Mannitol in water has no nutritional effect. Mannitol is a monosaccharide, which cannot be metabolized in the body, is not metabolized after oral intestinal absorption, is not absorbed by the renal tubules after glomerular filtration, and is excreted from the urine, so it has no nutritional effect. Mannitol is mainly used in medical treatment for dehydration, lowering cranial pressure, diuresis, etc. It is mainly used to treat cerebral edema caused by various reasons, prevent brain hernia formation, treat glaucoma, lower intraocular pressure, have osmotic diuretic effect, used to prevent acute tubular necrosis, and also used for intestinal preparation for gastrointestinal surgery. Improper use of mannitol can lead to water-electrolyte disorders, dizziness, blurred vision, difficulty in urination, allergy, osmotic nephropathy, etc. It is contraindicated in patients with acute tubular injury. Self-administration of mannitol to water for nutrition is prohibited.