A naturally lean body type will tend to remain skinny.
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Other body types can grow fat.
A diet an exercise program was prescribed for this body and for a few weeks it started to grow skinny.
However on the balanced calorie controlled diet he soon began to feel hungry all the time. Also he hated the long boring cardio exercise. He was then losing his strength and not growing skinny. He was strictly following the program but growing fat.
He decided st give up on following his diet and exercise program.
Eating as much delicious natural foods as he wanted and replacing the long boring cardio with short high intensity exercise he regained his strength and continually grew skinny until he reached his ideal weight two years later. He stayed skinny for years.