..we tested if calculated serum osmolarity (Sosm) and adequate intake (AI) of water among women (n = 1271) and men (n = 1235) ≥ 60 years old were associated with scores of immediate and delayed recall, verbal fluency, and attention/processing speed. Sosm was categorized as < 285 (hyperhydrated), 285–289, 290–294, 295–300, or > 300 (dehydrated) mmol/L. AI of water was defined as ≥ 2 L/day for women and ≥ 2.5 L/day for men.
..Women with Sosm between 285 and 289 mmol/L scored 3.2–5.1 points higher on the Digit Symbol Substitution test (DSST) of attention/processing speed than women in other Sosm categories (P values < 0.05)..
..Meeting an alternative AI on water intake of ≥ 1 mL/kcal and ≥ 1500 mL, but not the sex-specific AI, was associated with scoring one point higher on a verbal fluency test (P = 0.02) and two points higher on the DSST (P = 0.03) among women. Significant negative associations between dehydration or inadequate water intake and test scores were not observed among men.
Hydration status and water intake were moderately associated with attention/processing speed among females. Future work should consider the effects of both dehydration and overhydration on cognitive function and investigate potential sex differences in cognitive responses to hydration status.