Disclaimer: This article is intended solely for informational and educational purposes only. It does not constitute medical advice.
For anesthesiologists, surgeons, and perioperative nursing staff, the term “clear liquid” has significant clinical implications, since it determines what a patient may safely consume before elective anesthesia and when they may do so. As of 2026, the operative definition of a clear liquid is provided by the American Society of Anesthesiologists (ASA) 2023 Practice Guidelines for Preoperative Fasting, a modular update of the 2017 ASA guideline (Joshi et al., 2023). Alongside broadly consistent statements from the European Society of Anesthesiology and the Canadian Anesthesiologists’ Society, the ASA definition represents the governing US standard.
The definition of a clear liquid is a beverage that is transparent or translucent enough to read print through, contains no pulp, particulate matter, or solid content, and leaves no significant residue in the stomach. Under the ASA guidelines, this includes water, black coffee, tea with no milk, sugar, or other additions, carbonated beverages, fruit juice without pulp, and now explicitly extends to carbohydrate-containing clear liquids, whether simple (such as glucose- or fructose-supplemented drinks and clear juices) or complex (maltodextrin-based carbohydrate-loading beverages) (Joshi et al., 2023).
Liquids containing less than 10 g of carbohydrate per 100 mL are not classified as carbohydrate-containing for guideline purposes, and oral rehydration solutions are grouped with simple carbohydrates. Protein-containing clear liquids, always studied in combination with carbohydrates rather than in isolation, are also recognized as a distinct subcategory, though the task force found insufficient evidence to recommend them preferentially over other clear liquids (Joshi et al., 2023).
Milk and dairy-containing drinks fall outside the clear liquid definition because their caloric and fat content slows gastric emptying in a manner comparable to a light solid meal; the same is true of any beverage containing pulp or particulate matter. This distinction matters clinically because the 2023 update, echoing the 2017 guideline, reaffirms that healthy adults without conditions predisposing them to delayed gastric emptying or aspiration may drink clear liquids, with or without carbohydrate, up to two hours before elective procedures requiring general anesthesia, regional anesthesia, or procedural sedation, with a strong recommendation and moderate strength of evidence for carbohydrate-containing formulations (Joshi et al., 2023).
Up to 400 mL of liquid is considered to be an appropriate volume, reflecting the median volume ingested in the trials underpinning the recommendation. Updated guidelines provide greater specifications while balancing clinical recognition of the increased risk of dehydration and slower recovery under stricter preoperative fasting requirements with the need to minimize the risk of aspiration.
The clinical rationale is informed by gastric physiology rather than volume alone. Ultrasound and magnetic resonance studies cited across the literature demonstrate that water clears from the stomach with a half-time of about 10 minutes and is essentially fully emptied within 30 to 40 minutes, even at relatively large ingested volumes, because clear liquids exert minimal osmotic and caloric drag on gastric motility (Friedrich et al., 2020).
This physiological behavior underlies the guidance that clear liquids, unlike milk, solids, or fried and fatty meals, do not meaningfully increase residual gastric volume when taken up to two hours preoperatively, and it is the basis for the ASA’s continued distinction between clear liquid fasting (2 hours), breast milk (4 hours), infant formula and non-human milk (6 hours), and a light meal or fried, fatty food (6 to 8 hours) (Joshi et al., 2023).
The 2023 modular update to the guidelines does not alter the two-hour threshold for adults; instead, it clarifies which specific beverages qualify and confirms that adding carbohydrate—and, to a lesser extent, protein—to a clear liquid does not compromise gastric emptying or increase aspiration risk in adults while offering measurable reductions in preoperative hunger and thirst (Joshi et al., 2023).
It also addresses chewing gum, concluding that gum causes only a minimal, clinically insignificant rise in residual gastric volume and should not delay surgery once consumption ends. The definition applies specifically to the ASA’s “healthy patient” population, excluding those with gastroparesis, bowel obstruction, significant reflux disease, obesity, pregnancy, or emergency presentation, for whom clinical judgment and individualized fasting intervals remain necessary (Joshi et al., 2023).
Some argue for the further review of current guidelines for pre-anesthesia, suggesting that clear liquids may be safe for patients to consume up to 1 hour before a procedure. Several anesthesia societies now recommend a one-hour threshold for pediatric patients, although the task force found insufficient evidence to adopt this practice for the 2023 guidelines, and some researchers are advocating for research in adults (Morrison et al., 2020). For now, clear liquid for pre-anesthesia fasting adults continue to allow the consumption up to 2 hours in advance and maintain stricter thresholds for other liquids and solid foods.
References
- Joshi, G. P., Abdelmalak, B. B., Weigel, W. A., Harbell, M. W., Kuo, C. I., Soriano, S. G., Stricker, P. A., Tipton, T., Grant, M. D., Marbella, A. M., Agarkar, M., Blanck, J. F., & Domino, K. B. (2023). 2023 American Society of Anesthesiologists Practice Guidelines for Preoperative Fasting: Carbohydrate-containing Clear Liquids with or without Protein, Chewing Gum, and Pediatric Fasting Duration—A Modular Update of the 2017 American Society of Anesthesiologists Practice Guidelines for Preoperative Fasting. Anesthesiology, 138(2), 132–151. https://pubs.asahq.org/anesthesiology/article/138/2/132/137508/2023-American-Society-of-Anesthesiologists
- Friedrich, S., Meybohm, P., & Kranke, P. (2020). Nulla Per Os (NPO) guidelines: time to revisit? Current Opinion in Anaesthesiology, 33(6), 740–745. https://journals.lww.com/co-anesthesiology/Fulltext/2020/12000/Nulla_Per_Os__NPO__guidelines__time_to_revisit_.7.aspx
- Morrison, C. E., Ritchie-McLean, S., Jha, A., & Mythen, M. (2020). Two hours too long: time to review fasting guidelines for clear fluids. British Journal of Anesthesia, 124(4), 363–366. https://www.bjanaesthesia.org/article/S0007-0912(19)31004-9/fulltext