Why Can Some People Eat Anything They Want and Never Gain a Pound?

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The following are both actual and paraphrased versions of questions I regularly get from readers:

If grains are so bad how can you explain the leanness and good health of Clarence Bass?

How is it that this guy can eat 30 bananas a day and not gain weight, or this guy can eat nothing but potatoes for 60 days and lose 20 pounds?

How do the Kitavans or Okinawans maintain good body composition despite a higher carb diet?

Mark, how were you able to maintain a low body fat percentage despite eating a half gallon of ice cream a day?

Why can my brother eat anything he wants and never gain a pound?

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All of these examples seem contrary to what we say in the Primal Blueprint. How can they be explained? Are they anomalies? Tails of the bell curve? Is something else at work?