Dr Zoë Harcombe, PhD set out in the late 1980s to find out why the obesity epidemic was occurring and how dietary guidelines were involved. The purpose of her PhD in public health Nutrition, was to conduct a systematic review and meta-analysis of the evidence supporting the introduction of dietary fat recommendations in 1977 and 1983. This talk was given at the Public Health Collaboration conference 2017 in Manchester.
Dr Harcombe begins her talk by outlining some key facts about fat. Interestingly, with the exception of a few foods such as avocados and nuts, natural, ‘real’ foods generally do not contain both carbohydrate and fat in appreciable quantities. It is the ‘fake food’ industry that produces processed foods with large amounts of both. A common myth is that animal products, especially red meat, contain predominantly saturated fat. Firstly, any food that contains fat, contains all three types of fat: saturated, monounsaturated and polyunsaturated, though these are in varying proportions. Secondly, dairy is the only food group that is predominantly saturated fat. Thirdly, dietary guidelines suggest eating oily fish in place of red meat, such as steak, for the purpose of reducing total fat and saturated fat, however, oily fish actually contains more of both. Finally, olive oil is touted as a healthy food, yet one tablespoon contains more saturated fat than 100 grams of pork chop. Whilst all of these foods can certainly contribute to a healthy diet, the confusion over the different types of fat is a sign that current guidelines are poorly informed.
New dietary guidelines were implemented in the USA in 1980, and in Britain in 1984. These guidelines ran counter to the old way of thinking, and set limits on the amount of dietary fats, particularly on saturated fat. In Britai, it was recommended that no more than 30% of energy should come from fats, and 10% from saturates. Since protein is relatively constant in the diet, the recommended proportion of dietary carbohydrate increased to 55% to make up the lost calories. The changes in dietary fat guidelines came about because of the rate of heart-related deaths in the USA in 1950, which stood at 40% of deaths (0.59% of the population).
The new dietary fat recommendations were made for the USA and Britain based on the inconclusive results of six secondary randomised controlled trials (RCTs). In fact none of these studies recommended change, and most actually cautioned against it. The epidemiological evidence at the time, also from six studies, did not support the changes either. None of these found a significant association between total fat and coronary heart disease (CHD), and only one found an association between saturated fat and CHD. This was the Seven Countries Study, conducted by American physiologist Ancel Keys, which found no associations for smoking, physical activity or weight – things which we now know to be heavily associated with CHD. Dr Harcombe goes on to address weaknesses in today’s evidence against saturated fat, of which there are many.
Dr Harcombe finishes by highlighting how much of the population’s dietary saturated fat comes from processed foods, both in the UK and the USA, concluding that the single most important guideline that should be implemented is to “eat real food”.