Lactose, a sugar found in milk, aids in the absorption of calcium, which in turn is required for bone repair  In addition to there being shortages of milk in tropical areas, inhabitants of these areas lose the ability to absorb lactose, unlike people from nontropical areas
Yet inhabitants of tropical areas have no more problems with bone repair than do people who inhabit nontropical areas
The abundant sunlight in tropical areas causes the human body to produce vitamin D naturally, which aids in the absorption of calcium.