Maybe Popeye had it right: Spinach makes you stronger. Researchers at Washington University School of Medicine in St. Louis have found the high nitrate content in the leafy greens like spinach, as well as beets, improves muscle performance.

In a new study, published in the journal Circulation: Heart Failure, investigators found drinking concentrated vegetable juice – in this case made from beets – increased muscle power in nine patients with heart failure.

"It's a small study, but we see robust changes in muscle power about two hours after patients drink the beet juice," said Linda R. Peterson, M.D., associate professor of medicine. "A lot of the activities of daily living are power-based – getting out of a chair, lifting groceries, climbing stairs. And they have a major impact on quality of life.

“We want to help make people more powerful because power is such an important predictor of how well people do, whether they have heart failure, cancer or other conditions. In general, physically more powerful people live longer."

Nitrates in beet juice, spinach, and other leafy green vegetables such as arugula and celery are processed by the body into nitric oxide, which is known to relax blood vessels and have other beneficial effects on metabolism.

The results of the study found that two hours after the treatment, patients demonstrated a 13 percent increase in power in muscles that extend the knee. The researchers also pointed out that participants experienced no major side effects from the beet juice, including no increase in heart rates or drops in blood pressure, which is important in patients with heart failure.

"The heart can't pump enough in these patients, but that's just where the problems start," said Peterson, a cardiologist and director of Cardiac Rehabilitation at Washington University and Barnes-Jewish Hospital. "Heart failure becomes a whole-body problem because of the metabolic changes that happen, increasing the risk of conditions such as insulin resistance and diabetes and generally leading to weaker muscles overall."