Laird Harrison

May 30, 2014

ORLANDO, Florida — The consumption of more caffeine might help sedentary people become more physically active, but differences in genes could determine who will get a boost from the substance, a pair of new studies suggest.

In one study, sedentary people counseled on exercise were more likely to increase their physical activity if they also drank a lot of coffee.

In another study, women were able to increase the amount they could pedal on a stationary bicycle after the consumption of caffeine, but only those with a specific form of a gene.

Both studies were presented here at the American College of Sports Medicine 61st Annual Meeting.

In the first study, sedentary people were assigned to a 1-year intervention; they were given advice about exercise and wore accelerometers to measure their physical activity. Regular caffeine consumption was determined through a self-administrated dietary history questionnaire.

The 93 people who consumed more than 1.5 g of caffeine per week (at least 2 cups of coffee per day) increased the mean time they spent in moderate to vigorous activity from 43 to 55 minutes per week. The 85 people who consumed less than that amount of caffeine increased their mean moderate to vigorous activity from 46 to 52 minutes per week.

This increase in weekly activity was significantly greater in the group that consumed more caffeine than in the group that consumed less (12 vs 6 minutes; P = .033).

"I have no clue as to what the mechanism is," lead researcher Julien Tripette, PhD, from the National Institute of Health and Nutrition in Shinkjuku, Japan, told Medscape Medical News.

The second study presented shed some light on how caffeine boosts activity by measuring performances during a cycling trial after participants consumed caffeine or placebo.

A Caffeine Gene?

Bryan Loy, MS, from the University of Georgia in Athens, and his colleagues took their inspiration from previous research that showed that differences in sensitivity to caffeine correspond to variations in the ADORA2A gene, which codes for the adenosine A2A receptor.

That receptor affects both the sensation of pain and the dopamine system.

The researchers recruited 22 women who reported being sensitive to caffeine and who consumed less than 200 mg of caffeine per day, on average. They defined "sensitive" as experiencing effects such as being kept up late at night after drinking coffee.

The women were categorized as having a dominant C allele (CT and CC) or a homozygous recessive allele (TT). Previous studies have shown that people with T allele homozygosity in the ADORA2A gene show greater sensitivity to caffeine than carriers of the C allele.

On day 1 of the study, baseline measures were taken during the cycling trial. After a warm-up period on an exercise bicycle, participants were coached to increase their speed to a level of moderate effort for 20 minutes and then to a level of maximum effort for 10 minutes.

On day 2 of the study, half of the CT/CC carriers and half of the T carriers were randomly assigned to receive 5 mg of caffeine per kg of body weight; the other half received placebo. An hour after consuming the caffeine or placebo, participants performed the cycling trial.

On day 3, the cycling trial was repeated, except those in the caffeine group on day 2 received placebo, and those in the placebo group on day 2 received caffeine.

In the CT/CC group, there was no significant difference in total work performed during the cycling trial after caffeine consumption or after placebo consumption.

However, in the TT group, total work increased after caffeine consumption.

Table. Kilojoules of Work

Allele Group After Caffeine After Placebo
CT/CC 63.40 66.10
TT 57.95 51.10

 

"It appeared that people were motivated to do more exercise," Loy told Medscape Medical News.

Participants were asked about the amount of pain they were experiencing, but the difference was not significant after caffeine or placebo consumption.

This led the researchers to hypothesize that caffeine improved dopamine transmission in the TT group but not in the CC/CT group.

They caution that the study was small and the results should be replicated before anyone uses them to determine who might experience an exercise boost from caffeine.

The genetic finding is intriguing, said Ildu Ahmetov, PhD, a researcher at the Volga Region State Academy of Physical Culture, Sports, and Tourism in Kazan, Russia, who was not involved in either study.

"In terms of novelty, it's very original," he told Medscape Medical News. "And it has a very practical future because we can't tell every athlete to consume caffeine. We should search for those who are sensitive."

The World Anti-Doping Agency removed caffeine from its list of prohibited substances in 2004, and many athletes use it in competition. But it can have adverse effects, particularly in older people, such as an increased risk for stroke, arterial hypertension, and headache, said Dr. Ahmetov.

It is too early to advise anyone to take caffeine. "At this stage there is no recommendation from me," said Dr. Tripette.

He and his group plan a future study in which exercise counseling is coupled with a caffeine prescription.

American College of Sports Medicine (ACSM) 61st Annual Meeting. Abstracts 423 and 316. Presented May 28, 2014.

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