Tuesday, April 28, 2009

Mediterranean Diet Score - What else does the model tell us?

A further analysis of the expected (from the model) vs the observed numbers within each diet score category and eating particular amounts of each food group throw up more interesting observations. This doesn't mean that we are claiming that people should behave like the model and that we expected that their food choices would not be internally correlated in some way. In fact, we expected that they would be (so do these researchers which is why they are trying to describe a dietary pattern); this data gives us information on what these correlations are.

We have already noted that meat & poultry intake is actually largely independent of the diet score. So, when researchers claim that a Mediterranean diet is low in meat, we will know that that is not borne out by this data.

Comparison of expected vs actual numbers in the low score (0-3) group shows that there are more people getting a low score because they score 0 for vegetables, fruit & nuts, legumes or fish consumption (i.e. they don't eat 'enough' of these). Conversely, in the high scoring group (6-9) there are more people than expected scoring a 1 in these same categories. There are also more people than expected scoring a 0 for meat & poultry, dairy and cereals in the high scoring group, i.e. they get a high score while still eating 'bad' amounts of these food groups. The suggestion here then is that the diet score is mainly a reflection of vegetable, fruit & nut, legume and fish consumption and is much less related to dairy and cereal consumption as well as being independent of meat consumption.

What about the predicted correlation between meat & poultry, dairy and the monounsaturated/saturated fat ratio? Whether this has any impact is very hard to determine. The main reason for this is that the postulated effect is obscured by the data presentation. To see why consider this thought experiment:

Let's assume our subject has a score of 6 so far and we have meat & poultry, dairy and the fat ratio still to determine. If either meat & poultry or dairy score 0 (i.e. greater than median consumption), this increases the chance that the fat ratio will also be scored 0, giving a final score of 7; if both meat & poultry and dairy score 0, then there is an even greater chance of fat ratio being 0, giving a final score of 6. But, we can't see the difference (i.e. that there are proportionally more scores of 6 and 7 and fewer of 8 than expected) because all scores 6-9 are lumped together. The same argument works in the other direction, making the scores 2 and 3 potentially more frequent than scores of 1, but again this effect is obscured when the low score includes 0-3. Perhaps this is why?

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