Nutrient status is defined as the balance between the nutrient supply of the diet and the nutrient requirements of an animal or a population of animals. Nutrient status can be determined directly by measuring the concentrations of nutrients in blood or specific tissues or by measuring quantitative net nutrient retention in the body. The latter is often measured indirectly in balance studies in which nutrient input via the diet and nutrient losses via faeces, urine and expired air and as heat, in case measurement of energy retention, is considered. Nutrient retention is calculated as nutrient intake via the diet minus its excretion via faeces, urine and expired air or otherwise, in case of energy as heat. Alternatively, concentrations of nutrients, their metabolites or biological compounds or expression of genes involved in their metabolism can be measured as indicator of the nutrient status in either specific pools, organs and tissues in the body or in excretal products. Quantitative techniques for measuring nutrient status require facilities and equipment to quantitatively collect faeces, urine, expired air, and heat loss mostly over a period of several days. These techniques are therefore time consuming and costly but generally provide proper estimates for nutrient retention. Alternatively, nutrient retention in the animal itself and in the animal end product can also be measured directly by analysing the nutrient of interest in homogenized sample of the whole body of small animals and, if applicable, in the mentioned end products. Also this approach is generally time consuming and costly, and in certain cases, such as in dairy cows and large ruminant species, not feasible given the size of the animal.