A very interesting study was recently published in the British Medical Journal that looked at the ability of a “high protein high calcium” diet to prevent fractures. Well, I say high protein high calcium – what they were actually testing was what happens if you give frail elderly people (the people at greatest risk of fractures) a diet high in dairy – which doesn’t sound either as scientific or as sexy as “high protein high calcium”…
Anyway, this was a randomized trial involving 7,195 nursing home residents spread across 60 nursing homes in Australia. The trial was cluster-randomized, which means that rather than individual nursing home residents being randomized to one diet or the other, entire nursing homes were randomized to follow either a high dairy diet or to continue with their current diet. So 30 nursing homes followed a high dairy diet, and 30 continued doing what they were doing previously.
Since it’s hard to blind people to what they are eating, this was an unblinded study. That should always make us more cautious about a study’s results, because it makes manipulation of the data easier. However, the outcome being analyzed was radiographically confirmed fractures, which is a hard outcome without any subjectiveness – either you’ve broken your hip or you haven’t – so the lack of blinding shouldn’t necessarily be that big a deal.
One thing that’s important to note about the study is that it was largely funded by various dairy farmers’ associations and lobbying groups. It’s been shown many times that studies funded by groups with vested interests usually show more positive results than studied that are independently funded. This lack of independence might in fact be the single biggest weakness with the study – it’s certainly a reason to be sceptical of any positive results. However, it’s rare to see randomized trials in nutrition science – usually all we have is crappy observational studies that can’t say anything about cause and effect, so it’s nice to have a proper randomized trial to look at for once. The fact that this was a randomized trial is likely the reason it ended up being published in the top-tier British Medical Journal instead of one of the many low-quality nutrition journals.
The nursing homes that were randomized to a high dairy diet were required to reformulate their menus so that the residents received more dairy products that were high in protein and calcium (i.e. milk, yogurt, and cheese) in their diets. This was accomplished, for example, by replacing biscuits with yogurt as an afternoon snack. Here we immediately stumble upon a major issue with this study, that in fact plagues most diet studies – whenever you add something to a diet, it generally means that you’re taking something else away from the diet. If we add dairy and remove refined carbs, and we then see a benefit, is it due to the addition of dairy or the removal of refined carbs? It’s impossible to say.
Let’s get to the results.
Average calcium intake among the participants in the intervention group increased from 562 mg to 1142 mg per day, while average daily protein intake increased more modestly, from 0.9 grams per kg of body weight to 1.1 grams. No change in intake was seen in the control group, as you would expect.
The participants were followed for an average of 13 months, and during that time period 5.2% of participants in the control group experienced a fracture, compared with only 3.7% in the intervention group. That might not sound like a very big difference, since the absolute difference in fracture risk was only 1.5%, but if you consider that the participants were only followed for one year, it’s not bad. It represents a 33% reduction in the relative risk of a fracture. The absolute benefit grew bigger and bigger with each additional unit of time, and would likely have continued growing bigger if the study had gone on for longer. The effect size at one year is equivalent to that you would see with a bisphosphonate, which is the type of drug most commonly used to treat people with an increased risk of fractures. Considering that we’re talking about an easy, cheap, safe (and tasty) intervention, that sounds pretty good. Personally, I’d rather eat dairy than take a bisphosphonate.
Let’s assume for a second that the researchers haven’t somehow manipulated their data, and that the reduction is real. Then we have to answer two questions: first, what is responsible for the reduction in risk? And second, how is it having its beneficial effect?
As mentioned, it’s impossible to say whether the benefit is due to adding dairy or removing bad things like biscuits and cakes, which the dairy has now come to replace. And since dairy is itself a collection of lots of different substances, if we assume that it is in fact the addition of dairy that’s having a beneficial effect, it’s still impossible to say whether it’s the increase in calcium that is beneficial, or the increase in protein, or something else entirely. Trials of calcium supplements have generally found much more modest reductions in risk than was found here, so I’m inclined to think that protein is more important for fracture prevention than calcium.
There is one problem with attributing the benefit to protein though, and that is that protein intake only increased modestly, from 0.9 grams per kilogram of body weight to 1.1 grams. However, if 0.9 grams is insufficient for people to maintain their present amount of muscle mass, while 1.1 grams is sufficient, then that small difference in intake could actually result in large downstream consequences.
This is supported by the fact that the dairy group maintained its lean mass over the course of the study, while the control group lost on average 300 grams of lean mass. Again, this might not sound like a lot, but if the control group is losing 300 grams more lean mass per year than the intervention group, then there is quickly going to be a very big and noticeable difference in strength between the groups. More muscle mass means greater strength and thereby a greater ability to avoid falling, and also more dampening tissue on top of the bone if a fall does happen.
The dairy group both suffered fewer falls than the control group (suggesting an effect on muscle mass and thereby strength), and when they fell, they had fewer fractures (suggesting an effect on bone strength). The dairy group did in fact appear to have better bone density overall than the control group at the end of the study, when measurements were taken from a subset of study participants. This could perhaps be explained by the increase in calcium intake, or the increase in protein intake (protein is a central component of bone), or something else entirely. Dairy is a pretty good source of vitamin A, to take one example.
That all assumes the benefit seen in this study is real. Is it though?
The fact that this was an industry funded study makes me very sceptical. Even if everything seems kosher on the surface in a short study report in a medical journal, there are so many little easily hidden manipulations that can happen along the way as a trial progresses, which can sway the trial in favour of showing benefit. One thing that I find particularly concerning is that much of the data in the trial report is presented in text form only, without accompanying tables. That makes it much harder to analyze the data. It makes me wonder if the authors are trying to hide something. So, from my perspective, this study is encouraging, but hardly constitutes conclusive proof. For that, you would need a truly independent trial that confirms the findings of this study. In the meantime, have some cheese.
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