Errors in the estimation of hydration status from changes in body mass

Abstract

Hydration status is not easily measured, but acute changes in hydration status are often estimated from body mass change. Changes in body mass are also often used as a proxy measure for sweat losses. There are, however, several sources of error that may give rise to misleading results, and our aim in this paper is to quantify these potential errors. Respiratory water losses can be substantial during hard work in dry environments. Mass loss also results from substrate oxidation, but this generates water of oxidation which is added to the body water pool, thus dissociating changes in body mass and hydration status: fat oxidation actually results in a net gain in body mass as the mass of carbon dioxide generated is less than the mass of oxygen consumed. Water stored with muscle glycogen is presumed to be made available as endogenous carbohydrate stores are oxidized. Fluid ingestion and sweat loss complicate the picture by altering body water distribution. Loss of hypotonic sweat results in increased osmolality of body fluids. Urine and faecal losses can be measured easily, but changes in the water content of the bladder and the gastrointestinal tract cannot. Body mass change is not always a reliable measure of changes in hydration status and substantial loss of mass may occur without an effective net negative fluid balance.

Efeito de suplemento hidroeletrolítico na hidratação de jogadores juniores de futebol

“…Athletes who start prolonged exercises dehydrated, especially in tropical climate, are more prone to adverse effects in the cardiovascular function, temperature regulation and exercise performance (16) . Body mass loss during sports practice is a real measurement of hypohydration in athletes, besides urinary density (17) . It is considered dehydration when the body mass percentage was higher than 2% of the initial body mass (14,18) .…”

A Review on Opportunities To Assess Hydration in Wireless Body Area Networks

“…Its validity relies on a predetermined stable and healthy body mass. Some researchers have argued that using the variability in body mass as indicative of immediate change in fluid balance could be erroneous because body mass loss could occur from substrate oxidation which adds water to the body [31]. However, body mass measurement continues to enjoy popularity in both clinical and heat stress hydration measurements, in conjunction with other hydration indices, mostly because of the simplicity and ease of measurement.…”

Author's Reply

“…Exercise-induced weight loss is always accompanied by alterations in fluid dynamics and losses from most, if not all, body fluid compartments. 7 A reference is cited, suggesting dissociation between body weight and body water losses during exercise and that total body water may be maintained despite significant reductions in body weight during exercise. In fact, this reference (which is a proceedings poster abstract) is based on an ultraendurance exercise lasting more than 10 hours, suggesting that not all body weight loss during exercise can be attributed to body fluid losses.…”