1.7.4 Nitrogen balance
In a healthy person, there should be a balance between the input of nitrogen to the body and its output. The input is mostly from protein, though some other foods may contain small quantities of nitrogen too. The output of nitrogen is mainly in urine, with a little in the faeces from undigested protein, from cells shed from the gut lining and from bacteria that live in the colon. Nitrogen is also lost from the body in the proteins of skin cells which are continuously shed, and in hair and nails, which consist of the protein keratin. Studies indicate that if there is no protein in the diet, 0.34 g protein per kg of body mass is lost from the body each day.
For a person with a body mass of 60 kg, how much protein would be needed in the diet each day to replace the amount lost?
The amount of protein needed would be (0.34×60) g=20.4 g.
However, for good health, more protein than this theoretical amount is needed. For example, for females aged between 19 and 50, the estimated average requirement, EAR, is 36.0 g per day, while the reference nutrient intake, RNI, is 45.0 g per day.
Why is the EAR value lower than the RNI value for protein?
The EAR is the average amount of protein required for the whole population, so half of the population (50%) would need more than this value (and half would need less). The RNI on the other hand, is the protein intake that would be sufficient for 97.5% of the population, so only 2.5% would need more than this. So inevitably the EAR value will be lower than the RNI value.
Extra dietary protein is needed by people who are suffering from injury, infection, burns and cancer, as all these conditions increase the rate of loss of protein from the body. The upper safe limit of protein intake is probably around 1.5 g per kg of body mass per day. Higher intakes may cause loss of minerals from the bones (which can result in more fractures), possibly due to the loss of more calcium in the urine, and may possibly contribute to a decline in kidney function, although this effect has yet to be confirmed.
The Expenditure and Food Survey (DEFRA 2001/2) gives a value of 71.3 g for the daily intake of protein in the UK. How much greater than the RNI is this value for a female aged 40, expressed (a) as an amount and (b) as a percentage of the RNI?
(a) RNI for a female aged 40 is 45.0 g of protein per day. So the daily intake of protein is 71.3 g – 45.0 g=26.3 g greater than the reference nutrient intake.
(b) Expressed as a percentage of RNI, this value would be
So the average daily intake of protein is over half as much again as the reference nutrient intake for a female aged 40.
For a woman weighing 60 kg, what is the safe (maximum) limit of protein intake?
How many times greater is this value than the EAR value?
Use Table 2 to calculate how much protein is present in a 250 g (about 8 oz) lean steak and thus whether eating this amount of meat on a daily basis would exceed the safe limit.
(a) The safe limit for protein intake for a 60 kg woman would be 60 × 1.5 g protein per day=90 g
(b) The estimated average requirement EAR is only 36 g per day. So, the safe limit is times greater=2.5 (or two and a half) times greater than the estimated average requirement.
(c) Table SP4.2 shows that lean beef is 20% protein. So, a 250 g steak contains protein=50 g protein. So this alone does not exceed the safe limit for protein, though for other reasons it might not be the basis for healthy diet, and of course, significant amounts of protein would be consumed in other foods during the day.
For women over 50, the EAR for protein is 37.2 g and the RNI is 46.5 g. For men over 50, the corresponding values are 42.6 g and 53.3 g. For men between 19 and 50, the values are 44.4 g and 55.5 g.
Construct a table to present the values clearly for both genders and the two age ranges.
Identify which values are appropriate to you, or another adult whom you know. Use the information from Table 2, and perhaps information on the protein content from food labels, to estimate that person's daily protein intake and compare it to the values in your table.
(a) Your table might look something like Table 6, though you may have chosen to put the EAR and RNI values as the row headings and the different genders and ages as the columns, which is just as good. The data for women between 19 and 50 is given in the preceding text.
Table 6 Dietary reference values for protein for adults
|Gender and age||Estimated average requirement, EAR/g per day||Reference nutrient intake, RNI/g per day|
|over 50 years||42.6||53.3|
|over 50 years||37.2||46.5|
(b) Your answer to this part will depend on the person you chose and their diet. If they ate foods which are rather different from those given in Table 2, you may not have been able to estimate the daily protein intake very accurately. Here is an example:
|Meal||Food eaten||Estimated protein content/g|
|Breakfast||Cereal with milk (8 g per 40 g serving, with milk, according to packet)||8|
|Lunch||White bread, 100 g (8% protein)||8|
|Cheese, 50 g (26% protein)||13|
|A little butter (less than 1% protein)||negligible|
|6 cherry tomatoes||unknown|
|Piece of fruit cake||unknown|
|Banana, 120 g fruit (1% protein)||1.2|
|Evening meal||Beef stew, made with 150 g beef (20% protein)||30|
|Baked potato, 150 g (2% protein)||3|
|Frozen green beans, 75 g (1.3 g per 75 g serving, according to packet)||1.3|
|Yoghurt, 150 g (5 g protein per 100 g)||7.5|
|Hot drinks||Made with about 150 ml milk (about 150 g, with 3.4 g protein per 100 g, according to carton)||5.1|
|Snack||Apple, 100 g (less than 1% protein); cheese, 25 g (26% protein)||about 6|
|Total protein||about 83|
This person (female, over 50) has eaten about 83 g protein during this day, significantly more than both the estimated average requirement (EAR) of 37.2 g protein and the reference nutrient intake (RNI) of 46.5 g protein. It is also more than the 71.3 g estimated daily protein intake in the UK determined by the Expenditure and Food Survey, referred to in Question 2, but less than the safe limit of 90 g per day, calculated in Question 3 for a woman weighing 60 kg.