Dairy Products and Osteoporosis
Bone tissue is constantly renovated, throughout all life skeleton is continuously destroyed and rebuilt through a process known as remodeling mediated by bone cells: osteoblasts operating matrix deposition and osteoclasts that carry out bone resorption. Under physiological conditions, within the third decade of age, physical activity, the correct intake of calcium and the hormones action promote bone formation exceeding resorption, in the middle age the balance of bone remodeling remains neutral, switching instead in favor of bone resorption in old age. Osteoporosis is the condition in which the skeleton is subject to bone mass loss and is caused by nutritional, metabolic, pathological factors and/or by aging. Thus, the skeleton becomes subject to a greater risk of pathological skeletal failure (fractures), following the reduction of bone density and alteration of the microarchitecture of bones (Harvey et al., 2009). For many years researchers have been studying therapeutic approaches, including food and supplements intake, to counteract bone mass loss.
It is well known that milk, dairy products, and cheese have a high calcium content, which is an important nutrient for the prevention and treatment of osteoporosis; nevertheless, the currently available data concerning the relationship between dairy products and osteoporosis are conflicting.
Dairy products are commonly known as the major source of calcium in our diet: Recent studies indicated that 1000-1300 mgs is the right amount of calcium intake per day, as recommended by the Food and Nutrition Board, National Academy of Sciences (USA). However, the consumption of higher amounts is not linked with a fracture risk decrease; thus, studies performed on post-menopausal women demonstrated that a high-calcium diet did not appear to reduce risk (Feskanich et al., 2003). Mainly, the efficacy of calcium intake on osteoporosis and fracture risk prevention is conditioned by the concomitant assumption of Vitamin D, which is lacking in all dairy products. Without adequate Vitamin D consumption, only 10% to 15% of the calcium can be utilized for building new bone (Holick and Garabedian 2006).
Vitamin D has a crucial role in bone metabolism. When the blood calcium level decreases, the organism promotes the conversion of vitamin D into its active form, which in the intestines activates the absorption of calcium and in the kidney stimulates its reabsorption. Thus, in order to be useful to bone health, dairy products have to be supplemented with vitamin D, especially in countries above 40° degrees of latitude where the sun ultraviolet irradiation is reduced.
In children during the early growing years, avoidance of milk consumption for a long period is of relevant consequence for their bone growth (Black et al., 2002). Volunteers of both sexes aged 3 to 10 years who had a history of avoiding consumption of cow's milk for more than four months at some point in their lives have been object of study. The children who avoided milk had a very low calcium intake in their diet and lower bone mass compared with children who habitually drank milk. The milk avoiders were also significantly lower height and had a higher Body Mass Index (BMI) than children of control. However, a recent study indicated that, some years later, in adolescents (13-18 years) increased consumption of milk was not associated with a lower risk of hip fracture when they become older adults (Feskanich et al., 2014). The association between milk consumption and bone mineral density augmentation is effective in childhood rather than in adult age, in fact, during the growth, an adequate nutritional calcium intake is essential for the acquisition of strong and healthy bones (Heaney et al., 2000). Two large Harvard studies found no association between high calcium intake and fracture risk in adulthood. A 12-year prospective study examined if a higher intake of milk and other calcium rich-foods could reduce the risk of osteoporosis in women (Feskanich et al., 1997). In the study, 77,761 women aged between 34 and 59 years were analyzed. Among them, women who drank two or more glasses of milk/day had the same risks for hip and forearm fracture compared to them who consumed one glass or less per week. Analogous results were achieved by a similar study carried out on male individuals (Owusu et al., 1997). In that study, 51,529 male health professionals, who were 40 to 75 years of age were followed for 8 years. Men who consumed more than 2.5 glasses of milk/day (600 mL) showed no protective effect against either forearm or hip fracture compared with men which drank 1.0 (240 mL) per week.
Recently, a study conducted in northern Europe on two large Swedish cohorts, one with 61,433 women (39-74 years) and one with 45,339 men (45-79 years) indicated that high milk intake was associated with higher mortality both in women and men; moreover, only in women was it associated with an higher fracture risk. Differently, women with a high intake of dairy products compared with women with a lower one showed lesser mortality and fracture rates. All the participants of the study constantly described their consumption of milk and dairy products. The most accredited hypothesis for these results depends on the high content of lactose in milk, which is much lower in most of dairy products. Lactose, and in particular its derived galactose, if given experimentally to animals determine premature aging with consequent shortened life span for induced oxidative stress and chronic inflammation (Michaelsson et al., 2014).
Concluding nowadays, although further studies are needed, high milk consumption could be recommended during childhood but is useless and probably even contraindicated in the rest of life. Dairy products intake, which is useful in the first part of life, remains safe in aging, but for both milk and milk derivatives it is advisable to supplement with vitamin D to obtain skeleton benefits.