Result of being too careful about food -- too slim

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A friend (male) of mine weighs only 105 pounds.
He eats little meat (mostly without skin), more vegetables, an apple a day, no snacks, drinks Chinese tea, not much coffee or tea (often with half bag of sugar), and three meals a day.
He has to put on more clothes than others because he is slim and has no fat under his skin.  He wishes to gain weight and try to eat more but not effective.
Everyone says he is too slim and worries when he gets sick there would be no fat to sustain his health.