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Test positive for pot? Blame that darned diet you are on!

CANNABIS smokers beware: stress or dieting might trigger "reintoxication", resulting in a positive drug test long after you last used the drug.

The main psychoactive ingredient of cannabis is tetrahydrocannabinol (THC), and once in the body it is readily absorbed into fat cells. Over the next few days it slowly diffuses back into the blood. Since THC is taken up by fat more readily than it diffuses out, continual intake means some THC can remain in the fat cells.

It has been suggested that stored THC can be released at a later date in situations where the body's fat is rapidly broken down. This is based on anecdotal reports of spikes in blood cannabinoid levels in people who have not taken the drug recently but have experienced extreme stress or rapid weight loss.

Jonathon Arnold at the University of Sydney, Australia, cites the example of an athlete who swore he hadn't smoked cannabis in months but who had rapidly lost 4 kilograms just before a positive drug test.

To investigate whether rapid breakdown of body fat could have been responsible, Arnold and colleague Iain McGregor first exposed THC-laden fat cells taken from rats to the stress hormone ACTH. They found that the hormone increased the speed of release of THC from the cells.

Then they injected rats with 10 milligrams per kilogram of THC (equivalent to a person smoking between five and 10 cannabis cigarettes, depending on their strength) every day for 10 days. Two days later, they injected a third of the rats with ACTH, deprived another third of food for 24 hours, with the rest as controls.

Subsequent blood tests showed that rats that were food deprived had double the blood level of THC acid, a metabolite of THC, compared with the controls. Those that were exposed to ACTH also showed a statistically significant increase in THC acid levels. The study has been accepted for publication by the British Journal of Pharmacology.

However, the new work did not find evidence of increased THC acid levels more than two days after the last THC dose. Arnold thinks this could be due to the short-term nature of the experiment. He suspects that if doses were given over a longer period of time, sufficient THC levels could build up in body fat to explain abnormally high levels of THC metabolites in people who claim not to have taken the drug recently.

The main implication of the work could be for legal cases in which athletes or employees have tested positive for cannabis but claim they haven't recently consumed it, Arnold says. "But the clincher would be for us to show these results in humans."

Issue 2720 of New Scientist magazine

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