Net protein utilisation

The product of BV and digestibility is termed the net protein utilisation (NPU) and is the proportion of the nitrogen intake retained by the animal. 

Biological value and net protein utilisation with rats. 

Proteins have different biological availabilities in the human body and a number of methods have been introduced to evaluate and measure protein utilisation and retention. Two types of measurements are used to estimate protein quality biological assays and chemical analysis. The most common measures of protein quality included Biological Value (BV), Protein Efficiency Ratio (PER), Net Protein Utilisation (NPU), Amino Acid Score (AAS also known as chemical score, CS), Essential Amino Acid Index (EAAl) and Protein Digestibility Corrected Amino Acids Score (PDCAAS), amongst other procedures and modifications. 

The net protein requirement for milk production is the protein content of the milk. Up to day 22 of lactation, the crude protein content of milk (g/kg) can be estimated as 10 X (3.43 - 0.066 d), where d = days in milk. After day 22, milk protein concentration is assumed to be 19.6 g crude protein/kg. Using an efficiency of utilisation of 50 per cent and a digestibility of 79 per cent results in a dietary crude protein requirement for milk production of approximately 50 g/kg milk. Previous protein requirements were based on digestible crude protein, but more recently NRC (2007) argues that the lack of information regarding digestible crude protein content of feed-stuffs commonly fed to horses does not justify its use, and that requirements should be based on dietary crude protein. The crude protein requirement (g/day) for maintenance is therefore calculated as 1.44 W and total requirements for milk production as CP requirement = 1.44W + 50Y. For a 500 kg mare producing 15 kg of milk, the daily crude protein requirement (g/day) is estimated as 720 + 750 = 1470. 

In order to calculate the quantities of metabolisable protein required to satisfy these net requirements, factors for the efficiency of utilisation of metabolisable protein for maintenance, dermal losses, lactation and growth and the conversion of mobilised body protein to milk protein are required. The following are recommended by AFRC (1993) ... 

The same method could also be applied to the net substrate oxidation rates. Due to the - at least partial - mutual exchangeability of the substrates as source of energy, this approach was not fully functional for the substrates. In [29] a diet containing 37 % of protein, 4 % of fat and 51 % of CHO (The remainder consists of dietary fiber, water, minerals, etc.) was used. In this case by increasing the protein intake by 1 g the CHO intake was automatically increased by 1.4 g. And in energy metabolism CHO is preferred, as long as there is enough CHO from both intake and body stores available. So the relation of the substrates as fuel differed at different levels of diet intake. This resulted in a fat utilisation factor of up to 800 % and CHO utilisation factors of as low as 33 %. The CD s, however, remained at levels of 92 - 99 %. Only for carbohydrate and... 

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