What makes us concious about our diet ??

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We all either eat to live or live to eat. Whatever maybe our understanding about eating. What is the factor that makes us feel concious that we are either eating too less or a little more than normal ? How does one make the vital decision between more or less ? I myself find it very difficult to judge. It's just when i stand next to the mirror that I realize what i had been upto last supper...:-)