This question for the paleos may sound rather similar to the last one, but I believe it is important enough to warrant its own spot on the ledger. It concerns the composition of human breast milk.  Let’s recall that every mammal, from the tiny pygmy shrew to the giant blue whale, which is the largest creature alive and that ever lived, produces milk for its young, and these milks vary widely in composition. And, the balance of nutrients in the milk reflects the balance of nutrients in the future diet of the infant because there needs to be continuity. For instance, cats, are pure carnivores; they eat 100% meat, which is a very high-protein diet. So, correspondingly, their milk is very high in protein- about 11% by weight.  Cows, in contrast, live on grass, which is lower in protein, and their milk has only 3.4% protein by weight.

Judging by the protein content of our milk, humans must be very low-protein feeders, but the early protein researchers didn’t look at it that way. They looked at other things- which misled them. For instance, Chittendon, a turn-of-the-century protein researcher, declared that humans need 132 mgs of protein a day. That’s because he based his conclusions on nitrogen excretion, and he found that vigorous young males who were fed less than 132 mgs of protein daily went into “negative nitrogen balance.” That means that they were excreting more nitrogen than they were taking in.  But, what Chittendon did not realize is that when protein consumption is reduced, the body- over time- slows down its protein turnover such that nitrogen balance is regained at the lower level of consumption. So, the negative nitrogen balance is only temporary- it goes away.  Today, we know that people can adjust to amazingly low levels of protein consumption. For instance, I have known individuals who avoided high-protein foods completely- including plant proteins. They were vegetarians who ate no beans or nuts or grains, and they ate no animal foods. They ate only fruits and vegetables- including avocadoes- and nothing else.  And, as expected, they got rather thin, but perhaps not as thin as you might think.  They still had some muscle shape and tone, and they were fully active.  And, they lived that way for years.  It’s extreme, I admit, and I’m not recommending it. However, it goes to show how little protein the human body can get by on- if it has to.  And the point is that the amount of protein that you eat sets the pace of your protein metabolism, and that pace starts with the amount of protein you get from your mother’s milk.  And, I think it’s safe to say that the transition from nursing to weaning is supposed to be smooth- not jolting.  So, mammals who eat high-protein diets get high-protein milks, and animals who eat low-protein diets, get low-protein milks.

How much protein is there in human milk? I used to say 1% by weight, making it the lowest protein milk on the planet. However, for a long time, some analyses have come in at .9%, and now I’m finding some analyses coming in at .8%. Compare that to the milk of a cat. It’s a 14-fold difference! It’s funny that Dr. Loren Cordain, a prominent paleo writer, has suggested that humans are “evolving” towards becoming increasingly cat-like, but apparently, nobody told the human mammary glands.

A paleo might respond by saying that humans take so long to mature (on average, boys continue to grow until the age of 18) that it’s the long, protracted growth period that accounts for the low-protein content of breast milk. After all, 18 years of growth is longer than the whole lifespan of some mammals. However, that doesn’t change the fact that humans grow very fast as infants.  Typically, babies double their birth weight in 6 months. It is the most rapidly growing period in human life. And, they do all that growing on a diet of low-protein mother’s milk.

Another way to look at it is as protein as a percentage of calories, and on that basis, human breast milk derives 7% of its calories from protein. And, as foods go, 7% of calories from protein is low. Among plant foods, all provide a higher percentage of calories from protein- except for some fruits.  In contrast, green vegetables, on that basis, are extremely high in protein (33% of calories), and even grains provide about 13% of their calories as protein-which is almost twice as high as human milk. Beans, of course, are very high in protein, as are most oil-seeds and nuts. So, unless you are eating too much fruit at the expense of other foods, you can’t fail to obtain enough protein from a plant-based diet- so long as you are getting enough food.

Human milk is not only low in protein, it is also very high in carbohydrate. It is, by far, the sweetest of the milks with over 7% lactose by weight. No other milk comes close. That not only conditions the infant’s body to receiving a steady supply of sugar, it also conditions the baby’s tongue to a sweet taste. That’s why the foods that babies are most accepting of in weaning are sweet foods, such as blended cantaloupe and mashed banana.

The bottom line is that the composition of breast milk shows that humans are not high-protein feeders- not even when we are undergoing our most rapid period of growth.  But, as a 60 year old man, I am not growing at all.  I am just maintaining. So, how much protein do I need?

So, our question for the paleos is: Since human breast milk is the lowest protein milk on the planet, containing as low as .8% protein by weight, and yet babies grow just fine on it, doubling their birth weight in only six months, why should a high-protein diet be needed later in life, especially after growth has stopped?