Vitamin C May Stabilize Levodopa Levels in Patients Using Laxatives

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by Steve Bryson, PhD |

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Vitamin C may help to stabilize levodopa and carbidopa levels in the bloodstream of Parkinson’s disease patients who take laxatives containing magnesium oxide (MgO), which is thought to break down this common Parkinson’s treatment, a study suggested.

Larger studies are needed to validate the benefits of vitamin C for patients using MgO-based laxatives to ease constipation, the researchers recommended.

The study, “Ascorbic acid can alleviate the degradation of levodopa and carbidopa induced by magnesium oxide,” was published in the journal Brain and Behavior.

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Parkinson’s is marked by abnormally low levels of the nerve cell signaling molecule called dopamine, leading to both motor and nonmotor symptoms.

Constipation is a common nonmotor symptom, and it is often treated with laxatives that contain MgO.

However, some studies suggest that MgO can degrade standard Parkinson’s therapies such as levodopa, a precursor molecule that is converted to dopamine in the brain, and carbidopa, a medicine taken alongside levodopa to slow dopamine’s breakdown.

Ascorbic acid, or vitamin C, has been shown to stabilize levodopa and carbidopa in solution. But whether ascorbic acid can prevent the degradation of levodopa and carbidopa induced by MgO has not been explored.

Investigators at the Ehime University Graduate School of Medicine in Japan measured the stabilizing effects of ascorbic acid on levodopa and carbidopa in combination with MgO in solution. They also tested the concentration of levodopa in the bloodstream of a Parkinson’s patient receiving levodopa, MgO, and ascorbic acid.

The team mixed solutions containing levodopa and carbidopa; levodopa, carbidopa, and MgO; levodopa, carbidopa, MgO, and ascorbic acid; and, levodopa, carbidopa, MgO, and lemon juice, which contains small amounts of ascorbic acid. The solutions were incubated at room temperature, and samples were collected and analyzed at repeat timepoints ranging from zero to 40 minutes.

Under all conditions, the concentrations of levodopa remained virtually constant across timepoints.

In contrast, the concentration of carbidopa fell over time with the addition of MgO. This degradation effect was blocked by ascorbic acid in a concentration-dependent manner, but degradation continued with the addition of lemon juice.

Although the pH of the solution containing ascorbic acid was higher, or more alkaline, than that with lemon juice, the “differences in action on carbidopa were attributable to the difference in the content of [ascorbic acid], not to the alkaline pH condition,” the researchers noted.

Researchers then evaluated levodopa levels in the bloodstream of an 86-year-old woman with Parkinson’s, who was receiving levodopa/carbidopa every two hours and MgO every four hours via a feeding, or PEG, tube, with or without 200 mg of ascorbic acid.

Over an eight-hour period, her blood levels of levodopa were 53 micromole-hour per liter without ascorbic acid and 67.27 micromole-hour per liter with ascorbic acid.

Although carbidopa concentrations were not measured in the woman, these findings “may be caused by the reduced carbidopa concentration,” the researchers wrote.

“In our data, the effect of 250 mg of MgO on 10 mg of carbidopa was considerably mitigated by 200 mg of [ascorbic acid], suggesting that the combination of [ascorbic acid] stabilized carbidopa in the patient,” they added.

No apparent difference in clinical symptoms were evident with or without the addition of ascorbic acid.

Researchers noted that including only one patient was a limitation to the study, as well as not measuring bloodstream levels of carbidopa. “Further large-scale studies, including measurement of carbidopa concentrations, are required to validate the effect of [ascorbic acid] on levodopa and carbidopa suspended with MgO” in Parkinson’s patients, they wrote.

“Although MgO causes degradation of carbidopa, [ascorbic acid] can mitigate this effect and may contribute to improving the bioavailability of levodopa,” the team concluded.