A number of nutritional deficiencies can occur during rapid weight loss after either operative treatment or nonoperative starvation regimens. Nutritional monitoring and vitamin supplementation is essential; especially if very limited intake is combined with recurrent vomiting, acute thiamine deficiency can occur. Multiple cases have been described after both gastric bypass, gastroplasty, and prolonged fasting. Signs and symptoms can include memory loss and other signs of dementia, paralysis of extraocular muscles, nystagmus, and ataxia. Peripheral neuropathy has also been described. If this condition is suspected, treatment should be parenteral thiamine combined with other vitamins prior to infusion of glucose solutions. Administration of carbohydrate calories results in further depletion of thiamine in patients with marginal thiamine stores. Thus glucose infusions given without adequate thiamine replacement can lead to a rapidly worsening condition that may become irreversible. Gastric bypass results in impairment of enteral absorption of calcium and iron, and these elements need to be supplemented in any patient who has a bypass. However, inadequate intake of calcium leads to osteoporosis and not to hypocalcemia as long as parathyroid function is normal. Iron deficiency anemia can occur but does not lead to neurologic symptoms. Both hypochloremic alkalosis and volume depletion can occur with prolonged vomiting, but do not typically present with the neurologic findings described here. They have more obvious physical findings such as postural hypotension, tachycardia, and oliguria.