One of most important prescription of enhanced recovery after surgery protocols is the reduction of preoperative fasting time in opposition to the traditional recommendation of overnight fast. This changing from prolonged fast to only 2-3h after the ingestion of 200-400 mL of carbohydrate (CHO)-enriched drinks in not only safe but recommended by various societies of anesthesia and nutrition. A long list of beneficial effects associated with the abbreviation of the preoperative fasting time with CHO beverages is reported ranging from alleviation of thirst, hunger and stress of the patient to metabolic benefits such as the improvement of insulin resistance. However, new formulas for preoperative beverages continue to evolve during the last years aiming to achieve better results. The addition of glutamine, anti-oxidants e recently whey protein to CHO-enriched formula has been tested and associated with a better reduction of insulin resistance, decrease of acute phase inflammatory response, and increasing production of glutathione. The effect of whey protein-enriched preoperative drinks on the postoperative functional capacity of patients are awaited. Nevertheless, only a few studies have tested the gastric emptying of drinks containing whey protein in addition to CHO Thus, the aim of this study is to investigate in healthy volunteers the gastric residual volume (GRV) 3h after ingesting a formula containing CHO and whey protein.

Methods This is an experimental pilot crossover clinical trial design.The data will be collected in three phases. Firstly, after a fasting period of eight hours, the participants attended the Radiology Service of the Hospital de Cancer, Cuiaba, Brazil to perform MRI images of the upper abdomen with the objective of measuring the fasting GRV (phase 1). In this phase, adequate stomach motility will be observed to exclude cases of possible gastroparesis that could interfere with gastric emptying and expected results. GRV (cmˆ3) will be measured in the fasting state as explained below. In phase 2, each participant while still inside the MRI exam room will ingest 200mL of the study drink described below. The individual will be immediately positioned in the supine position and another MRI scan will be carried out. In this second phase, GRV will be evaluated by tracking the same region of interest (ROI) in each slice, forming a volume by the sum of all ROIs. Finally, in phase 3 a third MRI scan of the upper abdomen to measure again the GRV as previously described will be done after a 3-hour pause.

Intervention oral supplement The oral supplement used as intervention for the study will be Nutren Fresh (Nestle, São Paulo, Brazil). This oral non-residual supplement (clear fluid) has 100% whey protein isolated from total proteins, with various vitamins (B1, B6, C, D, niacin and folic acid) with "lemon tea" flavor. In addition, the formula contains no lactose or lipids; is formulated with a volume of 200ml, with a caloric density of 1.28kcal/ml; protein: 10g in 200ml of product (100% whey protein isolate); and carbohydrate: 54g in 200ml of product (74% glucose syrup and 26% maltodextrin).

MRI Protocol All exams will be performed using the same MRI equipment which was a Magnetom Essenza 1.5T device; Siemens Healthcare, Brazil. Each volunteer will be placed in a supine position, with a body coil (Body Matrix) to capture the signal. Cross-sectional acquisitions with 40 slices (perpendicular to the longitudinal axis of the body) were performed in HASTE (echo-planar fast spin echo) sequences with a slice width of 4.0 mm at 3.0 mm intervals. This STEM sequence (TR \[repetition time\] 774 msec, TE \[echo time\] 92 msec) visualizes fluids with hypersignal as opposed to low signal from adjacent organs. Each acquisition of a set of images was performed during 15 seconds of apnea. The images will be captured by an industry technician with MRI experience and reviewed by a radiologist to confirm that the images covered the entire stomach. The GRV will be assessed by tracking the region of interest (ROI) in each slice, forming a volume by the sum of all ROIs. GRV measurements in cmˆ3 will be performed manually by tracing regions of interest on each slice using Osirix MD ANVISA v.12.0.3 software and summing the slices to determine total gastric volume and the results entered into an ExcelR data sheet for statistical analysis.