Water is an essential nutrient - it is required in amounts that exceed the body's ability to produce it. All biochemical reactions occur in water. It fills the spaces in and between cells and helps form structures of large molecules such as protein and glycogen.
Most foods contain water. The body can usually get 20% of its total water requirements from solid foods alone. The human body can last weeks without food, but only days without water.
The digestion process also produces water as a byproduct and can provide around 10 per cent of the body’s water requirements.
Water is a transparent and nearly colorless chemical substance. Its chemical formula is H2O, meaning that its molecule contains one oxygen and two hydrogen atoms.
Water covers 71% of the Earth's surface. It is vital for all known forms of life. On Earth, 97% of the planet's crust water is found in seas and oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps. Only 2.5% of this water is freshwater, and 98.8% of that water is in ice (excepting ice in clouds) and groundwater. Less than 0.3% of all freshwater is in rivers, lakes.
Safe drinking water is essential to humans and other lifeforms even though it provides no calories or organic nutrients. ~One billion people still lack access to safe water. It was estimated that by 2025 more than half of the world population will be facing water-based vulnerability, and by 2030 water demand in some developing regions of the world will exceed supply by 50%.
Approximately 70% of the freshwater used by humans goes to agriculture.
The protein digestibility-corrected amino acid score (PDCAAS) has been adopted by FAO/WHO as the preferred method for the measurement of the protein value in human nutrition.
PDCAAS = Amino Acid Score x Digestibility
The method is based on comparison of the concentration of the first limiting essential amino acid in the test protein with the concentration of that amino acid in a reference (scoring) pattern. This scoring pattern is derived from the essential amino acid requirements of the preschool-age child.
Although the principle of the PDCAAS method has been widely accepted, critical questions have been raised in the scientific community:
the validity of the preschool-age child amino acid requirement values (more than 4 times greater than the EAA requirement for an adult),
the validity of correction for fecal instead of ileal digestibility,
the truncation of PDCAAS values to 100%.
The reference scoring pattern was based on studies performed more than 25 years ago on a limited number of 2-year-old children recovering from malnutrition.
According to the current official recommendations, a 2-year old child needs ~ 3x higher essential-to-non-essential amino acid ratio, and needs essential amino acids in different proportions than adult. Methionine/cysteine is the limiting essential amino acids for adults, and for children it is lysine or tryptophan.
The use of fecal digestibility overestimates the nutritional value of a protein because amino acid nitrogen entering the colon is lost for protein synthesis in the body and is, at least in part, excreted in urine as ammonia.