Research, recently published in the journal PNAS, studied changes in human bone density over thousands of years of evolution. It discovered that the human skeleton has become much lighter since the advent of agriculture, when human lifestyles changed from hunter-gathering to the more sedentary behaviour involved in farming. Results showed that human hunter-gatherers from around 7000 years ago had bone densities comparable to that of Orang Utans, whereas farmers who inhabited the same areas 6000 years later had a significant reduction in bone density of around 20%, making their bones more susceptible to fracture.
Both dietary differences, as well as differences in body size, were ruled out as possible reasons for this decrease, and the conclusion was that the effect was due to a reduction in the vigorous exercise which was central to the lifestyle of the hunter-gatherer. Further research has suggested that this trend has continued to the modern day and has, if anything, accelerated over the past 50 to 100 years. As a consequence, bone densities in some populations have reached dangerously low levels.
It was found that the honeycomb-like trabecular structure within the bone was much thickened in hunter-gatherers, with a high ratio of bone to air. This thickening is the result of constant loading on the bone through physical activity of the hunter-gatherers’ constant activities in seeking food. Their exertions resulted in regular minor bone damage, which thickened and strengthened after repair. The modern-day human lifestyle does not sufficiently challenge our bones, and they are lighter and more prone to fracture later in life as a result.
Recent studies on children demonstrated a relationship between time spent engaging in vigorous exercise and increased femoral neck strength, both in terms of shape and mineral density, independent of other factors such as diet and body size. This supports the argument that increased physical activity in childhood is likely to improve childhood skeletal bone development. UK guidelines for daily levels of physical activity in children were revised in 2011, with the minimum recommendation of one hour per day of vigorous exercise. This is a potentially important public health strategy towards reducing the incidence of osteoporosis in later life. It has been estimated that half UK seven year olds fail to meet this minimum requirement.
Other research has demonstrated further links between exercise and improved bone health. For example, mesenchymal stem cells within the bones can differentiate into either bone or fat cell. In studies involving mice, a correlation was shown between increased exercise, and selective stem cell differentiation into bone, rather than fat cells. Bone cells improve the climate for blood stem cells to produce blood, and have a positive impact upon general health. Sedentary behaviour, on the other hand, leads to a build-up of fat cells which, when they start to fill up the bone marrow cavity, cause the blood stem cells to become less productive, and conditions such as anaemia can result.
References
1. Timothy M. Ryan and Colin N. Shaw. Gracility of the modern Homo sapiens skeleton is the result of decreased biomechanical loading. PNAS, December 22, 2014 DOI: 10.1073/pnas.1418646112
2. L. J. Griffiths, M. Cortina-Borja, F. Sera, T. Pouliou, M. Geraci, C. Rich, T. J. Cole, C. Law, H. Joshi, A. R. Ness, S. A. Jebb, C. Dezateux. How active are our children? Findings from the Millennium Cohort Study. BMJ Open, 2013; 3 (8): e002893 DOI: 10.1136/bmjopen-2013-002893
3. J. M. Baker, M. De Lisio, G. Parise. Endurance exercise training promotes medullary hematopoiesis. The FASEB Journal, 2011; DOI: 10.1096/fj.11-189043
