Orange juice or other sources of vitamin C (ascorbic acid), may (or may not) benefit you in terms of health and exercise, but contrary to what many people thought previously, ascorbic acid doesn't seem to help physical exercise performance. And in terms of such endurance exercise as running, walking or cycling, whereas vitamin C reduces physiological "oxidative stress," that reduction didn't help performance for either men or women, whether they were in their 20s or 60s.
Earlier experiments in Canada, often described as "elegant," showed that when test subjects were given a drug to boost cardiac output (somewhat like exercise does), and then took the same drug and vitamin C, the increase in cardiac output was much greater. These results were widely interpreted as suggesting that since vitamin C decreased oxidative stress and improved heart performance, perhaps physical performance in exercise might be similarly boosted.
To study this effect in a "real life" exercise setting, physiologists at the University of Colorado, Boulder, gave vitamin C to a group of young (23 years old) and older (61 years old) adults prior to their performing exhaustive exercise on a treadmill.
The researchers predicted that "acute administration of ascorbic acid might improve/restore maximal aerobic capacity (MAC) and maximal cardiac output (MCO) in the sedentary older adults, thus creating the possibility that longer-term ascorbic acid supplementation could be used therapeutically to sustain the improvement."
Neither long- nor short-term vitamin C intake boosts exercise capacity
But reporting in the online edition of the Journal of Applied Physiology, the Colorado physiologists said "the age-associated decline in (MAC and MCO) is unaffected by acute or chronic (30-day) administration of moderate daily ascorbic acid supplementation" for either men or women.
The lead researcher, Christopher Bell, noted: "We did see a decrease in oxidative stress with large doses of vitamin C, but this decrease didn't improve aerobic abilities either for younger or older subjects."
The study, "Ascorbic acid does not affect the age-associated reduction in maximal cardiac output and oxygen consumption in healthy adults," was conducted by Christopher Bell, John M. Carson, Nathaniel W. Motte and Douglas R. Seals from the Department of Integrative Physiology, University of Colorado, Boulder.
Going into the experiment, the researchers had good reason to expect positive results. It's well known that MAC and MCO decline with age. Moreover, it's thought that this "age-associated decline in MAC may be partially mediated by the development of oxidative stress that can suppress beta-adrenergic receptor responsiveness and consequently reduce MCO." Although the exact physiological cause of this suppression isn't known, "increased reactive oxygen species (ROS) may be involved," they thought.
Furthermore, physiological or non-diseased aging "in adult humans is associated with increased oxidative stress as a result of increased production of ROS, a reduction in antioxidant defenses, or both. Recent evidence suggests that increased ROS tonically suppresses myocardial beta-adrenergic receptor responsiveness in middle-aged and older adult humans, as this can be reversed by acute intra-coronary administration of the powerful antioxidant ascorbic acid."
However in the current experiment, Bell et al. found that after a month of ingesting 500 mg daily of vitamin C or getting infused with acute amounts just before exercise, the blood results were surprisingly uniform between ages and sexes. Indeed, the only major difference found throughout the experiment was that "irrespective of age, MAC, MCO and heart stroke volume were greater in men compared with women; however there were no sex-differences or age-sex interactions pertaining to the response of any of these variables to administration of ascorbic acid."
The study also showed that, "Plasma concentration increased in all subjects following the acute but not chronic administration, establishing that we were able to increase plasma concentration of ascorbic acid to supra-physiological levels in both young and older adults with acute administration."
The report noted that: "Oxidized low-density lipoprotein concentration was negatively associated with maximum heart rate and was greater in the older compared with young adults and was reduced in both groups following acute, but not chronic, ascorbic acid administration, establishing that we were able to reduce oxidative stress in both groups with acute administration." In another measure, maximal heart rate was lower in older adults compared with young, but again was unchanged with vitamin C; maximum stroke volume was similar in the older and young adults, and was unchanged after taking vitamin C.
According to Bell, the big difference in results came from the fact that in earlier studies some of the metabolic manifestations of exercise were chemically induced, while the current study actually tested the role of vitamin C in hard-exercising humans. This means "the body's physiologic stimulation is from actual exercise with input from the nervous system sensing an increased need for oxygen and sending more blood going to the muscles."
Also it was thought that regular exercise lowers even older persons' oxidative stress. So in order to set the stage for the greatest possible effect, "we used sedentary older people because we wanted the highest level of oxidative stress as possible. But they were still healthy," Bell noted, and some earlier studies had "suggested that antioxidant administration may only prove to be beneficial for populations with elevated baseline oxidative stress such as vascular disease patients, smokers or older adults."
Despite some conceded inherent holes in the methodology, Bell said the fact that taking 500 mg. of vitamin C a day for a month -- or acute doses just prior to exercise – "had no discernable effect on either younger or older men or women was pretty compelling evidence that when it comes to endurance exercising, ascorbic acid probably doesn't carry its own weight."