The Maasai of East Africa claim a staple[1] of bovine milk as their birthright. Such a diet in adulthood has no precedent among mammals. Furthermore, no physiological adaptation in the Maasai to such specialisation has been discovered. We investigated how humans might achieve such an extreme position in the food-web, finding some biological surprises and putting ourselves through more viral infections than seems reasonable.

Most readers will know that lactose, the sugar naturally occurring in milk, can be unhealthy. However, dietary experts seem ignorant of the broader biology of lactose. By experimenting boldly with our own diets, and living for a time among the Maasai, we’ve learned some salutary lessons.

But, before we can expose milk sugars for their real effects, we need to clear up several misconceptions about dairy products. The first of these is the real nature of milk sugar.

Lactose is a disaccharide, the digestion of which releases the monosaccharides glucose and galactose. Regardless of whether one can digest lactose or not, its galactose component is not nutritious for adults, with the possible exception of lactating mothers. Galactose is an essential building block for the growing brain of the infant[2] because it’s vital for the formation of the glycolipids and glycoproteins in the nervous system. But because nerve tissues tend to be permanent once formed, the dietary requirement for galactose is minimal after the age of about ten years in humans.

The second misconception is about lactose intolerance. The reputed inability of most adults to digest the lactose in bovine milk is misleading because it suggests that the necessary genes for the secretion of the enzyme lactase are absent. In fact, all adult humans – like adult pigs, dogs and monkeys – continue to produce a certain amount of lactase in the small intestine even in old age. The genes remain present but the genetic controls are subtle and seemingly perverse. The real problem is that the adult gut is – possibly owing to the specialised nature of galactose – extremely reluctant to produce more than a nominal amount of lactase even when this enzyme is demanded by the consumption of milk.

The only animals producing enough lactase to digest anything approaching a staple diet of milk in adulthood are certain bloodlines of humans in two areas, namely northern Europe and the Saharan region[3]. These lactose-tolerant peoples are, unlike the Maasai, not specialised pastoralists with bovine livestock. If anything unites them, it is that they live in climates that are either extremely cold in winter or extremely hot in summer. The rest of us, including the Maasai, retain the genes necessary for secreting lactase but our digestive system fails to respond when we consume more than a small amount of lactose.

The third and fourth misconceptions are both about pasteurisation.

Pasteurisation does not – contrary to popular belief – sterilise milk. It kills pathogenic bacteria and beneficial lactobacilli, but not the bacteria that eventually rot any pasteurised milk left to stand in a sealed container beyond its expiry date. That sulphurous stink of rotten milk is indeed bacterial, and milk does not rot in this way unless it’s been pasteurised. This is because, left unpasteurised, milk sours[4] instead of spoiling; this happens spontaneously without any inoculation and the result is as wholesome as yoghurt. Such fermentation leaves the balance of amino acids and vitamins intact and adds useful enzymes. There is consequently a trade-off in pasteurisation: the consumer is protected from certain pathogens at the expense of the capacity of milk to preserve its food value at room temperature by means of spontaneous fermentation.

It’s also untrue that pasteurised milk is as digestible as raw milk. In fact, heating reduces the digestibility of the protein by plasticising the casein[5]. And it reduces the digestibility of the fat in milk by denaturing the indigenous lipase[6] secreted in milk by the mammary glands. These are additional costs paid for the benefits of hygienic marketing of milk.

The final misconception is about saturated fat. The idea that the saturated fat in milk is harmful – whether via cholesterol or otherwise – has not stood up to scrutiny[7]. So one thing that’s not puzzling about their fondness for milk is that the Maasai are largely free of cholesterol-related degenerative diseases. If there’s a problem with the fat in milk, it’s more likely to be a deficiency in the essential fatty acids known as omega-3[8] .

With these misconceptions out of the way, our puzzle gets more, not less, intriguing.

How is it that the Maasai remain incapable of making enough enzymes to digest the milk sugar in what is ostensibly their staple diet? Actually, this question hardly does justice to the real biological puzzle of milk. More to the point is ‘How is it that most adult humans retain the genes for the digestion of milk sugar but are practically unable to digest more than small quantities?’ If humans and other mammals are generally intolerant of lactose in adulthood, could it be that milk sugar is, for some reason, not worth digesting?

It’s the lactose that makes digestion of milk in adulthood difficult for most people, including the Maasai. But the ultimate problem may be what the lactose contains – in particular galactose, which is unhealthy even when absorbed into the bloodstream.

When unpasteurised milk spontaneously ferments by means of lactobacilli, the refreshingly sourish product is called clabber[9]. This can be thought of as runny, self-made yoghurt; it remains appetising without refrigeration to those used to its taste and texture. However, this normal fermentation of milk is surprisingly ineffective in breaking down lactose, suggesting that even bacteria shun the galactose contained in the lactose.

The bacteria that convert milk to clabber and yoghurt are called lactobacilli because they make lactic acid, not because they make lactase. Hence the clabber routinely consumed[10] by the Maasai – like the natural yoghurt sold as a ‘health product’ by the dairy industry – perversely retains enough of the milk sugar to cause lactose intolerance if consumed as a main component of the diet. What is biologically remarkable is that these wholesome bacteria of dairy products cannot or will not use the sugar, galactose. It’s as if this monosaccharide, while essential for brain development in infant mammals, is toxic to adult mammals and even to microbial decomposers.

Now that we’re quite clear how puzzling is the Maasai claim – by implication – of tolerating lactose with impunity, we can refine our question further. If the ultimate problem is not lactose as such, but rather its component sugar, galactose, why is this monosaccharide so undesirable as a source of energy? Based partly on our own self-experimentation, we have indeed come to regard galactose as toxic for adults in more than small doses.

A hypothesis seldom discussed in medical circles is that the metabolism of galactose in the liver produces insoluble mucic acid, which the body purges as unhealthy mucus via viral ‘colds’[11] . As a way of testing this, one of us consumed an average of 0.8 litre of unpasteurised goat’s milk – regarded as a particularly healthy dairy product by educated consumers – daily for several years without cooking or deliberate fermentation. The result: we remained physically fit, continued to enjoy the taste, and experienced no lactose intolerance. However, we did suffer increasingly frequent ‘colds’ culminating in an average of one ‘cold’ every three months in the second-last year of the experiment. Each ‘cold’ lasted for at least one week. When we partially substituted the milk with the most natural[12] available brand of bovine yoghurt, this rate increased intolerably to one ‘cold’ every two months for the final year of our self-imposed ordeal.

What we think these trials revealed is that galactose is a mild toxin for adult humans. We don’t doubt that lactose is comfortably broken down in the small intestine in lactose-tolerant northern Europeans and Saharans and uncomfortably fermented in the colon (by microbial lactase) in most people. But either way it’s the released galactose that has negligible food-value in any adult – including, we suspect, lactose-tolerant individuals.

Apart from the technically challenging manufacture of certain kinds of cheese, we are left with few  ways to get rid of the galactose while processing milk for routine consumption. One method is to allow the clabber to settle and to discard the whey – in which the lactose remains dissolved – while consuming only the fatty and protein-rich component which floats to the top. Another is to allow the milk to continue to ferment spontaneously beyond the bacterial stage of clabber and into an unnamed yeasty stage – as we in fact proceeded to do in seven further months of self-experimentation. With thorough fermentation by yeasts without deliberate inoculation, it seems that the unwanted galactose is finally destroyed[13], for we’ve suffered no ‘cold’ since. But the fizzy, fatty final product – for which we know no name in English or any other language – is so sour that it would hardly be recognised as a dairy product by most people, including the Maasai.

It’s easy to assume that the lactic acid in clabber and yoghurt is produced by the fermentation of the sugar in the milk. So it comes as a surprise to find that yoghurt and clabber retain the galactose and indeed most of the original lactose[14]. Clabber, which is full of beneficial lactobacilli, may act as a healthy probiotic for the colon, alleviating the discomfort of lactose maldigestion[15] . But even for contented consumers of clabber a problem can remain that galactose is virtually toxic for adults, however comfortable the digestion of lactose into galactose and glucose.

Getting rid of the galactose in milk, in our experience, means leaving the brew on the kitchen shelf for several days for the fermentation to proceed to its yeasty, fizzy stage. This destroys any remaining sugars and wastes most of their energy content via the volatilisation of carbon dioxide[16] . The product is delicious a half-cup at a time with oats and fruit, but is too vinegary to be any kind of staple. The extreme acidity of this stuff, which is self-preserving like pickle and remains edible for months without refrigeration or other protection, is presumably owing to acetic, not lactic, acid.

However, as far as we know the Maasai neither discard the whey nor consume their milk at an extremely sour[17] stage of fermentation. So how do they cope with the unwanted galactose in their digestive system? How can the Maasai claim of reliance on milk in adulthood possibly be true?

Our answer is twofold. Firstly, the land of milk and milk in which the Maasai claim to live may be exaggerated. Secondly, their culture may be sophisticated enough to compensate medicinally for the galactose in their otherwise wholesome clabber.

We suspect that the importance of milk to proud pastoralists is as much ideological as biological. To the Maasai and culturally similar peoples such as the Tutsi of Rwanda, milk symbolises a certain traditional superiority of free-striding herders over grubbing horticulturalists. Milk is the idealised diet, not the exclusive food or even the healthiest food, of the Maasai. When interviewed by anthropologists, they may omit to mention humbler foods that they eat just as frequently. The Maasai may claim to specialise on milk but in our experience – while living among them in Tanzania – they have a varied diet in the longer term and the broader sweep. Pastoralism is a viable economy overall in parts of East Africa but the local bost[18] is small and frugal compared with the European breeds of the dairy industry, or even the milk-producing breeds of the river buffalo[19] in Pakistan and India. Because the Maasai are traditionally resident rather than nomadic or transhumant, they have periodically throughout their history been forced to accept herbaceous crops – usually by trading – when the supply of milk falters. Indeed, in the case of the Arusha people of northern Tanzania, the whole economy has shifted to horticulture without the loss of the original pastoral language, Maa. The Arusha are regarded as horticultural Maasai, rivalling Bantu people[20] as accomplished cultivators. We suggest that a switch in reliance from pastoral to cultivated products and back again has played out repeatedly according to circumstance in the history of the Maasai. And by drinking milk and clabber less frequently than they would like to, and obtaining omega-3 fatty acids from other sources[21], these people have been spared most of the problems of galactose or an imbalance in fatty acids.

Furthermore, the Maasai use poorly documented herbal medicines. Already investigated are saponins brewed in ‘soups’ from bark and roots of various trees, which have been assumed to medicate a diet too rich in saturated fats. However, it’s possible that certain herbal remedies of the Maasai are effective against an excess of galactose in the preferred diet of clabber. A hint to anthropologists and food-chemists: among the plants to investigate would be Commiphora, the same genus from which myrrh is produced farther to the north.

So, this is our best explanation. The Maasai may claim to consume a staple of milk from cradle to grave, but this does not seem realistic in our experience. We suspect that the Maasai have always consumed limited and fluctuating amounts of dairy products, resorting to various plant products and the fatty tissues of livestock to balance the diet and to tide them over in bad years. They routinely allow their milk to ferment to clabber, which – although it hardly avoids the ultimate need to excrete the metabolites of galactose – is possibly the most practical of dairy products overall. We don’t doubt that in the best seasons the Maasai can enjoy the high life, consuming little but fresh milk and clabber for weeks on end. But we suspect that they manage this luxury – of which an inevitable downside is excess galactose – by means of secret natural medicines that might be manna to pharmacists.


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Featured image: A Maasai herdsman grazing cattle by Muhammad Mahdi Karim (GFDL 1.2, Maasai Herdsman)

1In anthropological interviews, the Maasai claim to rely traditionally on milk to the degree that this is their preferred staple diet, with the implication that milk is the food that perennially supplies most of their energy and nutrients.

2 Whereas bovine milk does not contain indigenous lactase, human milk does contain indigenous lactase, possibly owing to the demands for galactose by the growing brain in the human infant.

3 including Arabia

4 by means of lactobacilli, which are bacteria naturally present in milk

5 Heat-treated milk protein is so similar to indigestible plastic that it can be used to manufacture fibres, textile fabrics, paints and glues. See

6 The indigenous lipase in unpasteurised milk allows autolysis in the stomach, easing digestion by sparing enzymes.

7 See Catalyst programme of Oct. 2013, Australian Broadcasting Corporation podcast by Maryanne Demasi,

8 Few foods other than cold-water fish and flax seed are rich in the fatty acid alpha-linolenic acid. Bovine muscle and particularly brain are far better sources of omega-3 fatty acids than bovine milk is. Even if the dairy animal receives supplements of omega-3 fatty acids, it fails to absorb these because they are degraded by the lactobacilli and other bacteria in the ruminant stomach.

9 This word, of Gaelic origin, is effectively obsolete in the urban world because English-speaking consumers of pasteurised milk mistakenly assume that unpasteurised milk becomes unwholesome once it ferments spontaneously.

10 The Maasai drink most of their milk as ‘kule naa-oto’, a sour but not yeasty product curdled by lactobacilli and best translated into English as clabber. Most of the bovine milk drunk by the Maasai is fermented by a spontaneous process requiring no special inoculation because the beneficial bacteria are freely available in the environment.

11 See ‘How much lactose is safe to use?’ by Walter Last.

12 free of sweeteners, smootheners, colourings, and flavourings

13 The galactose metabolism of Saccharomyces is extremely well-documented genetically.

14 Fresh bovine milk is known to contain about 4.5% lactose. Yoghurt, which represents a similar stage of fermentation to clabber, retains about 4% lactose.

15 The term ‘lactose malabsorption’ is misleading because lactose as such is not absorbed, even by persons who secrete enough lactase to avoid lactose intolerance.

16 Depending on the microbiological details, some alcohol may also be produced as a metabolic waste by the yeasts. The products of yeasty fermentations of milk are even stranger to the English language than clabber. In the Middle East and western Asia, the term ‘kefir’ includes fermented milk at both the bacterial and yeasty stages. ‘Kumis’ and ‘airag’ are mildly alcoholic beverages in central Asia, typically produced from equine milk, which is richer in lactose than bovine milk although poorer in this sugar than human milk. Equids grow slowly and have large brains relative to ruminants.

17 or carbonated, alcoholic

18 Bos indicus X taurus. See our blog-post ‘Why we won’t call a bost a bost’.

19 Bubalus

20 The slopes of Mount Kilimanjaro are cultivated by a Bantu group, the Chagga people, whereas the slopes of nearby Mount Meru are farmed by a Maa-speaking group, the Arusha people.

21 The Maasai would acquire considerable amounts of alpha-linolenic acid (omega-3 fatty acids) from the flesh and particularly the brains of the goat, which is seldom milked but invariably kept alongside the bost, which they are reluctant to slaughter.