A dietary perspective on nutrient dense foods for bone health
Bone is a living tissue that is remodeled throughout life. Sufficient intakes of the minerals calcium and phosphorus are essential for bone mass and bone quality throughout the life cycle. In the case of calcium deficiency, bone resorption is enhanced and bone mineralization is inhibited. The major purpose of these processes is to maintain serum calcium homeostasis as calcium fulfills essential roles in various body functions, such as contraction of muscles, nerve impulses, blood clotting and cellular communication. In other words, calcium homeostasis gets higher priority than maintaining bone structure and bone serves as an important calcium reserve.
Besides calcium and phosphorus, nutrients such as magnesium and zinc, the vitamins C and D, and protein are approved for bone health claims, especially in relation to bone maintenance.¹ In children normal growth and development of bones are highly dependent on calcium, phosphorus, protein and vitamin D.¹
Potassium is associated with bone health through effects on the acid-base balance in the blood and/or reduced excretion of calcium into urine.² A low vitamin B12 intake / status has been linked to elevated homocysteine concentrations which on their turn were associated with lower bone mineral density (BMD) and higher fracture risk in elderly.³ What strikes is that there are no permitted health claims on individual foods or dietary patterns, although it is well-known that some food categories, such as dairy, provide a number of these bone relevant nutrients in substantial amounts. The problems that investigators face when trying to establish firm proof for nutrition related disorders of health have been aptly addressed by Heaney (2008) in his Atwater Memorial Lecture.4 Most nutrients act in all tissues, all tissues need a lot of nutrients, and inadequate intake impairs many body systems. Nutrients work together, rather than in isolation, and often their effects will not develop when the intakes of other nutrients are suboptimal. Interdependencies among nutrients may well be a part of the explanation for the heterogeneity of results from different research centers and investigators. This “orchestra function of nutrition” needs to be taken into account when investigators try to establish evidence-based recommendations for endpoints of (bone) health.
Recently, a number of longitudinal studies have been published that take a dietary perspective on bone health, in young children, adolescents and adults5-9
Eating patterns of Dutch elderly participating in the prospective Rotterdam cohort study (n = 5144) were ranked according to a newly developed BMD-Diet Score.5 The BMD-Diet Score considered intake of vegetables, fruits, fish, whole grains, legumes/beans and dairy products as “high-BMD” components and meat and confectionary as “low-BMD” components. BMD of the study participants was assessed using dual energy X-ray (DXA) absorptiometry at baseline and 3 follow-up visits. The BMD-Diet Score was positively associated with BMD and its effect size was approximately three times as large as observed for a Healthy Diet score based on WHO dietary guidelines. The development of this BMD-Diet Score was based on a variety of study populations, including Caucasian and Asian subjects. This analysis further showed that the association between the BMD-Diet Score and BMD remained significant after additional adjustment for dietary calcium, which indicates that calcium intake is important, but does not fully explain the favorable association between the BMD-Diet Score and BMD.
Dietary patterns in relation to BMD were also analyzed in a study with Chinese postmenopausal women.6 Out of 6 dietary patterns identified only a cereal grains-fruits pattern and a milk-root vegetables pattern were positively associated with BMD of the spine and hip. Moreover, lean mass was a better predictor of BMD than fat mass.
An Australian study7 with adolescents revealed that a dietary pattern with relatively high intake of protein, calcium and potassium (correlating with low fat dairy products, vegetables and whole grains) at mid adolescence was associated with a more favorable BMD at the age of 20.
The Dutch Generation R study included 2850 children and investigated how food intake assessments of infants at 13 months correlated with DXA scans of bones at the age of 6.8 Principal Components Analysis was used to construct dietary patterns from food groups. Subjects were then ranked according to adherence to a certain dietary pattern, described as “potatoes, rice and vegetables” or “refined grains and confectionary” or “dairy and whole grains”. It was observed that a pattern with higher intakes of “dairy, whole grains” resulted in a higher BMD during childhood. This dietary pattern results in higher relative intakes of (animal) protein, calcium, phosphorous, magnesium and vit B12. Despite more use of vitamin D supplements in this dietary pattern group, the children in the highest quartile of the two other dietary pattern groups had higher vitamin D intakes. Open for further research remains the question why the beneficial effect on BMD in the “dairy/whole grains” group was more explicit in the children without vit D supplementation.
The same cohort was also analyzed with respect to dietary acid load and bone health development during childhood. No consistent associations were found, however, which confirms that animal protein, known to increase dietary acid load, supports bone mass accrual in young children.9
Finally, it should be realized that vitamin D and its metabolites can fulfill catabolic as well as anabolic functions for bone homeostasis. In case of sufficient calcium supply, vitamin D and its metabolites can improve the calcium balance and facilitate mineral deposition in bone matrix largely without direct effects on bone cells. In case of calcium deficiency, however, [1,25(OH)2D] enhances bone resorption, whereas simultaneously inhibiting bone mineralization, so as to defend serum calcium homeostasis at the expense of bone mass. This further underscores the fact that nutrients work together, rather than in isolation, and that individual foods or dietary patterns should be taken into account for endpoints of (bone) health.