getLatestLeanBodyMass().md

Get the most recent lean body mass. The value is a number representing the weight in pounds (lbs)

On success, the callback function will be provided with a leanBodyMass object containing the leanBodyMass value, and the startDate and endDate of the sample. Note: startDate and endDate will be the same as leanBodyMass samples are saved at a specific point in time.

 AppleHealthKit.getLatestLeanBodyMass(null, (err: Object, results: Object) => {
  if (err) {
    return;
  }
  console.log(results)
});

{
	value: 176,
	startDate: '2016-07-08T12:00:00.000-0400',
	endDate: '2016-07-08T12:00:00.000-0400'
}