3. Measuring diet is hard.

How do we, and in reality how can we, measure an individual’s diet over a lifetime?

Actually, it’s almost impossible to measure diet, with any degree of confidence, over even a single week. The so called,  ‘gold standard’ of dietary measurement (the best method) is known as a 7 day weighed intake. The idea is that a person records and weighs absolutely everything that passes their lips for a week and all that data is put into a computer program that translates the food into nutrients (fats, carbohydrates, protein, vitamins, and minerals). This ‘best’ method is, however, beset with inherent error.

All nutrition and all dietetics students complete and analyze one of these weighed intakes, as part of their studies; and they all, without exception, cheat (I did too). The task of weighing and recording everything is simply too onerous, so they lie; for example recording the same breakfast every day and simplifying their diets to make them easier to measure. Would you confess to scoffing 3 cream cakes and downing a bottle of wine (or two) in an evening? No, I don’t think so, it’s human nature and such dietary indiscretions are not recorded at all or at least under recorded. And what about going out to eat? You can’t really start weighing your food in a restaurant.

So what this means is that the so called ‘gold standard’ of dietary measurement is, at very best, an approximation and without a doubt, an incomplete and inaccurate record that can not measure a weeks food consumption accurately, let alone represent an individual’s ‘typical’ diet.

Did you know that all the main UK dietary surveys (Nation Diet and Nutrition Surveys), undertaken over the past 20 years, all show that the UK population doesn’t eat enough? The fact the population has been getting fatter, over the same period, clearly indicates that the dietary surveys are without doubt underestimated and wrong. Yet it is this very data that informs UK nutritional policy. Because the nutrition community recognized that the data was seriously flawed, these large national surveys moved from measuring the population’s diet using the 7 days weighed intake to a 3 day, un-weighed food diary. Just three days to represent a lifetime’s diet.   

In fairness, there are also studies that attempt to measure diet over the longer term. These rely on periodically administered dietary questionnaires, given to large numbers of people, over a number of years. The validity (accuracy) of these questionnaires for measuring diet is determined by comparing them to the ‘gold standard’ (the 7 day weighed intake), in a small group of people. But since the ‘gold standard’ is flawed; so are the questionnaires. In any case, it is recognized that questionnaires are an essentially blunt instrument and can not provide a quantitative estimate of dietary intakes and can only be relied upon to classify people as low, medium or high consumers of any particular food or nutrient. Rather like virtue signaling, people tend to overestimate the perceived good stuff and underestimate the perceived bad stuff.

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