Do children need more protein than adults?

The answer to the title question depends upon how you define “need”. There are three ways of expressing the needs of individuals for protein – the absolute need in grams per day (adults need more) – the relative need in grams per kg of body weight per day (children need more) – the protein level or concentration needed the diet (??).

Absolute requirement

This is the weight of protein required by an individual in grams per day. As young children are much smaller than adults it is not surprising that adults require substantially more than young children when need is expressed in this way. Using the published UK dietary standards as the guide, children in the 10-12month age group weigh on average 9.7kg and an intake of 14.5g of protein per day, termed the RNI, is given as the amount necessary to ensure that any child in this age group gets enough protein. For adults these standards suggest that the average weight of an adult man as 74kg and the protein RNI is estimated at 55.5g/day. For an adult woman the average weight is 60kg and the protein RNI 45g/day. So by this absolute measure adults require 3-4 times as much protein as one year olds. All this tells us is that big people require more protein than very small people and as one might expect this is also true for energy and all of the other essential nutrients. In the early 1950s the estimated needs of a one year old child were around 35g/day i.e. much more than double what is thought to be needed now. The estimated needs of adults have changed relatively little over the years e.g. from 65g/day for a man in the early 1950s to 55.5g/day now.

The relative requirement

This is the requirement corrected for differences in body weight and is the grams of protein needed per kg of body weight per day. Using the absolute figure above then the relative requirement of the children is 1.54 grams of protein per kg of body weight per day (g/kg/d); obtained simply by dividing the protein requirement (14.5g/day) by the average body weight (9.7kg). The relative requirement of protein for both adult men and women is 0.75g/kg/day i.e. when expressed in this way one year old children need about twice as much protein as adults. The relative needs of children then declines as they get older e.g. in 7-10 year olds the relative need is estimated at just a third (33%) more than the adult values.

The reason that children need relatively more protein than adults is because of the protein requirement for growth. In order to maintain their tissues and organs, adults need to replace the protein that they inevitably breakdown or lose each day. If adults are put on a protein-free diet then even after adaptation they will still have a net obligatory loss of protein** each day and this has to be replaced.

Children are growing and accumulating extra lean tissue so the amount of protein in their bodies is increasing as they get bigger. Children therefore need to eat enough protein not only to replace their obligatory losses and keep them in protein (nitrogen) balance but also enough to allow for extra protein to accumulate in tissues and organs as they grow. One would assume that this extra need for protein would be highest during rapid growth e.g. during infancy. For this reason, it has always been assumed that the relative protein needs of growing children are higher than those of adults. However, views about the scale of this difference has varied over the years. In the decades after World War II it was widely assumed that children needed up to five times more than adults and so children were considered as being very prone to being protein deficient. Now that the relative protein needs of children are estimated be only double that of adults in one year olds and much less in older children then children are seen as much less likely to be protein deficient.

** Note that protein is the only major dietary component that contains nitrogen and any nitrogen lost from the body in urine is derived from the breakdown of protein; this means that measuring the nitrogen content of food or the nitrogen content of urine is a quick way of estimating protein intake and losses.

The protein level/concentration needed in the diet

This is clearly the key question because this determines whether children need a more protein-rich diet than adults or whether a diet with adequate levels of protein for an adult will also be sufficient for rapidly growing children.

We saw in the previous section that children require more protein than adults when one corrects for differences in their for body weights. This is also true for energy (calories). The estimated energy requirement of 10-12 month old baby boy is 920 kcal/day which is about 95kcal/kg body weight/day. The average adult man’s requirement is estimated at 2550kcal/day which works at to be around 34kcal/kg body weight/day. Thus the relative energy needs of a one year old boy are almost three times those of a man. This means that these boys need to eat three times as much of any diet as an adult in order to get their calorie needs. This means that if they eat enough to fulfill their energy needs they will get three times as much protein even though they only need about twice as much as an adult. At face value this suggests that children should be able to manage on a diet that is less protein rich than an adult diet. The table below illustrates this more systematically; it shows the relative energy and protein standards (i.e. amount per kg of body weight) for different age groups expressed as a multiple of that for a standard adult – in all cases the children’s relative energy need is a higher multiple of the adult value than the protein value e.g. a 1-3 year old requires 2.9 times as much energy (i.e. food) but only 1.5 times as much protein.

Table 1 – Selected UK energy (EAR) and protein (RNI) standards on a weight-for-weight basis expressed as a multiple of the standard adult value

Age Energy (EAR) Protein (RNI)
   
4-6 months 2.7 2.2
10-12 months 2.8 2.0
1-3 years 2.9 1.5
7-10 years 2.1 1.3
11-14 years 1.5 1.3
15-18 years 1.3 1.2
19-50 years 1.0 1.0
75+ years 0.9 1.0

Protein, carbohydrate and fat are the energy supplying nutrients in the diet (plus alcohol for some adults). The usual way that scientists express the protein level in a diet is to calculate what percentage of the calories in the food come from protein. This is also how the fat and carbohydrate levels in the diet are usually expressed. This is in effect the concentration of protein in the diet. If one estimates how many calories a child requires and how much protein they need then one can calculate what percentage of their calories should be in the form of protein (a gram of protein yields 4kcal). This calculation suggests that a one year old requires about 6% of their energy to be in the form of protein but a man around 9%**. Again suggesting that adults require a more protein-rich diet than children. To put this into perspective, there is currently about 18% protein in the average UK adult diet. In practice this means that protein is very unlikely to be deficient in any but the most bizarre UK diet. Vegetarians and especially vegans have traditionally been seen as an “at risk” group but they can be reassured that their diets are very unlikely to be lacking in protein. Protein intakes have been increasing in recent years and are way above estimated requirements.

**Note that the ways in which energy and protein needs are estimated tends to slightly exaggerate this percentage figure.

This idea that children may need less protein in their diets than adults seems a little counterintuitive especially for those who grew up in the era when protein was considered a key nutrient for growing children and infants. Maybe the fact that human milk is relatively low in protein and only contains about 6% of the energy as protein is reassurance that current views are likely to be correct. Note that cows’ milk, like most animal milk, contains at least three times as much protein as human milk and only milk from other primates has levels as low as that found in human milk.

A couple of final questions to consider:

  • Why were protein needs of children so overestimated in the past?
  • What about differences in protein quality?

Reasons why the past protein needs of children were exaggerated (personal view)

Estimates of the absolute requirements for protein have more than halved since the 1940s and 50s and there was a common belief in this era that children required up to 5 times more than adults on a weight adjusted basis (compared to twice as much now). In my opinion, a focus upon experiments with laboratory animals may have contributed to this exaggeration of the protein needs of children. A baby rat grows much faster than a human baby and doubles its birth weight in about 6 days whereas in a wholly breastfed baby this takes 4-6 months. This suggests that a baby rat needs relatively much more protein than a human baby and much more than an adult rat. Sure enough if one analyses rat milk then one finds that about 25% of the energy comes from protein (c.f. around 6% in human milk). Around 80% of the protein intake of young rats is needed for growth and only about a fifth is needed for maintenance; consistent with the idea that baby rats need 5 times more than adults on a weight corrected basis. Did these sorts of observations about the high requirement of the young of laboratory animals encourage scientists of past eras to assume that this also applied to human babies? It is an odd “coincidence” that the five times higher relative requirement for baby rats was the same figure given as the relative requirement of human babies compared to adults.

Protein quality is probably of little significance in human nutrition

Protein is made up of amino acids, many of which are termed essential amino acids that must be provided ready-made in the diet because we have no capacity for producing them. Proteins differ in their content of these essential amino acids – some dietary proteins like eggs, milk, meat, fish and legumes contain good amounts of all the essential amino acids and so are termed first class proteins. Other vegetable and cereal proteins are often relatively low in one or more of these essential amino acids and so are termed second class proteins; wheat, for example, has a low level of the essential amino acid lysine. This was for many years considered an important factor that increased the protein needs of growing children if their diet was largely from non-animal sources; they needed to eat more of the low quality protein to ensure that they got enough of all the essential amino acids.

Two factors have lessened the importance attached to protein quality as a likely cause of human protein deficiency. Firstly, diets are not made up of single protein sources. Even low protein foods like fruits and vegetables contain some protein. Proteins from different sources have different profiles of essential amino acids and so the different foods in the diet complement each other to improve the overall protein quality of the diet; what is lacking in one food is present in another. This is termed mutual supplementation of proteins and it means that the protein quality of human diets varies much less than the protein quality of single protein sources. Secondly, when protein needs were thought to be very high, many diets would have been considered to have deficient or marginal protein levels and so low protein quality would have pushed down still further the intakes of essential amino acids. Now that protein intakes are seen as way above required levels, variations in amino acid complement of food protein seems unlikely to precipitate deficiency.

Protein may be a much more of a priority in agriculture where the aim is to get animals to slaughter weight as quickly as possible. In rapidly growing farm and laboratory animals, low protein intake is much more likely to limit growth and protein quality may significantly affect the animal’s ability to make use of a diet’s protein for growth. A modern pork pig grows from 1.5kg at birth to 55kg in around four months whereas a human baby may not even double its 3.3kg birth weight in this time.

Conclusions   

Most adults or children living in affluent countries would find it almost impossible not to meet their protein requirements and variation in protein quality is unlikely to alter this conclusion.

Bibliography

Much of this is textbook level material and so I have not referenced the individual statements.

Webb, G.P. 2012 Nutrition: maintaining and improving health. 4th edition. Oxford, Taylor and Francis. http://routledge-ny.com/books/details/9781444142464/

Department of Health (1991) Dietary reference values for food energy and nutrients for the United Kingdom: report no. 41 of the Committee on Medical Aspects of Food Policy.  London, HMSO.

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One thought on “Do children need more protein than adults?

  1. Pingback: The Protein Gap – one of the biggest errors in nutritional science? | Dr Geoff

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