Water and hydration: Physiological basis in Adults - Water in the body
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Water in the body
Water is the main component of the human body; it is distributed throughout the body, in every organ, inside and between cells.
Water represents on average 60% of the body weight in adult men, and 50-55% in women (EFSA 2010; IOM 2004). This means that, for a man of average weight (70 kg), body water content is about 42 liters.
This average value varies among individuals, primarily because of differences in body composition: while the water content in lean body mass is constant among in mammals, at 73%, adipose tissue (body fat) is only about 10% water (Peronnet et al. 2012; Sawka et al. 2005; Wang et al. 1999). Therefore, body fat relative mass directly influences total body water. This explains the influence of age, gender and aerobic fitness on total body water: women and older persons have lower total body water, because of lower fat-free mass. In contrast, athletes have relatively high total body water (IOM 2004; Marieb and Hoehn 2007; Watson et al. 1980).
Water is distributed throughout the body and organs. The water content of various organs depends on their composition, and ranges from 83% in blood to only 10% in adipose tissue (Figure 1).
Figure 1.Water composition of tissues and organs by weight.
Adapted from Pivarnik and Palmer 1994.
Water is distributed in the body among two main compartments: intracellular and extracellular. The intracellular compartment is the largest, representing about two thirds of body water. The extracellular compartment, representing about one third of body water, comprises plasma fluid and interstitial fluid (Armstrong 2005; Marieb and Hoehn 2007) (Figure 2). Plasma fluid and interstitial fluid have a similar electrolyte composition, the most abundant ions being sodium and chloride (IOM 2004; Marieb and Hoehn 2007; Robertson and Berl 1996).
Other compartments also contain water, such as lymph, eyeball fluid and cerebrospinal fluid for example. These compartments make up a relatively small water volume, and are usually considered to be part of the interstitial fluid (Marieb and Hoehn 2007).
Figure 2. Distribution of total body water among compartments.
After ingestion, water is absorbed in the gastrointestinal tract. It then enters the vascular system, goes to interstitial spaces, and is transported to every cell (Figure 3). Intracellular water comprises 65% of all total body water.
Figure 3. Water’s journey from ingestion to cells.
After leaving the stomach, water is absorbed mostly in the early segments of the small intestine, the duodenum and the jejunum. A small portion of all water absorption occurs in the stomach and the colon (Shaffer and Thomson 1994): the small intestine absorbs 6.5L/day, whereas the colon absorbs 1.3L/day. These amounts correspond to the water ingested daily, in addition to the water produced by secretions from salivary glands, stomach, pancreas, liver and the small intestine itself (Zhang et al. 1996). The absorption process is very rapid: a recently published study showed that ingested water appears in plasma and blood cells as soon as 5 minutes after ingestion (Peronnet et al. 2012).
Water passes from the intestinal lumen into plasma mainly by passive transport, regulated by osmotic gradients. Water molecules are then transported via blood circulation to be distributed all over the body, to the interstitial fluids and to cells.
Water moves freely in the interstitial compartment and moves across cell membranes via water specific channels, the aquaporins. Fluid exchanges between compartments are regulated by osmotic and hydrostatic pressure, and water flows in accord with changes in the extracellular fluid osmolality (Marieb and Hoehn 2007).
The body water pool is renewed at a rate depending on the quantity of ingested water: the more a person drinks, the faster body water is renewed. For a man drinking 2L of water per day, a molecule of water stays in the body on average 10 days, and 99% of the body water pool is renewed within 50 days (Peronnet et al. 2012).
The renewal of body water is driven by ingested water, replacing the constant losses the body is facing. This allows for the maintenance of body water balance.
Take home messages
Water represents on average 60% of body weight in adult men. However, this percentage decreases along with lean body mass.
Most of the organs and tissues contain more than 70% water: blood and kidneys consist of 83% water, and muscles 76% water. However, adipose tissue contains only 10% of water.
Two third of the body water is intracellular. The extracellular fluid consists of plasma and interstitial fluids.
Ingested water is absorbed mainly in the small intestine. It appears in the blood as soon as 5 minutes after ingestion.
The body water pool is renewed at a rate depending on the quantity of ingested water. For a man drinking 2L of water per day, a molecule of water stays in the body on average 10 days, and 99% of the body water pool is renewed within 50 days.