1. Lactose intolerance

Intolerance to milk-containing foods is common, and increases with age. This is thought to be because as we get older, our need to digest milk declines (which we would originally have only consumed from our mother’s breast). The majority of the world’s population starts to decrease their ability to digest milk by the age of 3-5 years. This trend then continues as we get even older; in one study, around half of adults over 50 were lactose intolerant, and around a quarter were under the age of 50, although this does vary between ethnic groups, and Northern Europeans in fact seem to have one of the highest lactose tolerances (1) (2).

Milk intolerance is caused by low levels of the enzyme ‘lactase’ which breaks down the milk sugar known as ‘lactose’. This is usually due to a genetic trait, which means that we don’t produce enough of the lactase enzyme anymore (2).

This matters because lactose sugar needs to be broken down into two smaller sugars, known as glucose and galactose, before it can be absorbed as energy;

If this breakdown doesn’t happen, then the lactose travels onwards from the small bowel into the large bowel, or colon. In those who don’t have good lactase enzyme activity, around 75% of the unabsorbed lactose sugar will pass into the large bowel (2).

Unfortunately, when unabsorbed lactose sugar reaches the colon it is fermented by the gut bacteria, producing a gas. This can distend and stretch the bowel walls, leading to discomfort and bloating, two of the classical signs of lactose intolerance. The higher sugar load of the large bowel also draws water into the gut, which can lead to loose stools or diarrhoea (2).

The symptoms of lactose intolerance may include (noticed after eating at least two servings of dairy / day)*

• Long-term problems with diarrhoea
• Abdominal pain or discomfort
• Bloating
• Flatulence

*Note: These symptoms are very non-specific, so if you are suffering from any of the above, or are worried you might have lactose intolerance, do see your GP to just check there is nothing else that might be going on. This is especially important in children.

2. Galactose

Studies have shown that a high intake of one of the milk sugars we’ve looked at above, galactose, may not be all that beneficial to our health. Long-term exposure, particularly to high amounts of galactose, has been shown to hasten biological ageing in animal models (in this instance, in mice).

These changes include damage from oxygen radicals (known as oxidative stress), chronic inflammation, degeneration of the nervous system (including the brain), reduced immune function and changes in the way that genes are read and copied (which is potentially a risk factor for cancer development) (3) (4) (5). Such effects have been estimated to potentially affect humans with milk consumption of just 1-2 glasses per day (6).

Now, it is important to point out that there are obviously big differences between studies in mice, and what this would mean for humans, but this initial research offers us a warning shot as to why milk might not turn out to be quite so great for us after-all. A study of over 100,000 people, published in the British Medical Journal in 2014, showed that drinking milk was shown to increase levels of certain inflammation markers in both men and women (6).

There is a glimmer of good news for dairy lovers though. It is thought that cheese and fermented dairy products (such as kefir or yoghurt) would have less of these effects, because of their lower lactose sugar, and therefore galactose, content. In fact, a high intake of fermented milk products has been associated with a decreased risk of heart disease, as well as reducing the markers of oxidative stress and inflammation (6).

Galactose sugar is present in other foods, mainly cereals, vegetables, and fruits, but the amount is very small compared to that found in milk (7).

3. Fractures

Contrary to popular belief, the idea that high milk consumption reduces the risk of broken bones, particularly as we age, has been rightly questioned. In fact, it was found in this BMJ study that high milk consumption does not necessarily protect you against fractures, and for women in particular, might actually increase your risk (6). These results are only preliminary, however, so more research is needed (especially amongst different ethnic groups, who may have higher or lower rates of lactose intolerance), to be sure of the findings.

4. Mortality (risk of death)

Without too much time spent lingering on this rather morbid topic, it was found in the same BMJ study (6), that mortality increased across both sexes as milk intake increased. This pattern was not found, however, with dairy products other than milk. As is often the case, there is some disagreement in the scientific community on this topic too – some previous studies have shown the opposite effect. The discrepancy might be due to differences in the way that the studies were set up and analysed, so further research is probably needed before this question is answered for sure.

A skeptical aside: When looking at scientific research, it is really important to not get too confused with the difference between cause and effect. These are what are known as observational studies, so we cannot know for sure that the milk is what is increasing the risk of death, or whether milk drinkers might have another risk factor (perhaps it is all that caffeine they are drinking in their lattes, for example!) which is the real culprit.

5. Milk quality

Organic milk has been found to contain more of the healthy omega-3 fats (although less of the essential minerals iodine and selenium) than non-organic milk (8). Unfortunately, we don’t yet know what the overall effect of any residual chemicals (such as antibiotics, pesticides etc.) and hormones found in milk has on the human body.

6. A1 vs. A2

A major source of the protein found in milk is called Beta-casein, of which there are two subtypes – named A1 and A2. It is now possible to buy milk that has only the less common A2 protein (most supermarket milk also has A1). You may have seen some adverts for this on the television or in magazines.

Some studies have found that, in people who have milk intolerance, A2 milk may give them fewer gastrointestinal symptoms (such as bloating or pain), compared to regular milk. This suggests that there may be some people who, instead of being intolerant to the lactose sugar, are in fact intolerant to the A1 milk protein casein (9), (10) (11). However, these studies have been rather small, and have generally been funded by the A2 industry, so to be certain of the effects, a larger, independent study is certainly needed.

There is also still rather a lot of uncertainty about the potential effects, good or bad, of A1 vs. A2 milk (12) (13), but research is still going on to try to unpick the facts to help guide our decision of the future.