Osteoporosis is a condition in which the skeleton loses bone mass and minerals, and bones become more porous, thinner and weaker, leading to an increased risk of a fracture. Osteoporosis is on the increase in New Zealand, partly due to the ageing population, but also to dietary and lifestyle influences. People usually equate osteoporosis with a poor intake of calcium, the menopause in women and older age. This is certainly true - but excess alcohol, protein malnutrition, smoking, some medications, lack of exercise and other lifestyle factors also impair bone health.

The important message about getting enough calcium in your diet, at all stages in your life, is often heard (see recommended intakes below). Most people already know that dairy products and other foods rich in calcium are essential for developing and maintaining strong bones. Correct nutrition for bone health means making sure everyone has adequate calcium and vitamin D, two key nutrients in bone health.

Calcium

Calcium is a vital bone-building material - protecting against osteoporosis - but it cannot do its job without adequate vitamin D. Bone also provides the "calcium reservoir" for maintaining correct calcium levels in the blood.

Recommended daily calcium intake*

• Children 1-7 years: 700-800mg
• Children 8-11 years: 800-900mg
• Boys 12-18 years: 1000mg
• Girls 12-18 years: 800-1000mg
• Men 19 years and older: 800mg
• Woman 19-54 years: 800mg
• Women pregnant: 1100mg
• Women breastfeeding: 1200mg
• Women after menopause: 1000mg

*Osteoporosis and Bone Related Diseases National Resource Centre, Washington DC

Milk and dairy foods are among the richest and most easily accessible sources of calcium - a 250ml glass of milk contains about 300mg calcium. Three servings of dairy foods a day usually ensures adequate calcium intake of around 1000mg per day. Other good sources are foods and beverages fortified with calcium, small fish with bones, nuts, tofu and certain vegetables (eg, broccoli, bok choy).

In one vulnerable group - teenage girls - calcium intake fell well short of the recommended 1000mg, a recent New Zealand study showed. Many adults also did not consume two to three dairy servings a day and were probably not meeting the recommended intakes. Bone density and strength can be compromised later in life if there is inadequate calcium consumption, particularly during the growing years and early adulthood.

Vitamin D

Vitamin D occurs naturally in small amounts in some foods (see below) but diet is not the main source - vitamin D is mainly created through the action of sunlight on the skin. Most diets (without supplements) are not thought to provide sufficient vitamin D. An international study of over 2500 postmenopausal women who have osteoporosis has found most of these women are deficient in vitamin D.

Vitamin D was first identified as being an essential nutrient when it was discovered children given cod liver oil (high in vitamin D) did not develop rickets (early age bone deformities). Later, it was found that exposure to sunlight had a similar effect (see below). And, even recently, a Waikato study has reported that, while figures are no longer routinely kept, cases of rickets in children are still appearing. A vitamin D deficiency during pregnancy is believed to be an important cause (the infant also starts life with a deficiency), as is extreme protection of children from sunlight.

Vitamin D and calcium should be considered of equal importance for maintaining healthy bones throughout your entire life. Vitamin D has many functions but bone health is an important one.

• Vitamin D allows calcium absorption; without it your digestive system cannot absorb as much calcium from food. 
• Vitamin D is needed for the proper mineralisation and growth of bone. 
• Studies have shown that correction of vitamin D and calcium deficiencies in elderly women reduces fracture rates.
• Correction of vitamin D deficiency separately reduces body sway and the risk of falling in elderly women, perhaps by improving nerve-muscle function.
• Evidence increasing points to a role for vitamin D in the immune system and regulating blood pressure.

Foods containing vitamin D

• Fatty fish, eg, salmon, mackerel, sardines (rich source)
• Cod liver oil (rich source)
• Liver
• Some fortified foods (moderate source), eg:
  -margarines
  -milk
  -dairy foods
  -cereals.

In New Zealand, foods fortified with vitamin D include some dried and liquid milks, yoghurts, dairy desserts, oil spreads and legume beverages (eg, soy milk).

Adequate vitamin D intake for most healthy people*

• Infants and children (up to 19 years): 5 micrograms per day (200IU)
• Men and women (19-50 years): 5 micrograms per day (200IU)
• Men and women (51-70 years): 10 microgams per day (400IU)
• Pregnant women (14-50 years): 5 micrograms per day (200IU)
• Breastfeeding women (14-50 years): 5 micrograms per day (200IU)
• Men and women (71 years and older): 15 micrograms per day (600IU)

*Institute of Medicine of the National Academy of Sciences (US)

Vitamin D: the "sunshine" vitamin

Vitamin D is often referred to as the sunshine vitamin - normally, 90% of our requirement comes from the action of sunlight (ultraviolet light; UV), which penetrates the skin and activates pre-vitamin D into vitamin D (through reactions in the skin, liver and kidneys). Vitamin D manufacture by the skin is normally much greater than the amounts obtained from food.

Exactly how much vitamin D is obtained from the ultraviolet light in sunlight, and therefore how much sun exposure is needed each day, depends on: 

• Skin pigmentation - darker skin needs more UV to produce vitamin D
• Season, time of day - the less intense the sun, the more is needed
• Duration of exposure to sunlight
• Coverage by clothing or sunscreen
• Whether sitting behind glass or plastic.

Consequently, those at risk for a vitamin D deficiency include: 

• Darker skinned adults
• Dark-skinned children, particularly if they are also iron deficient
• People whose skin is largely always covered for religious reasons or custom (especially women and children)
• Vitamin D deficient pregnant women who may predispose newborns to vitamin D deficiency
• Infants entirely breastfed beyond six months without sunlight exposure or vitamin D supplementation
• Older adults who are seldom outside (eg, housebound or in residential care) and are not receiving vitamin D supplementation.

How much sun exposure is needed?

It is hard to say exactly - estimates vary for each person, where they live and when they go outdoors. The Working Group of the Australian and New Zealand Bone Mineral Society (pdf file) makes the following estimates for people with moderately fair skin exposing their arms, hands and face to the sun (just from incidental sun exposure).

Auckland
Dec-Jan, 11am or 3pm  6-8 min
Jul-Aug, 10am or 2pm 30-47 min
Jul-Aug, 12 noon  24 min

Christchurch
Dec-Jan, 11am or 3pm  6-9 min
Jul-Aug, 10am or 2pm 49-97 min
Jul-Aug, 12 noon  40 min

If the adequate sun exposure is not possible a daily vitamin D supplement of 400IU (10 micrograms) is recommended. Dark-skinned people will require more sun exposure than fair skinned people. However, deliberate sun exposure between 11am and 4pm is not advised from early October to late March because of the risk from high UV levels.

Ultraviolet light is the radiation most commonly associated with sunburn and skin cancer.

During summer (Oct to Mar) most people venturing outdoors exposing their face, arms or legs should be able to achieve adequate vitamin D levels through incidental outdoor UV exposure outside peak UV times. Someone who burns easily may only need five minutes sun exposure before 11am or after 4pm to achieve adequate vitamin D levels. Someone who tans more easily or has darker skin will need more time (eg, up to 20 minutes in summer). These times are estimates.

During winter, particularly in southern New Zealand, where UV levels are dramatically lower, vitamin D status may drop below adequate levels and a supplement may be recommended, particularly for those at risk of vitamin D deficiency (see below).

Comments in Australian medical journals describe frank vitamin D deficiency in up to 8% of younger women at the end of winter and in 90% of veiled, dark-skinned pregnant women attending an antenatal clinic in Melbourne. A similar proportion (87%) with deficiency was found in a study of 90 pregnant women of all backgrounds and ethnicities attending a Wellington health service. Other researchers in Australia found up to 46% of elderly people in nursing homes had very low levels of vitamin D. The elderly are most at risk of developing osteoporosis and having falls and fractures if adequate calcium and vitamin D are not maintained.

Doctors are well aware of the osteoporosis risks and can help you identify whether you are at risk of a nutritional deficiency or developing osteoporosis. If you have a deficiency, supplementation of calcium and vitamin D may be recommended.
 
Original article written by Kerry McIlroy, partner, Auckland Dietetic Consultants, and updated by everybody, February 2008.