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Do low fat diets really kill?

Date added: 30/08/2017

'Low fat diets could kill' screams the latest headline and, even more bizarre, 'fat has a protective effect'.

All of this from an observational study, which is basically a big soup of data stirred occasionally and left to settle. Let's look at some of these claims in detail:

Huge international study: yes, indeed, the study was international. In fact, it was a cohort study of 135,000 people from 18 different countries who were recruited between four to 14 years ago and checked occasionally to collect some data. However, it's worth pointing out that the UK was not included and, indeed, only two of the 18 countries (Canada, Sweden) had diets and lifestyles which were in any way similar to ours. Most of the participants were from Asia, the Middle East and Africa. This is why the results of the study can't be extrapolated to the UK situation.

Low fat kills: No interventions were done. No diets were tried. The people just did their own thing, which is why we can't say which particular aspect of their diet or lifestyle killed them in the end. Yes, it could be the low fat intakes but it could also be that they ate poor quality protein foods, too little fibre, not enough vitamin D, too much salt, etc etc. This is the whole point about an observational study - you simply can't determine cause and effect. Anyone who says you can is lying. The best you can do is check which factors were statistically associated with mortality. That's all.

How low is low? This gets us to the question about what levels of fat and saturated fat intake were associated with an increased mortality in this study. The graphs show that mortality started to rise when total fat intake fell below 20% of calories and saturated fats fell below 5% of calories. So, does this give Brits a licence to chuck out the low fat yogurt and gorge on butter, pies and ice-cream? No. If you check out UK dietary surveys, we get more than a third of our calories from fat and around 11-12% of our calories from saturated fats. That means we already eat more than enough fat. Indeed, there is room to switch some of the saturates for healthier unsaturated sources, such as oily fish, nuts, seeds and olive oil/rapeseed oil.

Dump the carbs and go high fat: According to some media, we should dump current dietary advice, which of course plays into the hands of the low carb lobby who can't wait to sell us their diet books. Let's check that Lancet study again ... the diets which seemed to increase risk had carb intakes well above 50% of daily calories. And remember that most participants were from China, Asia and the Middle East so we are really talking about diets high in rice. Again, this has no impact on our UK diets where we get our carbs from a variety of sources - potatoes, bread, cereals.

My advice, as ever, is to look at the whole picture not just one highly publicised study. What this tells us is that diets are complex and can't be distilled into a 'fat versus carbs' issue.

People eat food, not nutrients. So eat plenty of vegetables, wholegrains and fruit, then add some lean meat, fish, eggs and dairy, and keep those high calorie, less nutritious foods (cakes, biscuits, sweets, soft drinks, alcohol) for an occasional treat. It's not rocket science but, of course, it's a less sexier headline than 'low fat diets kill'.