High blood sugar can impair the ability to utilize oxygen for energy with cystic fibrosis (CF), which may explain why exercise can be especially tiring for those with the disease, a new study shows.

Researchers think the biological mechanism responsible for this phenomenon is that high blood sugar can contribute to a type of cell damage called oxidative stress, disrupting muscle cells’ ability to take up oxygen during exercise. Findings from a test in some patients suggest a supplement containing antioxidants — molecules that reduce oxidative stress — may increase the muscles’ ability to use oxygen during exercise.

“The present investigation has provided provocative proof-of-concept data to support that antioxidant supplementation and a subsequent reduction in oxidative stress may improve tissue [oxygen uptake] during exercise,” the researchers wrote. “It is plausible to expect that increases in skeletal muscle oxygen utilization, likely influenced by decreased oxidative stress, can increase exercise capacity.”

The study, “Muscle oxygen utilization and ventilatory parameters during exercise in people with cystic fibrosis: Role of HbA1c,” was published in Chronic Respiratory DiseaseThe researchers said future studies should test whether antioxidant supplements might improve exercise capacity in people with CF, and especially those with high blood sugar or CF-related diabetes (CFRD).

Many people with CF have problems exercising. Since CF typically causes substantial lung disease, conventional wisdom has long been that exercise problems in CF are mostly attributable to lung dysfunction. This may not be the case, recent research suggests.

In CF, thick mucus builds up in the pancreas, which can contribute to digestive symptoms and impair the ability to regulate blood sugar levels. It’s known that high blood sugar can contribute to oxidative stress in tissues. It’s not clear if there’s a connection between impaired blood sugar and exercise problems in CF.

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A trio of people run together in this exercise illustration.

Exercise capacity and high blood sugar levels

This study included 24 people with CF: 13 had higher than normal levels on a measure of blood sugar (specifically, HbA1c levels above 5.7%), while the other 11 had normal range blood sugar levels. Four had CFRD.

In a laboratory setting, all participants had a standardized exercise assessment, including how they used oxygen and expelled carbon dioxide during intense exercise. Their exercise capacity or peak VO2 — which reflects the maximum amount of oxygen absorbed during intense exercise — was also measured.

The average exercise capacity scores were significantly lower in patients with high blood sugar than those in the normal range. Other measures suggested the cycle of taking in oxygen and expelling carbon dioxide during breathing was less efficient with high blood sugar.

“Findings from the present study demonstrate that higher HbA1c can negatively impact exercise capacity in people with CF,” the researchers wrote.

During exercise, muscles need more oxygen to get energy to power their movements. In patients with high blood sugar, the ability of muscles to take up oxygen from the blood was reduced, the tests showed. This is likely because of oxidative stress in the muscle tissue, the researchers said.

Seven patients with high blood sugar participated in a small trial where they took supplements containing antioxidants (specifically 1,000 mg vitamin C, 600 international units vitamin E, and 600 mg alpha lipoic acid) for about a month. Measures of the muscles’ ability to take up oxygen were improved and appeared more similar to patients with lower blood sugar levels, results showed.

“Present findings demonstrate that supplementation with an [antioxidant] for [four] weeks can improve skeletal muscle oxygen utilization during exercise in a subset of individuals with elevated HbA1c,” the researchers wrote, emphasizing that more research is needed to verify and expand on these results because they came from a small study. “Larger, multi-center, randomized, placebo-controlled clinical trials over a longer period of time are needed to fully exploit this hypothesis and determine whether antioxidant supplementation can … improve exercise capacity.”

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