A recent study published in the British Medical Journal on milk consumption has generated some waves. Milk is widely advocated (by the Dairy Farmers for instance) for the prevention of osteoporosis and other general goodness of health. We give it to our kids, believing it will make them tall, strong and less obese. However, the evidence to back such beliefs is surprisingly scarce.
The Dietary Guidelines for America, for instance will advise that we should get 3 cups of milk per day in order to get enough calcium. Most people also include vitamin D as one of the benefits, but of course, milk does not naturally contain vitamin D. It is only added afterwards and not part of the milk itself. Advising people to drink milk to get their vitamin D is like advising people to drink Vitamin Water to get their vitamins. It doesn’t make any sense. If you want Vitamin D, then take a supplement. That’s the same as getting it with milk.
Anywho, we talked about dairy in the protein section recently. A quick recap. Dairy protein, while being low on the glycemic index, is surprisingly high on the insulin index. However, large scale correlation studies are equivocal on whether it causes weight gain or not. This is because dairy is a potent stimulus to the incretin effect, which also produces satiety through its effect on gastric emptying.
In this recent study, the authors, from Uppsala University in Sweden reviewed two very large cohorts – The Swedish Mammography Cohort and the Cohort of Swedish Men involving 10-20 year followup periods and over 100,000 participants. The subject here was not obesity, but fractures and total mortality.
Where did people get the idea that drinking milk was good for the bones? Mostly, from television. There was really very little data to support such a belief. It seems to be mostly based on the fact that milk has calcium and bones have calcium, so drinking milk must be good for the bones. And eating kidney is good for the kidney. Eating liver is good for the liver. Riiiight.
The whole calcium is good for the bones thing was kind of blown out of the water several years ago, anyway. In 2006, the Women’s Health Initiative produced the results of a randomized trial of calcium plus Vitamin D supplements on hip fracture risk. Over 36,000 women participated in this huge and expensive trial. It’s too bad the results were completely ignored. While bone density went up (yay!), hip fracture rate did not.
Some point to the fact that reduction in hip fracture was almost significant (hazard ratio 0.88), they always neglect that there were more fractures elsewhere since the total fracture rate was close to unity (0.96). In other words, calcium supplements made the bone density look better but didn’t actually make anybody clinically better. So calcium made the doctor feel better that he/she was treating you, but it didn’t make the patient any better.
Furthermore, this large study was able to identify a major risk of supplementation in increasing kidney stones. While this is not a life threatening complication, having little rocks in one’s genitourinary system is no fun either. Even worse, it was all because of the stupid calcium pills that didn’t work anyway.
But, as always, when trials don’t show what we expect them to, we ignore them and make excuses. So calcium continued to be prescribed widely for several years. But the evidence was getting too much to ignore. Several years after that, there was increasing concern that calcium supplements were increasing the risk of heart disease by 25-30%.
Finally, it was time to pull the plug. The US Preventive Services Task Force removed calcium supplements from its recommended list in February 2013. While the official recommendation is “Insufficient Evidence”, it really takes a lot to get the rug pulled out from a longstanding recommendation like that. So just like that, calcium supplements were out. But the recommendation to drink milk persisted driven by its wholesome image. Extravagant claims abound in the literature that 3-4 glasses of milk a day will save 20% of the total health care costs of osteoporosis.
Of course, osteoporosis was never a calcium deficiency disease. Countries that had the lowest consumption of calcium had the lowest osteoporosis rates. The Japanese, for instance, had a daily calcium consumption approximately 1/3 of Americans. Yet their mortality from osteoporosis was barely 1/10 of that in the USA. It wasn’t until the Japanese move to the USA, with its higher intake of calcium that osteoporosis increases. That is not to say that the higher calcium caused osteoporosis, but the higher calcium certainly didn’t protect against it.
Since osteoporosis wasn’t caused by calcium deficiency, there is no reason to believe that extra calcium will protect against it.
So researcher in Sweden decided to look a little close. Milk, they reasoned, may also have detrimental effects. It is the principal source of D-galactose. In experimental animals, this substance was extremely harmful as it increases oxidative stress. Cheese and fermented milk products (yogurt) do not have this potential toxin. Most meta-analyses did not separate out milk from other dairy products.
What they found was a shock. Each additional glass of milk for women did not reduce the risk of fracture, it increased it! Drinking 3 or more glasses a day virtually doubled the risk of hip fracture. Not only that, but the risk of dying was substantially higher. Milk wasn’t good, it was bad. In men, the correlation was much weaker, although there was still a slightly higher risk.
Mind you, this is only a correlation study. This does not prove that milk causes fractures. This could be the result of reverse causality. That means that those women at highest risk of fracture were drinking the most milk to try to prevent it. A correlation study can only generate hypotheses.
What it means practically is that we should not be advising people to drink milk. We simply don’t know if it is harmful or beneficial. In the absence of knowledge, the prime directive is to ‘First, do no harm’. That means, if you like milk, drink it. If you don’t, then don’t drink it. After all, there is only one species of living organism that drinks milk after they are weaned from it. That’s us humans. Perhaps we think we are much smarter than Mother Nature. But somehow, I doubt it.