Vitamins from leafy vegetables and bone health
Targeting nutrition as a possible way to achieve optimal bone health
The World Health Organisation has identified osteoporosis as the second leading health care problem after cardiovascular diseases. The hallmark of osteoporosis is bone fragility. It affects some 200 million people, and the number of sufferers is expected to increase steadily as the human race experiences a progressive lengthening of life expectancy.
There is an overwhelming body of evidence indicating that nutrition is an important modifiable factor in the development and maintenance of bone mass. Although most studies have focussed on the effect of calcium and milk products on bone accrual, the role of vegetables and fruit in the diet is emerging, suggesting complementary lines of prevention. Indeed, the cause of osteoporosis is multifactorial. Oxidative stress induced by reactive oxygen species plays an important role as well in the pathophysiology of primary osteoporosis, and has been positively associated with the risk of fracture. This provides a rationale for investigating the role of dietary antioxidants in osteoporosis, which can act as free radical scavengers. Even though there was a lack of relation between plasma levels of vitamin and mineral antioxidants and bone mineral density in the Women’s Health Initiative trial, antioxidant intake such as ß-carotene, vitamin E or vitamin C has been shown to be lower in osteoporotic women and was even associated with reduced risk of fracture in smokers in the UTAH study. Thus, it is worthwhile to consider specific foods, like leafy vegetables which are typically rich in phytochemicals such as carotenoids, and high in vitamins C, B, E and K, and the mineral calcium, while being low in calories.
Leafy vegetables are a rich source of micronutrients that may be important for bone health
Nearly one thousand species of plants with edible leaves are known. Leafy vegetables most often come from short-lived herbaceous plants and share a great deal with other vegetables in nutrition and cooking methods. Basically, 100g of lettuce will provide 30% of the recommended dietary allowance (RDA) for folic acid, 18% for provitamin A and 10% for vitamin C. 200g of spinach will bring about 3% of the RDA for folic acid and half of the RDA for vitamin C.
Micronutrients and bone health
Carotenoids: Conflicting research suggests a potentially complex relationship with vitamin A and bone: a low bone mineral density (BMD) and an increased risk of hip fracture have been observed in women with a high dietary intake. Conversely, observational studies have revealed that nonsupplemented subjects with higher dietary intake of retinol (derived from both retinoids from animal food and carotenoids contained in vegetables and fruits) lose less bone with age than subjects with a lower intake. Moreover, in free-living elderly women, plasma levels of retinol and of all carotenoids tested have been consistently lower in osteoporotic than in controls. This suggests a bone sparing effect of retinol, to which the provitamin A activity of some carotenoids might have contributed.
B Vitamins: Current research indicates that increased dietary intake of folic acid along with the elimination of unhealthy lifestyle practices, will lower plasma levels of homocysteine (Hcy), a sulfur-containing intermediate product in the metabolism of the essential amino acid methionine. Certain forms of Hcy may cause damage through oxidative effects and negative protein interactions.
Regarding bone, an elevated level of Hcy is not only a risk factor but also a factor in abnormal bone metabolism. Indeed, genetic hyperhomocysteinemia is associated with skeletal abnormalities and osteoporosis (LASA, Rotterdam and Framingham studies). Many studies lend support to the idea that raising B-vitamin status might be beneficial to bone, because they are involved in the metabolism and clearance of Hcy.
Reduced levels of markers for bone formation have been reported in vitamin B12-deficient individuals. In the same way, both men and women with levels of vitamin B12 lower than 148 pm have lower BMD. Hip fracture patients have been shown to a reduced vitamin B6 and folate intake, as well. A recent double blind placebo-controlled intervention trial demonstrated that a Hcy-lowering therapy with 5 mg folate and 1.5 mg vitamin B12 impressively reduced the hip fracture rate in hemiplegic stroke patients (10 fractures per 1000 patients against 43 in the placebo group).
Vitamin C: Vitamin C is a known potent antioxidant that could reduce harmful effects of free radicals. Other mechanisms through which it may contribute to BMD are speculated to be related to its role in collagen formation of bone matrix, its effect on osteoblast growth or its role in promoting calcium absorption.
Vitamin E: Plasma vitamin E concentrations have been found to be lower in osteoporotic women. Among current smokers, there is an association between low intakes of dietary vitamin E and increased risk of fracture. It has been shown to reduce pain in RA patients as well, possibly by decreasing proinflammatory cytokines and lipid mediators.
Vitamin K: Vitamin K, usually associated with leafy vegetables is an important factor in proper blood clotting and bone metabolism, and has emerged as a potential protector against osteoporosis. Vitamin K mediates the alpha carboxylation of glutamyl residues on several bone proteins, notably osteocalcin. It exhibits antioxidant properties, as well. This may explain why cumulative epidemiological evidence suggests that subclinical vitamin K deficiency contributes to age-related bone loss and osteoporosis fractures. Moreover, low doses of phylloquinone (1 mg/d) have been demonstrated to exert protective effects on BMD in postmenopausal women.
Fruit and vegetables may have a potential promise for targeting the prevention of postmenopausal osteoporosis by maintaining or improving bone mass of human subjects. Thus, any dietary and behavioural recommendations targeted to the global health of the population should take a comprehensive consideration of their potential bone sparing properties.