There are two adult brothers, 5 years apart. The weight difference between them is minor, but remarkable and incomprehensible given the circumstances. As we cannot come up with a plausible explanation, I hope to find one here.

A more detailed description of both brothers:

  • Younger brother: late twenties, 69.5 kg at 174 cm height. Hits the climbing gym 2-3 times a week, plays soccer or goes running between 0 and 2 times a week. Nutrition mostly vegetarian, meat maybe once a week.
  • Older brother: early thirties, 67 kg and 180 cm height. Hits the climbing gym 3-4 times a week, no other sports. Comparable nutrition, probably less total intake than the younger brother.
  • Visual aspects, body type:
    • Generally, they share a similar body build as they are brothers.
    • If put side by side, there are some visual differences:
    • The younger brother is slim/lean with well defined muscles, but no bulky upper body. A little more body fat around the belly than the older brother. The fancy modern scale claims 12% body fat.
    • The older brother features a wider upper body/back/shoulder, noticeably more muscle mass in the upper body and arms. Body fat most definitely lower than the younger brother's.

Long story short: one would expect the taller and bulkier brother to be heavier than the younger brother. The difference in body fat is too low to compensate this difference.

What are plausible reasons for this seemingly contradictory weight difference?

  • @Paparazzi Well, it is possible, but I think it's unlikely. The younger brother has 8.34 kg of body fat. If we assume 9 % body fat for the older brother, he has about 6.03 kg. So, if the younger brother loses about 2.5kg of body fat to reach the same ratio of body fat to height, he would weigh the same as the older brother while still being smaller and less bulky.
    – maxv
    Mar 12 '17 at 22:39

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