Why DO human beings drink milk?

We’ve all seen the sensationalist posts on social media claiming that dairy products are simply not good for human beings – it’s unhealthy and causes more harm than good. But how true is this? If human beings have been consuming dairy for thousands of years you’d kind of assume that it must have some benefits. Surely it can’t really be the poison many claim it to be.

Of course if the dairy industry is to be believed, milk and other dairy products are absolutely critical to human health. Many health experts and even the medical profession seem to be of the same view; heck, we even had the CSIRO telling us recently that some Australians are – gasp! – moving away from dairy WITHOUT proper medical advice.

https://www.csiro.au/en/News/News-releases/2016/Dairy-avoidance-reaches-dangerous-levels

Hmmm…  That does seem a bit unlikely to me. A food that has been in the human diet for less than 10,000 years and to which nearly 2/3 of the global population is intolerant is THAT critical? I don’t know about you but I regard that suggestion with some suspicion.

Of course, dairy farmers are adamant this is the gospel truth. In fact, in conversation recently a dairy farmer made the claim that “several leading nutritionists attribute the fact that developed countries’ average age is around 80 is due to the fact modern homo sapiens drink cow’s milk”.

Now, I don’t think I’ve ever seen such an ambitious claim before, nor have I ever seen any peer reviewed research that says that. But then again, I am hardly an authority of the subject. I did a quick Google and did indeed uncover an article that referred to a 2009 study that argued that drinking milk can lessen the chances of dying from illnesses such as coronary heart disease (CHD) and stroke by up to 15-20%. It seems this study was some kind of meta-analysis rather than novel research, as the article refers to the “review (bringing) together published evidence from 324 studies of milk consumption as predictors of coronary heart disease (CHD), stroke and, diabetes”.

https://www.sciencedaily.com/releases/2009/07/090722083720.htm

This seems to be rather similar to a recent study along the same lines which uncovered the same general result from a meta-analysis of current research.

http://www.foodandnutritionresearch.net/index.php/fnr/article/view/32527.

I was not able to locate the original 2009 paper, but my guess is that these are largely the same papers given the involvement of the UK’s University of Reading in both. I don’t know how this conclusion bears up to critical scrutiny, but I do note substantial funding from the dairy industry in the more recent study.

By contrast, I found two recent papers that suggest that consumption of dairy products may contribute to increased mortality in some cases.

“High milk intake was associated with higher mortality in one cohort of women and in another cohort of men, and with higher fracture incidence in women. Given the observational study designs with the inherent possibility of residual confounding and reverse causation phenomena, a cautious interpretation of the results is recommended”.

http://www.bmj.com/content/349/bmj.g6015.long

“The role of dairy product consumption in mortality generally appeared to be neutral in men. In women, dairy fat intake was associated with slightly increased all-cause and IHD mortality. More research is warranted on a possible protective effect of fermented milk on stroke mortality”.

http://ajcn.nutrition.org/content/93/3/615.abstract

I won’t pretend this is any kind of exhaustive analysis of the research on my part, I merely use these examples to illustrate the confusion of evidence around just what benefit dairy might offer. Generally speaking it seems there are arguments for both benefits and risks, although I do note that many studies are, in my opinion, confounded by the fact that dairy industry funding is involved.

To be honest, I can’t reach any sound conclusion from the various articles and papers I’ve read. On the whole, dairy consumption in Australia might offer some benefits, particularly around protection from cardiovascular disease, yet it is definitely implicated in increased risks of some cancers. It just isn’t clear to me whether dairy is worthwhile purely in health terms and I am not at all convinced it confers any sort of benefits in terms of longevity.

So, what about all those memes on social media about the awful risks of dairy consumption – how it’s responsible for cancer and bone fractures and all sorts of ailments, or even that it’s full of blood and pus? Well, there’s clearly a grain of truth in most of these but I suggest there is rather a lot of exaggeration going on. Still, dairy consumption does seem to be not without risks.

What all of this does lead me to wonder about though, is exactly why it is that humans consume milk and other dairy products at all. After all, we are the only mammal that has adapted to drinking mother’s milk after weaning. How did this happen and what are the evolutionary drivers for this behaviour and what benefits can we expect from dairy consumption in a modern setting as a result. What I uncovered is quite remarkable, at least to me.

Here, in a relatively quick overview, is an explanation for human consumption of dairy products in a purely evolutionary context. Of course, my usual caveat – this is what I’ve found from my own research, interpreted in my own fashion. I may be quite wrong. But I also think I am not overtly misrepresenting the science. I welcome any comments, clarifications or rebuttals. Please note too that I am addressing the health benefits of dairy consumption in an evolutionary context – I am not addressing the ethical question of dairying in modern Australia.

Mammals express a gene that causes production of an enzyme called lactase that allows them to digest the lactose in mother’s milk until they are weaned. Once they are weaned, the gene “switches off”. After that, drinking milk leads to a variety of stomach/digestive issues. This has evolved because while milk is a great food for a baby, it is obviously too costly for adults to suckle from mothers. So adult mammals get their nutrition from environmental foods.

In human beings, we have evolved a genetic adaptation such that the gene doesn’t switch off. This means we can continue to digest lactose throughout life, and that is called lactase persistence (because the gene allows us to continue producing lactase into adulthood).

The really interesting thing is that this genetic change is quite recent and only arose between 5000 and 10000 years ago. It seems to have arisen several times in slightly different ways. The most common version appeared in Europe maybe 8000 years ago and is closely associated with the spread of farming and domestication of cattle.

Before this, people had been using milk from animals like goats and camels in Africa and the Middle East – in these conditions milk quickly soured so the typical foods eaten were yoghurt and cheese. In both cases, the fermentation process breaks down the lactose which makes it possible for people to eat such foods even with lactose intolerance.

The spread of farming into Europe meant that milk was able to keep for longer, but it was also unlikely to be consumed given the lactose intolerance thing. Eventually, somewhere or other (maybe near Turkey) someone had a gene mutation that turned on the lactase persistence gene. This probably happened more than once, but it needed some kind of opportune event for that mutation to spread.

Once it began to spread, selective pressures were of such high order that it spread quite quickly. Today we can see how that worked by looking at the frequency of this genetic change in populations. In some parts of Europe, up to 100% of the population have lactase persistence, while in others very few do. And of course there hasn’t been either enough time nor cultural changes for it to spread very far. As a result, most of the global population remained lactose intolerant until quite recently – eg native Americans, many Asian populations, a majority of Australian Aborigines etc.

The big question is why did humans develop this adaptation? The answer appears to be that no-one really knows. There are a few theories but further research is needed.

To understand this, we need to know a little about evolution. Evolution isn’t a directed process as such, nor does it have our best interests at heart. All it “cares” about is whether we produce more children. In other words, whatever happens, it is having more surviving offspring that makes the difference and fixes genetic change in populations. Regardless of how the change arose.

In the case of milk, it seems there is something about it that gave peoples in Europe an adaptive advantage. Some of the ideas relate to presence of calcium and Vitamin D but this is not highly regarded nowadays. It might be that it provides some kind of insurance against drought or famine, though quite why or how people would keep their cows alive in times of drought is a little hard to guess.

One very good possible explanation is that we ourselves created the selective pressures by our lifestyles. Nomadic hunter/gatherers tended to eat natural diets that were largely healthy, or they adapted to whatever foods were available (they had to, if they didn’t they would simply die off).

Once we began farming and settling down into large communities, neolithic farmers tended to develop poor health. In fact, agriculture has been described as one of the biggest mistakes in human history! I don’t know about that, but I get the point. It’s certainly one of the worst things that has ever happened as far as all the other animals and the environment are concerned…

Anyways, as settled farmers, neolithic peoples ate less diverse ranges of foods, increased the amount of animal flesh in their diet, and by being in large stable communities it was easier for disease to spread. In such a situation, it is possible that the higher nutritional value of cow’s milk made for a more constant nutritious food supply (for example, crops might fail, or there might be long periods between harvesting and so on).

So communities who dairy farmed had a sort of insurance policy against the very health risks they themselves had created.

What is very clear is that the lactase persistence gene follows the spread of dairy farming – where people farmed dairy cows, the people were (or became?) lactose tolerant…

Of course, this kind of adaptation to digesting lactose occurred elsewhere, for example in Africa, but there it was due to a slightly different mechanism and probably for different reasons. As all mammals produce milk for offspring, humans can probably get the benefit of milk from many different animal sources. The lactose tolerance thing applies to whatever source one makes use of. In other words, the adaptation isn’t about consuming dairy per se, it’s about being able to digest lactose.

So where does that leave us? Well, here is my summarised take on the whole picture.

Human beings can, and did, get all the nutrition they needed from natural sources without dairy, and indeed most of the world’s population still does. Until quite recently we had not evolved to consume dairy products at all. Like all other mammals, we evolved to wean off mother’s milk as we matured and thereafter eat from the natural environment. This appears to have been quite satisfactory in terms of survival, but again I will point out that survival doesn’t necessarily translate into a long, healthy life…

Lactase persistence is a genetic adaptation that must have offered some survival benefit such is the speed with which it spread throughout the population. A likely explanation for its strong selective pressure might be that it is an example of niche construction in evolution – that is, we probably created the conditions to encourage the spread of the genetic adaptation.

It is very likely therefore that the consumption of dairy is not specifically a dietary advantage at all times, rather it is more of a hedge against other environmental factors. Nonetheless, mother’s milk is a nutritionally sound food. This must be balanced against the clear point that evolutionarily speaking, no other animals consume milk past weaning, so nature has never had to ensure that milk confers health benefits over the long term for individuals. I suspect we have no idea how long term over-consumption of milk affects populations.

And that is one of the big risks in dairy. As a short term hedge strategy, it only has to provide sufficient nutrition for people and their offspring to survive and produce more offspring in difficult times. It doesn’t need to ensure a long life, and nor does it need to be consumed all the time. It might be that long term over-consumption of dairy brings with it health risks that are only seen at the far reaches of longevity, or in modern settings. As indeed current research seems to show.

Consider our neolithic farming communities. They would have eaten as they needed of things like animals, fruits, vegetables, perhaps fish, and dairy. Dairy may not have been a big part of their diet, certainly not to the extent we see in modern Australia, and in any case they didn’t have the kind of insights possible in modern health care and research. As they tended to have shorter lifespans, they would hardly have noticed whether dairy had any adverse long-term effects, though they would have noticed if dairy helped in tough times.

What has made the bigger difference in modern times as far as longevity goes is a range of factors such as better health care, less rigorous lifestyles, antibiotics and more generally available foods without the feast or famine kind of cycles. Dairy more than likely fitted in with that quite happily, and any possible risks would be masked by the generally improved health status of Western populations.

And really, it seems highly likely that moderate intakes of such forms of dairy as cheese and yoghurt are probably net beneficial. Milk itself is also likely to have some beneficial effect though here we are probably talking more about convenience – that is, it is easier to get certain nutrients (eg calcium) from dairy than to have to worry about maintaining a balanced diet complete with the right nutrition. In other words, milk consumption probably makes up for the typically poor Western diet.

If that were all there is to the story we might consider that dairy consumption in modern Australia is a health benefit. However the dairy industry encourages over-consumption of dairy products and so it seems likely to me that we are conducting a big experiment on our health by doing so. The results of that experiment might be the kinds of risks now being uncovered, such as increased risks of various cancers. As I noted above, perhaps over-consumption of dairy has health risks when viewed over the longer term.

The bottom line though seems to me to be that there is nothing in the story of dairy that would actually point to dairy being an essential element in the natural diet of a human being. I doubt very much that we have some improved longevity due to dairy – the best that could be said is that the inclusion of dairy in neolithic diets provided a protection against other environmental risk factors.

One curious possibility that occurs to me is that perhaps the same niche construction drivers exist today. Consider that as I observed above, the modern Western diet is generally acknowledged as rather poor. People eat the wrong foods, in particular processed foods, they eat far too much, and we have very sedentary lifestyles. Perhaps whatever benefits are claimed for dairy really only arise in the modern setting because its consumption helps to redress the balance against the same compromised health due to diet and density of stable populations that bedevilled neolithic farming communities.

And consider too the significant role that the livestock industry plays in encouraging people to partake of such poor diets. Could it be that the livestock industry is engaged in a circular feedback loop – compromise human health by encouraging an unhealthy diet/lifestyle and mask that through the consumption of dairy products?

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s