I try to stick to papers with original, primary data because summaries / editorials / even meta-analyses are so often, so very often, misleading. But here is a readable editorial by the great Jim Mann about being a vegetarian.
Mann makes the interesting comment that
'Although vegetarians, notably young women, do have lower iron stores than those who eat meat, there is little evidence that meat avoidance itself is associated with malnutrition.'
Let's unpack a couple of phrases in this - 'lower iron stores' and 'malnutrition'. Firstly, severe iron deficiency leads to anaemia, and one of the features of severe anaemia is extreme lassitude - you feel tired all the time. But note the two 'severe's in there. It is false logic to argue that if severe anaemia causes tiredness, then mild anaemia causes mild tiredness. This false logic leads to huge numbers of women being given iron supplements. They arrive at the surgery saying they feel tired all the time, a blood test shows mild iron-deficiency anaemia, a prescription follows. But lots of people (9% of a general population sample) have unexplained fatigue, so mild anaemia plus tiredness is at least as likely to be coincidence as cause and effect.
One of the hallmarks of good public health practice is a sense of proportion - whenever someone says that x% of people with a disease (or worse, some biochemical result) have tiredness / headache / depression / wheeze, you should immediately ask yourself - 'What proportion of the general population have this symptom?'
Indeed this raises the whole question of what we mean by 'anaemia'. If defined as 'outside the normal range' for a blood test, what is 'normal'? Values found in 95% of the population - in which case we have by definition declared 5% as ill, or at least as 'not normal'. Or is anaemia a value 'low enough to be associated with ill health'? Which takes us back to where we started.
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While we're on diet, here's a good story from Edwin Moore ("Lemmings don't leap' - 180 myths, misconceptions and urban legends exploded").
"in 1870 a Dr von Wolf put a decimal point in the wrong place and it became a 'scientific fact' (until the error was spotted in 1937) that spinach contained ten times more iron than it actually does."
Unfortunately Moore gives no citations so this story may itself be an urban myth.