https://www.ncbi.nlm.nih.gov/pubmed/9024231 https://www.ncbi.nlm.nih.gov/pubmed/9024231 > Adjusted bone density at various skeletal sites was 4.5-16.1% higher

https://www.ncbi.nlm.nih.gov/pubmed/9024231 https://www.ncbi.nlm.nih.gov/pubmed/9024231 > Adjusted bone density at various skeletal sites was 4.5-16.1% higher for black than for white men and was 1.2-7.3% higher for black than for white women Initially I thought that a bone-density increase of 16% would not significantly affect buoyancy, but the human body has relative-density of 0.98 compared to water... with a margin that narrow I can imagine that a 16% increase in bone density would tip someone to have a >1.0 relative-density compared to water - which then makes swimming difficult.