The effect of acute hot water immersion on cutaneous peripheral microvascular responses in males of White-European, Black-African and South-Asian descent.

Plain-language summary

This study looked at how the smallest blood vessels in the skin respond to a hot bath, and whether that differs by ethnicity. Thirty-one healthy young men (10 White-European, 10 Black-African, 11 South-Asian) each did a 60-minute immersion in hot water at 39 degrees Celsius and, on a separate occasion, in neutral water at 36 degrees. Blood flow in the forearm and the big toe was measured in two ways: after a cuff briefly cut off circulation and then released it, and while the skin was warmed locally to 42 and 44 degrees. At rest the three groups were the same. The differences appeared only under challenge: Black-African men reached a lower peak blood flow after the cuff release in both the forearm and the toe, and a blunted response in the toe during local heating. The hot bath itself helped everyone equally, improving toe blood flow during the strongest local heating regardless of group. Thirty-one men is a small study, and this measured one session rather than a course of heat therapy.

Key findings

• At rest, skin blood flow was similar across all three groups.
• After circulation was briefly cut off and released, Black-African men reached a lower peak blood flow in both the forearm and the big toe.
• Black-African men also showed a blunted big-toe response during local heating at 42 and 44 degrees Celsius.
• A single hot bath improved big-toe blood flow during the strongest local heating, equally across all three groups.