Previous work using mannitol and glucose indicates that mean (SD) SBWC at 40 minutes postprandial after ingesting 300ml glucose was 47 (SD 15) and using n=12 the investigators calculate the investigators can detect an increase of 20 ml in excess of this with 90% power which is very much less than the investigators are predicting. The investigators plan to use 18 to ensure the investigators can assess our secondary endpoints for which there is no data with which to perform a power calculation.