Which of the following does NOT affect skin hydration?

Dietary fat intake does not directly affect skin hydration in the same manner as the other options listed. While dietary fat plays a role in overall skin health, particularly in the maintenance of cell membranes and the production of sebum (the skin's natural oil), it does not directly influence the moisture content of the skin in the way that environmental humidity, temperature, and collagen levels do.

Environmental humidity affects how much moisture can be absorbed into the skin from the air; higher humidity levels can help the skin retain water, enhancing hydration. Temperature also plays a significant role; for instance, cold weather can lead to dryness and decreased hydration levels, while warmer temperatures often encourage perspiration that helps keep the skin moist.

Collagen is vital for skin structure and elasticity, and when collagen levels are adequate, they contribute to the skin’s ability to retain moisture. Hence, while dietary fat is important for skin health, it does not have the same direct effect on hydration levels as the other factors.