Can your cardio workout sabotage your attempt to pass a drug test?

In Part I of this series, we learned how the endocannabinoid system regulates and responds to physical activity. The endocannabinoid system is crucial for motivating people to exercise voluntarily, and exercise itself alters one's sensitivity to cannabinoids. In Part II, we saw that endocannabinoid receptors are present in the lungs, heart, and blood vessels – key tissues for exercise performance.

Endogenous cannabinoids are produced from certain dietary fats and regulate various aspects of metabolism. Plant cannabinoids like THC are fat-soluble molecules and some of the THC you consume accumulates in fatty tissue. This suggests that the amount of body fat you have and the rate at which you burn it could have an impact on how quickly THC is released from fat stores into the bloodstream.

Can burning body fat actually cause measurable changes in THC levels in the blood?

Read the other two articles in this content series

Effects of fasting and exercise on blood THC levels

Conclusion: THC is stored in fat and released when fat is burned.

The effects of fasting and exercise have been measured in a limited number of studies in both animals and humans. In a rodent study, rats were given THC daily for five days, followed by a three-day washout. Afterwards, some fasted for 24 hours. Compared to nonfasted rats, fasted rats had higher blood concentrations of THC-COOH, but not THC. THC-COOH is a metabolite of THC – what drug tests measure in urine samples. Increased levels of THC-COOH in the blood were also observed when rats fasted for 20 hours immediately after a single dose of THC.

In another rodent study, rats were given THC daily for ten days (twice as long as the last study), followed by a 24-hour fast. Compared to non-fasting animals, higher blood levels of both THC and THC-COOH were found in fasting animals, although the increase in THC was much smaller than THC-COOH.

A limited number of human studies have examined the effects of exercise and fasting on blood THC/THC-COOH levels in humans. In a small study of six chronic, daily cannabis users, they did 45 minutes of moderate-intensity exercise (treadmill running) followed by a 24-hour fast. No significant increase in THC or THC-COOH was detected.

Why should this be so, given the results of the above animal studies showing an increase in THC-COOH after fasting? One explanation is the length of the study: humans live about thirty times longer than rats. A 24-hour fast for a rat is roughly equivalent to a month-long fast for a human. It is possible that a short day of fasting is not enough to burn a lot of fat in humans.

Immediately after exercise, there was a significant increase in blood THC levels, but not THC-COOH.

Another human study made additional observations that shed further light on the matter. Fourteen regular cannabis users had their blood tested before, immediately after and two hours after 35 minutes of stationary cycling. Immediately after exercise, there was a significant increase in blood THC levels, but not THC-COOH. The effect returned to normal after two hours.

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Researchers made an additional, interesting observation: Body mass index (BMI) was significantly correlated with the change in blood THC levels after exercise. Individuals with a higher BMI (more body fat) tended to see greater increases in blood THC levels after exercise. This may explain why no change was found in the previous study – these people had an average BMI of around 21, while the majority in this study had a BMI over 21.

The experiments on rodents found that fasting can lead to an increase in THC-COOH levels in the blood, while there is little or no increase in THC levels. In contrast, human studies found that a combination of exercise and fasting could increase THC levels (but not THC-COOH), with greater increases observed in individuals with a higher BMI. Why should human studies find an increase in THC but rodent studies find an increase in THC-COOH? One possibility is drug metabolism. Rodents metabolize drugs much faster than we do, so THC may have less time to accumulate in fat before it is converted into THC-COOH.


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Could burning fat get you high or cause you to fail a drug test?

Conclusion: Are you getting high? Perhaps. Fail a drug test? Unlikely.

Assuming that the above results also apply in humans – that THC levels in the blood can be increased by fasting and exercise (at least in people with sufficient body fat) – could burning body fat lead to a psychoactive effect?

Without more robust data, it's hard to be sure, but it's entirely possible.

In the study that found an increase in blood THC levels in both fasting and non-fasting cannabis users after exercise, there was an average increase in blood THC levels of about 15%. In a study that measured blood THC levels after vaping cannabis, a relatively low THC dose of 10 mg – enough to produce psychoactive effects in the majority of study participants – resulted in an average increase in THC levels. Level in the blood of about 10%. A higher dose of 30 mg resulted in an increase of approximately 30%.

…a 15 percent increase in blood THC levels from exercise and fasting could potentially produce a psychoactive effect.

So a 15 percent increase in blood THC levels from exercise and fasting could potentially produce a psychoactive effect. Unfortunately, the study that measured this increase in THC in the blood after exercise/fasting did not also measure whether participants experienced psychoactive effects.

Overall, these results suggest that individuals with more body fat may be susceptible to higher levels of THC in the blood after fat burning. Burning more fat means more room for THC to accumulate over time. A prediction here would be that longer fasts or more intense exercise, which results in greater fat burning, could lead to a greater increase in blood THC levels, particularly in individuals with more body fat.


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As far as drug testing goes, results from human studies suggest there is no increased risk of failure because THC-COOH levels in the blood – what urine tests look for – were not elevated after fasting and exercise in humans. It's conceivable that more intense exercise or prolonged fasting increases THC-COOH levels, but I'm not aware of any studies looking at this.

Cannabinoids are fat molecules – our own endogenous cannabinoids are made from dietary fats, and plant cannabinoids can be stored in fatty tissue and influence our eating behavior and metabolism. These things are worth knowing about as you plan your lifestyle and crafting intentions for the new year.

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