Originally Posted by
cyccommute
It's not weight so much as it's vibration.
This quote
sums it up nicely.
Padded gloves, bar ends (I'd go with a longer one then the Cane Creek or Ergon), softer grips (mountain bikes come with torture devices not handlebar grips
) and a more relaxed grip on the bars would help.
The added benefit of a relaxed grip is that you are putting less weight on the bars too.
pain or numbness in the palm and thumb is usually the median nerve, which runs through the depression in the center of the palm at the wrist. (The "carpal tunnel", of carpal tunnel syndrome fame.) There's a lot going through a pretty small space (there are, as I recall, nine tendons and the median nerve.), and if anything is inflamed, the others can be adversely effected. The median nerve provides innervation to the palm side of the thumb, index, middle, and part of the ring finger, as well as the fingernails on these fingers.
Stress on the carpal tunnel can be caused by hand/wrist angle, putting weight on the wrong part of the hand, and a bunch of other things (like too soft grips that deform to put pressure on it). They're mostly fit related.