Reduced dietary intake induces body fluid hypotonicity via alterations in water and energy metabolism.

Maintenance of the body's fluid balance is essential for vital human functions. Although drinking water is widely recommended, the role of dietary intake in body fluids remains unclear. This study investigated whether overnight dietary restriction with adequate water consumption affects body water balance, its regulatory responses, and thermoregulatory and cognitive functions during exercise in a hot environment. Fifteen young adults experienced two conditions: sufficient meal intake (CON) and small dinner with skipped breakfast (RED), both with adequate controlled water consumption. Blood and urine samples, as well as indices of systemic circulation, thermoregulatory responses and cognitive functions, were obtained before, during and after a 60 min moderate-intensity treadmill exercise. RED induced lower serum osmolality and urine sodium excretion compared with CON (both P < 0.05), suggesting the development of body fluid hypotonicity with impaired urinary concentrating capacity. Water- and electrolyte-regulating hormone levels remained unchanged in RED (all P > 0.05). Systemic circulation and thermoregulatory responses (heart rate, blood pressure, skin blood flow and sweat rate) remained comparable between the conditions, leading to similar core body and skin surface temperature elevations (all P > 0.05). Moreover, cognitive performance (Go/No-Go, Stroop, reaction time and digit span tasks) was not changed in RED (all P > 0.05). Notably, RED increased oxygen uptake with a reduced respiratory quotient during exercise (both P < 0.05), indicating a metabolic shift toward lipid oxidation. These findings suggest that overnight dietary restriction induces body fluid hypotonicity with altered water and energy metabolism. KEY POINTS: Dietary intake provides substrates essential for water and osmotic balances, yet its role in maintaining hydration remains underexplored. This study assessed whether overnight dietary restriction (RED), with adequate water intake, induces dehydration and affects body functions during exercise in heat. RED induced body fluid hypotonicity, as evidenced by lower serum osmolality and sodium excretion, without changes in body weight or urine volume before and after exercise. RED led to reduced urinary concentrating capacity during exercise, which may reflect changes in the reabsorption of electrolytes and water in the renal tubules.