In a statement the NHS complained of newspaper "over-reaction" to a recent study at the University of California by Nogueira et al., with the Daily Mirror gushing, "Chocolate gives you a workout" and the Daily Express crowing, "How dark chocolate boosts fitness in the same way as jogging".
But the NHS said that the relevance of the mouse study to human was uncertain, where the research analysed the effects of epicatechin (a flavanoid found in cocoa, with higher levels in dark than milk chocolate) on the animals' muscle performance.
This found that mice receiving epicatechin for 15 consecutive days performed better on a treadmill testing endurance compared with mice that didn’t.
The NHS said the finding "provided evidence" that giving mice epicatechin could lead to increases in muscular performance similar to those obtained by regular exercise.
"[But] contrary to the statements made in the news reports, it is unclear whether the chemical would have the same effect in humans, and further research would need to investigate this," it added.
Moreover, although epicatechin is found in dark chocolate, chocolate was not tested in this research, the NHS added.
The health service said: "It is unclear how much would need to be consumed to get the levels that were given to these mice, or the appropriate level of epicatechin needed to get a similar response in humans."
The study also focused on muscular performance and failed to compare the chemical's effect on other health variables that exercise had a positive effect upon, the NHS said, such as stress reduction and improved cardiovascular health.
The NHS added that chocolate also contained a lot of fat and sugar, with health implications of over-consumption "well established".