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Heavy marijuana use was associated with poorer kidney function in young adults at the outset of an observational study, but there was no association 10 years later, a longitudinal study showed.

Compared with healthy young adults who never smoked pot, those reporting daily use at the start of the study had a significantly lower mean estimated glomerular filtration rate (eGFR) (-4.5%, 95% CI -8.1 to -0.7%, P=0.02), according to Julie Ishida, MD, of the University of California San Francisco, and colleagues.

Those reporting 5 or more years of cumulative marijuana use also had a significantly lower mean eGFR (-3.0%, 95% CI -5.6 to -0.4%, P=0.03), Ishida and colleagues reported in the

However, the study found no significant association between heavy marijuana use and mean eGFR 10 years later (-0.1%, 95% CI -0.7 to 0.5, P=0.72). Pot smoking was also not associated with changes in eGFR over time or albuminuria, the investigators said.

Why the differing results? "Because this young cohort had only small changes in kidney function longitudinally, we had much greater statistical power for the cross-sectional analyses at baseline," Ishida told 番茄社区app via email. "We must caution, of course, that our study was observational, so we cannot rule out the possibility of confounding due to unmeasured factors in the association between marijuana and eGFR at study initiation."

As the legalization of marijuana in the U.S. for medical and recreational purposes has become more common, and marijuana use has risen over the last decade, it's not clear how that has impacted kidney disease, the researchers said. While previous research has been limited to case reports of acute kidney injury associated with synthetic cannabinoid use and to small studies of relatively short duration, theirs is the first to look at the potential link between marijuana use and kidney function in the general population.

"Chronic kidney disease affects over 20 million Americans, and the identification of potentially modifiable risk factors for adverse kidney outcomes is of public health importance," they wrote.

The exact mechanisms through which marijuana might affect kidney function are not known, but previous research has shown that the endogenous cannabinoid system plays a role in regulating kidney hemodynamics and sodium transport, they added.

Ishida and colleagues analyzed data on 3,765 healthy young men and women who were part of the Coronary Artery Risk Development in Young Adults () study. Their mean age was 35 and their mean eGFR was 111 mL/min/1.73m². They were enrolled from 1985-1986 at U.S. centers in Chicago, Minneapolis, Birmingham, Ala., and Oakland, Calif.

The majority of the cohort (83%) reported they had smoked marijuana at some point in their life, but the proportion of regular users remained at approximately 15% during the study period, the investigators said.

Participants underwent follow-up exams at yearly intervals for up to 25 years. Marijuana use was assessed at each exam with a self-administered questionnaire. Cystatin C was used to calculate eGFR; urine albumin-to-creatinine ratio was also measured from spot urine samples.

The researchers found that the association between pot smoking and lower eGFR at year 10 did not differ by sex (P=0.76). When the data were analyzed by race, however, the association was stronger in whites (-6.0%, 95% CI -9.2 to -2.7, P<0.001) and non-existent in blacks (1.0%, 95% CI -3.1 to 5.2, P=0.62).

"This was a surprise finding," Ishida told 番茄社区app. "So we prefer not to speculate on whether the race difference reflects a true biologic difference or chance finding. Clearly, this is potentially very important and warrants further study."

The researchers noted that while their findings "were largely negative, we observed that higher marijuana use was associated with modestly lower eGFR_cys among adults with preserved eGFR. This result may not translate into a clinically meaningful difference and may be insufficient to inform decision-making concerning marijuana use. However, it is possible that the association could be stronger among patients with established kidney disease, and additional research to define the impact of marijuana use on kidney outcomes in other study populations is warranted."

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