Wednesday, May 28, 2014

Nutrition Fallacies: Deriving Adult Nutritional Needs From Human Milk Composition

I have seen a number of authors suggest that the nutritional composition of human mother's milk gives us important information about how adult humans should eat.  

The most common suggestions I have seen include:

1.  Since human milk provides 55% of energy as fat, and about 25% of total energy as saturated fats, this shows that adult humans should eat diets providing ~55% of energy as fat and ~25% as saturated fats.  This argument is often made by advocates of high fat diets (e.g. Paul Jaminet in The Perfect Health Diet.)
2.  Since human milk provides only 5% of energy as protein, this shows that adult humans do not need any more than 5% of energy as protein.   This argument is often made by advocates of very low protein plant-based diets (such as fruit-based diets) who want to play down the importance of protein. 

One can only come to these suggestions by taking the nutritional composition of human milk out of context.  Human milk has a specific nutritional purpose, namely, to support the growth and development of infants.  Infants have specific nutritional needs not had by adults.  

Human Milk Promotes Fat Gain?

An infant has an energy requirement approximately 2 to 3 times that of an adult, on a kcal/kg/d basis–80-115 kcal/kg/d for infant [1] vs. 43 kcal/kg/d for 70 kg active adult male expending 3000 kcal per day.  If the 70 kg human had the same energy requirements as the infant on a weight basis, he would need to ingest 6000 to 9000 kcal daily.  

Obviously, the infant's extraordinary metabolic rate creates a demand for a high energy density food.  Since fat provides more energy per unit weight than any other macronutrient, Nature provides the energy-hungry infant with a food high in fat.  Simply put, human milk provides a high proportion of energy as fat in order to meet the high energy demands of an infant. Since adults do not have the same energy requirements as infants, it is very unlikely that the same high fat intake serves adult needs.

During the first 18 months of life, breast-fed human infants increase their total body mass from an average of 3.5 kg to 11.0 kg, and their percent body fat from 16% to 25%. [2]   This represents a gain of 2.15 kg of body fat (rising from 0.6 kg to 2.75 kg), in relative terms a 300% increase in body fat mass.  So a diet of fat-rich human milk promotes a rapid rate of body fat accrual and a rapid gain in weight.  Is that what an ideal adult diet should do?  Since adults have only one-third the metabolic rate of infants, one might predict that modeling an adult diet on the nutrient profile of human milk might lead to similar or greater rates of gain of fat and weight in adults.  

Why don't those authors who advocate a high fat diet based on the composition of mother's milk provide this additional information?  If they told you that eating a 55% fat diet enables infants to triple their body weight and fat mass and increase their body fat percentage by 50% in just 18 months, would you consider that fat intake ideal for your purposes? 

As for the saturated fat content, besides having a high need for fats to build adipose reserves and use as energy, infants are also actively building new neural tissue, much of which incorporates saturated fats.  Once again, adults are not growing new neural tissue at the pace of infants, so they do not have the same requirements for fats. 

Protein Phobia

Some people claim that human milk provides the infant a diet low in protein because only 5% of the energy of human milk comes from protein.  However, protein requirements are properly expressed in relation to weight, not percent of energy consumed.  This especially applies here because infants have a much greater energy requirement than adults as discussed above.  Since infants have to consume a much larger proportion of their food as energy-yielding nutrients (fat and carbohydrate),

Infants up to 9 months of age actually have a higher protein requirement than adults when expressed in g/kg/d.  Using the current RDA, a moderately active adult who is not engaged in strenuous resistance or endurance training requires 0.8 g/kg daily.  Infants aged 1–2 months need 1.99 g/kg/d; aged 5–6 months, 0.92 g/kg/d; and 9–12 months, 0.8 g/kg/d. [3 

Obviously, in the first 6 months of life, human milk provides the infant with up to 2.0 g PRO/kg body weight which is necessary for the rapid accrual of lean mass involved in a doubling of body mass in ~ 6 months.  Hence, it is misleading to call human milk a "low" protein diet.

How can a food that provides only 5% of energy as protein provide a high protein intake? Although human milk supplies only 5% of energy as protein, the required large energy intake of an infant leads to a relatively high (compared to the RDA) protein intake expressed in g/kg.  If a 70 kg adult consumed energy at a rate similar to an infant, he would ingest up to 9000 kcal per day of which 5% (450 kcal) would consist of protein.  That would amount to 113 g protein for a 70 kg adult. That comes to 1.6 g PRO /kg or twice the US RDA for protein for an adult male.  Of interest this is similar to the amount recommended for adults engaged in resistance training with a goal of muscle hypertrophy – 1.33 g/kg/d. [4]  

However, since the metabolic rate and therefore total energy requirement of adults is one-third to one-half that of an infant, the adult needs to consume 12–15% of energy as protein to obtain the same intake of protein on a g/kg basis.  

5% of 9000 = 450 = 113 g protein
15% of 3000 = 450 = 113 g protein

Stated otherwise, although human milk provides only 5% of energy as protein, since the infant consumes a very large amount of energy from the milk, s/he obtains, in absolute figures, a moderately high protein intake expressed as g/kg in the first 6 months of life, and this tapers to an intake essentially equivalent to the RDA, i.e. adequate, not "low" in protein.  

To reiterate, at no point in the first year of life does human milk provide infants with any less than 0.8 g/kg protein.   A 70 kg moderately active male consuming 3000 kcal per day and 0.8 g protein/kg body mass will be consuming about 7 percent of energy from protein.  If he consumed only 5% of energy from protein, he would obtain only 38 g protein daily, which is only 0.5 g/kg – less than the Estimated Average Requirement (EAR) of 0.66 g/kg/d which is based on nitrogen balance studies.[5]  The EAR states the mean requirement, so for any individual, there is a 50% chance that his requirements are higher than the EAR.  

A 0.5 g/kg intake is also significantly less than estimates of protein requirements based on indicator amino acid oxidation studies, which suggest mean and population-safe requirements of 0.93 and 1.2 g/kg/d, respectively, in young men [6] and 0.85 and 1.15 g/kg/d in elderly women [7].  Using these estimates, a 120 pound elderly woman with an energy requirement of only 2000 kcal per day would probably need about 46 g and possibly as much as 63 g protein daily, which would be 9 to 13% of energy from protein. 

In other words, it is quite possible that while 5% of energy as protein is adequate for someone who has very high energy requirements and consumption (like the infant), but is inadequate for adults with lower energy requirements and consumption.  When talking about protein requirements, it is most informative to focus on the absolute requirements in g/kg, and to put all talk of protein intakes as a percent of energy in this more important context.  

In summary, since infants have very different energy requirements and metabolic activities from adults, it is misleading to use human milk as a model for adult fat or protein requirements. 


1 comment:

Steve Bates said...

I'm glad someone did a detailed post on this. The Perfect Health Diet is obviously based on pseudoscience and its lack of efficacy shows in the portly appearance of the two founders.