts of sun exposure. It has been suggested by some vitamin D researchers, for example, that approximately 5–30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial tanning beds that emit 2%–6% UVB radiation is also effective. Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement to achieve recommended levels of intake.1

Vitamin D is also available through a small number of foods. The best food sources of vitamin D are fatty fish (sardines, herring, mackerel, tuna and salmon), milk (especially milks with added vitamin D), and vitamin D fortified soy drinks, margarines and dairy blend spreads. Red meat and eggs contain very small amounts of vitamin D. It is important to know that dietary sources alone rarely provide enough vitamin D to meet daily requirements. 2

The third source of vitamin D is from a vitamin supplement. Vitamin D tablets, capsules and liquids are available to ensure you are getting enough D vitamin. Supplements are available in many forms including tablets, capsules and liquids. The strength will vary but vitamin D is commonly available in a 1000 IU dose.

Benefits of vitamin D

Benefits of Vitamin DVitamin D affects several areas in the human body. Importantly, Vitamin D is essential for the absorption and use of calcium and phosphorus from the small intestine and is vital for normal mineralisation of bone. This is a major function of vitamin D.

Pregnancy and lactation are times of increased calcium and vitamin D requirements. During pregnancy, maternal D vitamin requirements can increase up to 4 to 5-fold to facilitate the availability of extra calcium required for fetal skeletal growth.

The developing baby is dependent on the mother for adequate vitamin D nutrition, which is important for healthy development of the skeleton. Adequate intake of vitamin D is also important during early pregnancy as it’s been shown to help maintain healthy children’s normal language development.

The need for vitamin D continues into childhood where growing children require Vitamin D for optimal bone mineralisation especially during times when sunlight exposure is limited.

In the elderly additional dietary vitamin D may be required as the ability of the skin to make vitamin D with exposure to sunlight decreases with age. A study has shown that taking 700IU of vitamin D daily reduces the risk of a fracture in two ways: by decreasing falls and increasing bone density. Adequate Vitamin D and Calcium intake is an important part of management of osteoporosis.

In addition to bone health, vitamin D may also affect muscle strength. A dose of 400IU of vitamin D daily may help to improve muscle strength, walking distance and functional ability in the elderly. It may also decrease the risk of falls in this group due to the strengthening action.

Research also suggests that vitamin D may help maintain a healthy immune system and help regulate cell growth and differentiation, the process that determines what a cell is to become.

How Much vitamin D is adequate?

The adequate intake of vitamin D is 5-15 mcg (200 IU – 600 IU) a day for adults, assuming some sun exposure.4 Other researchers have recommended that, for good health, the daily supply of vitamin D from all sources should be greater than 20mcg (800 IU).4

Diet is usually a poor source of D vitamin; most Australians only get around 10% of their vitamin D from dietary sources.5 Therefore it is important that we boost our levels through sunlight exposure and if that is inadequate, a vitamin D supplement.

In terms of vitamin D from sunshine, table 1 provides a good guideline to getting the right amount of sunshine to meet our vitamin D requirements.

Skin colour   Light to olive skin:Fitzpatrick type I – IV Naturally dark skin: Fitzpatrick type V – VI
Infants, children, adolescents Summer Full sun protection with sunscreen/hat. Encourage active outside play during and after school/preschool Able to tolerate intermittent sun exposure without sunscreen, hat still recommended. Encourage active outside play during and after school/preschool.
Infants, children, adolescents Winter Sun protection recommendations vary with latitude/UV index. Encourage active outside play during and after school/preschool Sunscreen not needed in Southern states/New Zealand. It may not be possible to maintain vitamin D levels through sun exposure in southern states of Australia/New Zealand. Encourage active outside play during and after school/preschool

Summer 6 – 7 minutes with neck, arms and hands exposed mid-morning or mid-afternoon most days of the week 15 – 50 minutes with neck, arms and hands exposed mid-morning or mid-afternoon most days of the week

Winter 7 – 40 minutes (depending on latitude) with face and arms exposed at lunchtime most days of the week Depends on latitude. It may not be possible to maintain vitamin D levels through sun exposure in southern states of Australia/New Zealand
Table 1. Sun Exposure Requirements 6

How can I determine my vitamin D level?

How can I determine my vitamin D level?It is difficult to predict if your body’s level of vitamin D is sufficient because so many factors can influence its status. To obtain a reliable result you will need to visit your GP and arrange to have a simple Vitamin D blood test. Your GP will be able to discuss the results with you and organise treatment accordingly.

What to ask for at your GP

What to ask for at your GPIf you are concerned about your body’s vitamin D levels speak to your GP. He will be able to arrange a simple blood test to determine your D vitamin status. See table 2 for blood test result information.

If your levels of are insufficient, your GP may also be able to suggest ways of obtaining more vitamin D including supplement recommendations. With treatment, your vit D levels should increase and reach desirable levels. If you are taking supplements to increase your vitamin d levels, it is wise to have regular medical check-ups to monitor changes.

There is no Australian RDI for Cholecalciferol, only Adequate Intake is advised

Vitamin D Status Blood Serum level
Mild vitamin D deficiency 25–50 nmol/L
Moderate vitamin D deficiency 12.5–25 nmol/L
Severe vitamin D deficiency less than 12.5 nmol/L
Table 2. Vitamin D Blood Serum Level Guideline6

Recommended daily intakes

There is no Australian RDI for Cholecalciferol, only Adequate Intake is advised. Table 3 shows vitamin D adequate intake amounts.

Age Group Adequate Intake
Children 1-18 years 5 mcg (200IU)
Men 19-50 years 5 mcg (200IU)
Men 51-70 years 10 mcg (400IU)
Men >70 years 15 mcg (600IU)
Women 19 – 50 years 5 mcg (200IU)
Women 51 – 70 years 10 mcg (400IU)
Women > 70 years 15 mcg (600IU)
Pregnancy 14 – 50 years 5 mcg (200IU)
Lactation 14 – 50 years 5 mcg (200IU)
Table 3. Adequate Intakes for vitamin D