Gastric emptying after ingestion of clear fluids and nutritional liquid

INTERVENTION: Twenty volunteers will fast at least for 6 hours for food and for 2 hours for clear liquid prior to this study. Gastric ultrasound is done to determine the baseline gastric residual volume (GRV). The volunteers are then randomised into two groups, one receiving 250mls of nutritional liquid (Up and Go, which contains protein 8.3g, total fat 3.8g, saturated fat 0.5g, carbohydorate 30.3g, sugars 19.0g, sodium 163mg, dietary fibre 3.8g, monounsaturated 1.5g, polyunsaturated 1.8g, potassium 538mg, folic acid 100mcg, viatminA 128mcg, Vitamin B1 0.28mg, Vitamin B2 0.28mg, Vitamin B3 2.5mg, Vitamin B6 0.40mg, Vitamin B12, Vitamin C 10.0mg, calcium 400mg, energy 198cal) and the other 250mls of clear fluid (water). Gastric ultrasound is done immediately post ingestion then every 10 minutes for the 1st 30 mins then every hour until the gastric volume returns to baseline. After each volunteer's gastric volume has returned to baseline, he/she will then crossover to the other group and the process is repeated. CONDITION: Aspiration pneumonia PRIMARY OUTCOME: Primary outcome 1: The duration required for gastric emptying after ingestion of fluid with the same caloric composition(Up&Go) as a standard meal and clear fluid(water) of equal volume, which will be assessed by gastric ultrasound. ; ; Primary outcome 2: Whether ingesting water prior to ingesting Up&Go influences the speed of gastric emptying or not which will be assessed by gastric ultrasound only when water is ingested first. The order of treatments is randomised. SECONDARY OUTCOME: Correlation of scanning technique between two operators. The technique will be assessed by the results of the calculated cross sectional area of stomachs. INCLUSION CRITERIA: Healthy volunteers