Diet. One flawed study purported that Chocolate, french fries, potato chips and sugar, among others, affect acne. A recent review of scientific literature cannot affirm either way. The consensus among health professionals is that acne sufferers should experiment with their diets, and refrain from consuming such fare if they find such food affects the severity of their acne.
But how is someone supposed to know what foods increase or decrease his acne? Try to pay attention to what he eats and what changes in diet are linked to what effects? How will he know which food did it, and how will he know what the time delay between diet changes and acne changes is? (If acne changes, was the it due to the food 2, 4, 6, or 20 days ago? Or not due to food at all?) Scientists trying to do controlled studies haven't figured anything out yet. A person who goes on his own anecdotal evidence will almost certainly be creating unscientific superstitions for himself to follow. This should not be encouraged. People have enough hang-ups about food already. The only responsible advice for scientists to give is, "don't worry about it, eat what you want."
That some scientists would encourage people to act on anecdotal evidence in this way suggests they are not competent to perform studies themselves.
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A person trying to influence their health should rely mostly on their own anectotal experiences. Scientific studies can only show trends - 75% of people who ate xx food ha xxx reaction. A study can help you determine where to start and to know what you would be most likely to expect, but your own experience can only dtermine if something works for YOU an not someone else
The question at issue is how to apply your own observations of yourself. Suppose on day 1 you eat 7 different foods. on day 2 you eat 6 foods, 3 of which are the same. on day 3, you eat 7 foods, some of which are the same. on day 4 ... anyway on day 7 you think you have more acne (but you didn't write down daily measurements, so you may be wrong). On day 12 you think you have less acne. Are you really supposed to be able to figure out which food caused it? And what makes you think a food did cause it?
Correlation does not imply causation, so what are you going to go on?