[...]the cells of people with CFS stop making as much energy from sugar as usual, and start relying more on lower-yielding fuels, such as amino acids and fats. This kind of metabolic switch produces lactate, which can cause pain when it accumulates in muscles.

Together, this would explain both the shortness of energy, and why even mild exercise can be exhausting and painful.

Øystein Fluge of Haukeland University Hospital in Bergen, Norway, and his colleagues studied amino acids in 200 people with CFS, and 102 people without it. The levels of some amino acids in the blood of women with CFS was abnormally low – specifically for the types of amino acid that can be used by the body as an alternative fuel source.

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Both sexes had high levels of several enzymes known to suppress pyruvate dehydrogenase (PDH), an enzyme vital for moving carbohydrates and sugars into a cell’s mitochondria – a key step for fully exploiting sugar for energy.

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The result is not unlike starvation, says Armstrong. “When people are facing starvation, the body uses amino acids and fatty acids to fuel energy for most cells in the body, to keep glucose levels vital for the brain and muscles as high as possible.”

“We think that no single enzyme in metabolism will be the answer to CFS, just as no single enzyme is the ‘cause’ of something like hibernation,” says Robert Naviaux of the University of California at San Diego, who has found depletion of fatty acids in patients suggesting they were diverted as fuel.

So what could flick the switch to a different method of metabolism? Fluge’s team thinks that a person’s own immune system may stop PDH from working, possibly triggered by a mild infection.