Fruit and vegetables: potential role in building better bones
Global incidence of and risk for osteoporosis
In countries such as North America, Europe and Australasia it is estimated that 1 in every 3 women and 1 in every 10 men aged 55 years and over suffer from osteoporosis. Osteoporosis means ‘porous bones’ and describes the thinning of the inner honeycomb structure of bones which leads to an increase in bone fracture risk. Fractures are most common at the wrist, spine, and hip. Globally, around 1.7 million hip fractures occur each year; this is expected to increase four fold by 2050(1). Hip fracture rates are highest in Caucasian women living in temperate climates, slightly lower in women from Mediterranean and Asian countries and lowest in women living in Africa(2). The lifetime risk of fracture in women aged over 50 years is greater than the risk of breast cancer and cardiovascular disease(3). As the incidence of fracture increases with age in both sexes, osteoporosis also becomes a major problem for older men too. In countries with high fractures rates, 20% of symptomatic spine fracture and 30% of hip fractures occur in men(4).
Osteoporotic fractures are a major cause of morbidity. Fractures often result in loss of mobility and long-term functional incapacity which leads to loss of independence. In some cases, fractures can lead to premature death. As well as the huge personal cost to the individual, it is estimated that the associated annual health costs for osteoporotic fractures in Europe is over 13.9 billion Euros.
Diet, fruit and vegetable intake, and risk for osteoporosis
Bone health is influenced by genes, hormonal status and lifestyle factors such as levels of physical activity, smoking and dietary intake. Diet is an important, modifiable risk factor for osteoporosis in later life. It is well known that an adequate intake of dietary calcium and vitamin D status is important for bone health. However, fruits and vegetables are emerging as another important food group.
In an ancillary study of the Dietary Approaches to Stop Hypertension (DASH) trial, men and women (aged 23-76 years) who consumed a diet rich in fruit and vegetables over three months had significantly lower markers of bone turnover(5). Several other observational studies also demonstrate this association in more specific age groups. In a cross-sectional study, intakes of zinc, magnesium, potassium, fibre and vitamin C were associated with higher bone mass in premenopausal women(6). These relationships were independent of factors such as weight, height, total energy intake, smoking and physical activity. In older adults (69-93 years) in the Framingham Osteoporosis Study, those who had a diet rich in fruit and vegetables had significantly higher bone mineral density than individuals whose diets were lower in fruit and vegetables and higher in confectionery(7).
The observational data for younger people is slightly less consistent, although the trends are in a similar direction. McGartland et al. found that higher intakes of fruit resulted in higher heel bone mineral density in girls aged 12 years but not boys(8), whereas Valanparast et al. found that fruit and vegetables significantly predicted total body bone mineral content in boys but not girls aged 8-20 years(9).
Potential mechanisms for fruit and vegetable intake and reduction in risk for osteoporosis
It is not entirely clear how fruits and vegetables might have positive effect on bone. There are likely to be several plausible mechanisms. Humans eat foods that produce and consume hydrogen ions that affect acid-base balance. Foods such as cereals, dairy products and meat are associated with a higher dietary acid load relative to alkali forming foods such as fruit and vegetables. The tight control of extracellular fluid between a pH of 7.35 and 7.45 is essential for survival and it has been known for several decades that alkaline bone mineral contributes to the maintenance of the body’s pH. The skeleton acts as a buffer by liberating calcium from bone that is ultimately excreted in urine. Diets rich in fruit and vegetables may result in a more alkaline extracellular fluid which, in theory, reduces the need to draw on skeletal calcium supplies. In the DASH study, increasing fruit and vegetable intake from 3.6 to 9.5 portions a day decreased urinary calcium excretion from 157 mg/d to 110 mg/d(10).
The positive effect of fruits and vegetables on bone health may not be entirely due to the effect on acid-base balance. Fruit and vegetables are also rich sources of vitamins C, K1 and beta-carotene. Vitamin C plays a role in collagen formation and plays a regulatory role in osteoblast differentiation. Vitamin C and beta-carotene are antioxidants. Oxidative stress has been shown to be negatively associated with bone mineral density in men and women(11). In an observational study a significant positive association was found between fruit and fruit juice (a major source of vitamin C) and bone mineral content in boys aged 16-18 years(12).
Osteocalcin plays a role in the calcification of bones. Vitamin K1 is essential for the alpha-carboxylation of osteocalcin, this allows the protein to function properly. The richest source of vitamin K1 is green leafy vegetables. Population studies have shown that low dietary or circulating vitamin K are associated with low bone mineral density. Supplementation of postmenopausal women (n=45) with low bone mineral density with 80 μg of vitamin K1 daily for 12 months resulted in an increase in alpha-carboxylated osteocalcin to similar levels observed in women with normal bone mineral density(13).
Osteoporosis presents a growing public health problem and the associated personal and health care costs are considerable. Fruit and vegetables are already a key part of public health strategies across Europe to reduce the risk of cancer and CVD(1). Emerging evidence of the protective effect of this food group on bones adds to the case for initiatives to encourage greater consumption of fruit and vegetables. However, more research is also needed to elucidate the mechanisms of action and perhaps most importantly, into effective methods to increase consumption among a population who need more than just research to persuade them to eat more fruit and vegetables.
- WHO Technical Report Series 916. Geneva, 2003
- Royal College of Physicians. Osteoporosis, Clinical Guidelines for Prevention and Treatment. London, 1999.
- WHO Technical Report Series, Assessment of fracture risk and its application to screening for post menopausal osteoporosis.1994;843: 1-129.
- Prentice A. Public Health Nutr. 2004; (1A):227-243.
- Lin PH et al. J Nutr. 2003;133:3130-3136.
- New SA et al. Am J Clin Nutr. 1997;65:1831-1839.
- Tucker et al. Am J Clin Nutr. 2002;76:245-252.
- McGartland CP et al. Am J Clin Nutr. 2004;80:1019-23.
- Vatanparast H et al. Am J Clin Nutr. 2005;82:700-706.
- Appel LJ et al. NEJM. 1997;336:1117-1124.
- Basu S et al. Biochem Biophys Res Commun. 2001;288:275-279.
- Prynne CJ et al. Am J Clin Nutr. 2006;83:1420-428.
- Schaafsma A et al. 2000;54:626-631.