ABSTRACT
Water is essential for survival, but one in three individuals worldwide (2.2 billion people) lacks access to safe drinking water. Water intake requirements largely reflect water turnover (WT), the water used by the body each day. We investigated the determinants of human WT in 5604 people from the ages of 8 days to 96 years from 23 countries using isotope-tracking (2H) methods. Age, body size, and composition were significantly associated with WT, as were physical activity, athletic status, pregnancy, socioeconomic status, and environmental characteristics (latitude, altitude, air temperature, and humidity). People who lived in countries with a low human development index (HDI) had higher WT than people in high-HDI countries. On the basis of this extensive dataset, we provide equations to predict human WT in relation to anthropometric, economic, and environmental factors.
Subject(s)
Drinking , Life Style , Water , Female , Humans , Pregnancy , Exercise , Humidity , Social Class , Water/metabolism , Infant, Newborn , Infant , Child, Preschool , Child , Adolescent , Young Adult , Adult , Middle Aged , Aged , Aged, 80 and over , Drinking/physiologyABSTRACT
The thermoneutral zone (TNZ) defines the range of ambient temperatures at which resting metabolic rate (MR) is at a minimum. While the TNZ lower limit has been characterized, it is still unclear whether there is an upper limit, that is, beyond which MR during rest increases, and if so, what physiological upregulations explain this. We take the first step to fill this knowledge gap by measuring MR and multiple physiological variables in participants exposed to ambient heat stress while resting. Thirteen participants were exposed for an hour to 28â-50% relative humidity (RH) air, and both 40 and 50â each in 25% RH and humid (50% RH) conditions. Core and skin temperatures, blood pressure, sweat-, heart-, and breathing-rate, minute ventilation, and movement levels were recorded throughout each condition. MR increased 35% (p = .015) during exposure to 40â-25% RH compared to baseline and a further 13% (p = .000) at in 50â-50%RH. This was not explained by increased fidgeting (p = .26), suggesting physiological upregulation. However, while greater heat stress invoked increases in heart rate (64%, p = .000), minute ventilation (78%, p = .000), and sweat rate (74%. p = .000) when comparing 50â-50% RH with baseline, the exact size of their relative energy cost is unclear and, therefore, so is their contribution to this increase in MR. Our study shows clear evidence that resting MR increases in humans at high temperature-there is a metabolic upper critical temperature, at least as low as 40â. Further studies should pinpoint this value and fully explain this increased MR.
