Although calcium and vitamin D are important for building and maintaining bone health, there are a number of other important nutrients.
Vitamin K is needed for the production and functioning of a compound called osteocalcin. Osteocalcin is the second most abundant protein in bone after collagen, and it is required for bone mineralization. Some evidence suggests that low dietary intake of vitamin K or low vitamin K status could contribute to low BMD and increased risk of fragility fractures in the elderly, via a reduced functioning of osteocalcin. However, the possible mechanisms whereby suboptimal vitamin K intake and status affect bone metabolism are not well understood at present, and there is not yet adequate clinical trial evidence that adding vitamin K would be effective in either preventing or treating osteoporosis. Good food sources of vitamin K include leafy green vegetables such as lettuce, spinach, cabbage and kale, liver, and some fermented foods such as fermented cheeses and natto (fermented soybeans).
B vitamins and homocysteine
Recent observational studies suggest that high homocysteine levels in the blood may be associated with lower BMD and increased hip fracture risk in older persons. Homocysteine is an amino acid (amino acids are the building blocks of proteins) that has been linked with increased risk of cardiovascular disease. It is thought that it might also have adverse effects on bone, by interfering with the formation of the main protein in bone, collagen. Homocysteine levels in the blood may rise if there is inadequate intake of vitamin B6, vitamin B12 and folic acid, which play a role in the chemical reactions that change homocysteine into other amino acids for use by the body, and therefore help to keep it out of harm’s way. No intervention trials have yet been carried out to determine whether supplementation with any or all of these B vitamins reduce fracture risk, so it is not yet known whether deficiencies in these vitamins are potentially modifiable risk factors for osteoporosis.
The mineral magnesium is involved in calcium homeostasis and in the formation of bone mineral. Severe magnesium deficiency results in abnormal bone structure and function, but rarely occurs in those who follow a balanced diet. Magnesium is fairly easy to ingest in foods like green vegetables, legumes, nuts, seeds, unrefined grains, and fish. Older people can potentially be at risk of mild magnesium deficiency, as magnesium absorption decreases and renal excretion of magnesium increases with age. Older persons are also more likely to be taking certain medications that can increase magnesium loss in the urine, such certain diuretics, cancer medications or antibiotics. There are currently no studies which demonstrate that magnesium supplementation is useful either in preventing bone loss or reducing fracture risk.
The mineral zinc plays a role in the regulation of bone turnover. Zinc is also needed for the correct functioning of an enzyme called alkaline phosphatase, which is required for bone mineralization. Zinc deficiency is usually seen in severely malnourished children and can be associated with impaired bone growth. Milder degrees of zinc deficiency have been reported in the elderly and could potentially contribute to poor bone status. Zinc is most abundant in lean red meat and meat products, with poultry, whole grain cereals, pulses and legumes also being significant sources.
An adequate dietary protein intake is essential for bone health. Insufficient protein intake is detrimental both for the acquisition of bone mass during childhood and adolescence, as well as for the preservation of bone mass with ageing. Poor nutritional status, particularly with respect to protein, is common in the elderly, and appears to be more severe in patients with hip fracture than in the general ageing population. Insufficient protein also leads to less muscle mass and strength which in itself is a risk factor for falls. Randomized clinical trials in elderly patients with hip fracture have demonstrated the beneficial effects of giving protein supplements on the clinical outcome following surgery to repair the fracture. Protein supplementation resulted in fewer deaths, shorter hospital stays, and a greater likelihood of return to independent living.
Despite the research evidence that dietary protein is beneficial for bone health, and for recovering hip fracture patients, there has nevertheless been speculation that a higher dietary protein intake could have negative effects on calcium metabolism and possibly induce bone loss. This relates to the hypothesis that the ‘acid-base balance’ of the diet is a potential risk factor for osteoporosis. As foods are digested, absorbed and metabolized by the body, they produce chemicals which are acidic, neutral or basic. When acid is produced, it needs to be buffered (neutralized) in order to maintain the blood pH at optimum levels for the cells in the body. This buffering occurs by the action of the kidneys (excrete the acid substances) and the lungs (exhale carbon dioxide). Foods can be ranked according to whether they produce acid or base on a scale referred to as their Potential Renal Acid Load, or PRAL50.
For example cereals, grains, rice, pasta, certain hard cheeses, fish and meat are acid-producing and have a higher PRAL value than fruits and vegetables, which are almost all alkaline, base-producing foods (they contain alkaline salts of potassium, calcium and magnesium). It is sometimes stated that milk is an acidic food that ‘leaches’ calcium from the skeleton, but this is not the case – milk is in the category of foods that are essentially neutral, neither particularly acidic nor basic. Milk is a rich source of dietary calcium, protein and other nutrients, and supplementing the diets of children or adults with milk has been shown to improve bone mineral density. It has been theorized that if the diet provides predominantly acidic foods (which includes key protein sources) and does not contain sufficient alkali-rich basic foods, then the alkaline salts of the skeleton may be drawn on in order to help with the buffering process, following which some of the calcium from bone is lost in the urine.
In summary, the majority of scientific evidence – including that from clinical trials – supports beneficial effects of protein intake on bone health, and highlights the risks associated with protein insufficiency and malnutrition.
Fruits and vegetables
In population-based observational studies, higher fruit and vegetable consumption has been demonstrated to have beneficial effects on bone mineral density in elderly men and women. This could be related to the fact that fruits and vegetables supply alkaline salts in the diet, which assist in maintaining the acid-base balance of the body by helping to buffer the effects of the more acid-producing foods. However, fruits and vegetables contain a whole array of vitamins and minerals, antioxidants and possibly other types of bioactive compounds, so the exact components which may confer a benefit to bone are still to be clarified. Evidence for a beneficial effect of fruits and vegetables on bone health was provided by the Dietary Approaches to Stopping Hypertension (DASH) intervention trials, carried out in men and women of ages 23-76 years.
Although the DASH studies were designed to investigate how diet could prevent heart disease, one of them investigated whether the total dietary pattern could also influence bone health. In the bone study, half of the subjects were asked to modify their whole dietary pattern, and consume a diet rich in fruits, vegetables and low-fat dairy products and low in sodium (the DASH diet), and the other half continued with their regular diets. Over a period of a few months, the DASH diet improved markers of bone and calcium metabolism (chemicals in the blood), which could potentially result in improved bone density if continued over the long term. Higher fruit and vegetable consumption has been demonstrated to have beneficial effects on bone mineral density in the elderly.