I’ve had a few emails this week about a study which appeared in the British Medical Journal last year . In short, it purported to show that Swedish women eating a ‘low-carbohydrate, high-protein’ diet over a period of 16 years had greater risk of cardiovascular events such as heart attacks or stroke. I was well aware of the study at the time, but chose not to write about it because, as I explain below, I believe it really is a rubbish piece of ‘research’ which tells us essentially nothing about the impact of low carbohydrate diets on health. Also, it came on the back of another study based on the Swedes that was similarly useless. One reason for writing about the more recent study now is to make clear just how rubbish it is, seeing as I’m getting asked about it. Another reason for this post is to draw attention to the utter drivel that some doctors and commentators can talk, suppoesedly in the name of science.
Let me start by describing the study before I point out what I think some of the problems are with it.
In the early 1990s a large group of Swedish women were asked to fill out a diet questionnaire asking them to remember and log how often they had eaten specific foods. After eliminating some women with evidence of cardiovascular disease or who hadn’t filled out their questionnaire properly, about 43,000 women were then followed for 16 years and monitored for cardiovascular events such as heart attacks and strokes.
On the basis of their dietary logs, the researchers assigned the women a ‘low-carbohydrate, high-protein’ score. Here, women with the lowest amount of carbohydrate in their diets and the most protein ended up with a score of 20. Women with the most carbohydrate and the least protein ended up with a score of 2. Women with a score of 16 or above were found to be at 60 per cent increased risk of cardiovascular events than those with a score of 6 or less, which led to the conclusion that ‘low-carbohydrate, high-protein’ diets are bad news. End of.
OK, here, in no particular order, are what I see as the main problems with this study:
1. It’s epidemiological
This means it looks at the association between two things (diet and cardiovascular disease). But just because two things are associated with each other, does not mean one is causing the other. This study, and the way it was reported, inferred that low-carb, high-protein diets cause cardiovascular disease, and this sort of study just can’t support that sort of conclusion at all.
2. The study used dietary recall
Do you remember what everything you ate and drank a week ago today? Most people struggle withw this sort of thing. Just imagine, now, trying to give an accurate representation of what you’ve eaten over the last 6 months. Can you see some room for inaccuracy?
Also, I don’t want to come over unduly cynical, but there’s a tendency (unconsciously or consciously) for people to under-report their consumption of foods perceived as unhealthy, and over-report their consumption of food with healthy reputations.
Oh, and the women’s diets were assessed only once during the 16-year period of the study, because we all know people never change their diets over time, right? Again, here we see enormous potential for the dietary logs not to reflect what the women actually ate.
3. The ‘low-carbohydrate’ women weren’t very ‘low-carbohydrate’ at all
Women with the highest low-carbohydrate score were, according to their dietary logs were consuming up to about 125 g of carbohydrate a day. In reality, the low-carbohydrate diets that are the whipping boys of just this sort of study often allow less than 50 grams of carbohydrate a day. In other words, this study was not able to determine the effect of genuinely low-carbohydrate diets.
Very rarely is a piece of science perfect and beyond criticism, but the failings of this study make it almost not worth the paper it’s written on. Yet, this week has seen me getting a steady stream of emails about it. Why?
Well, it turns out that this study was specifically cited in a piece run by the Guardian newspaper, in which several doctors were asked what things they wouldn’t do. You can read this piece here.
Here, you’ll find Dr Tom Smith writing the following advice to us all:
I would never go on a low-carbohydrate, high-protein diet like Atkins, Dukan or Cambridge. Why? Because although you will probably lose weight, they may kill you. Don’t take my word for it – read about the 43,396 Swedish women followed for an average of 15 years. Those who stuck to low carbs and high protein had a rising risk of dying from heart attacks and strokes, depending on how strict they were and for how long they endured them. There was a staggering 62% higher risk of such illnesses among the women eating the strictest diet over those who ate normally. Eating is for enjoyment; these diets turn food into medication, and it’s patently the wrong medicine – it is often lethal.
Notice here how Dr Tom shows no apparent appreciation of the manifold deficiencies of this study and the likely errors in it. Notice also how he seems to fail to appreciate the inability of epidemiological evidence to determine cause and effect. Notice also how Dr Tom appears to conclude causality when it is simply incorrect to do so.
But should we be too surprised? Well, my experience tells me that while we may rely doctors for our scientific and rational minds (and some of us pride ourselves on having such things), the reality is some of us have a tenuous grasp of scientific principles and methodology. But another reason for us not being too surprised is that Dr Tom has form in this area.
Last year, I wrote about him specifically because I’d heard similar utter nonsense spout from his mouth at a scientific meeting. You can read about this here. In short, he lauded quite useless epidemiological evidence again in his ‘theories’ about Alzheimer’s disease. I wonder if Dr Smith is aware that a major theory regarding the development of dementia concerns what is being termed ‘type 3 diabetes’ which, in all likelihood, is driven by the over-consumption of carbohydrate. I’m wondering also if Dr Smith might personally (and professionally) benefit from cutting back on carbs for the sake of his mental faculties.
1. Lagiou P, et al. Low-carbohydrate-high protein diet and incidence of cardiovascular diseases in Swedish women: prospective cohort study. BMJ 2012;344:e4026