Gastric emptying antacid effects

Levodopa, a dopamine precursor, is the most effective agent for PD. Patients experience a 40% to 50% improvement in motor function. It is absorbed in the small intestine and peaks in the plasma in 30 to 120 minutes. A stomach with excess acid, food, or anticholinergic medications will delay gastric emptying time and decrease the amount of levodopa absorbed. Antacids decrease stomach acidity and improve levodopa absorption. Levodopa requires active transport by a large, neutral amino acid transporter protein from the small intestine into the plasma and from the plasma across the blood-brain barrier into the brain (Fig. 29-2). Levodopa competes with other amino acids, such as those contained in food, for this transport mechanism. Thus, in advanced disease, adjusting the timing of protein-rich meals in relationship to levodopa doses may be helpful. Levodopa also binds to iron supplements and administration of these should be spaced by at least 2 hours from the levodopa dose.1,8,16,25... 

The effect of antacids on gastric emptying rate is a factor which makes difficult a direct physicochemical analysis of the problem. The difficulty in predicting the effect of antacids is clearly shown by studies with naproxen, a... 

Anticholinergics should be taken before meals to decrease the acid secretion that occurs with eating. They should not be taken with antacids because antacids can slow the absorption of antichloinergic administration. Side effects include dry mouth, decreased secretions, tachycardia, urinary retention, and constipation. However, because gastric emptying time is delayed, gastric secretions can be stimulated and actually aggravate the ulceration. 

Aluminium/magnesium hydroxide gel reduces the absorption of quinine from the gut of rats and reduces quinine blood levels by 50 to 70%. This appears to occur because aluminium hydroxide slows gastric emptying, which reduces absorption, and also because magnesium hydroxide forms an insoluble precipitate with quinine. However, there seem to be no clinical reports of a reduction in the therapeutic effectiveness of quinine due to the concurrent use of antacids. 

Vats TS, Hurwitz A, Robinson RG, Herrin W. Effects of antacids on gastric emptying in children. PediatrRes (1973) 7, 340. 

Antacids are cleared from the empty stomach in 30 minutes. However, the presence of food is sufficient to elevate gastric pH to about 5 for 1 hour and to prolong the neutralizing effects of antacids for 2-3 hours. 

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