Compared with young adults who had never smoked marijuana, those who had used it more than 100 times were 40% more likely to develop prediabetes by the time they were middle aged, in a new observational study.

However, heavy use of pot in young adulthood was not associated with an increased risk of developing diabetes in middle age.

These findings, based on an analysis of data from the Coronary Artery Risk Development in Young Adults (CARDIA) study — which looked at more than 3000 participants when they were roughly 32 years old and then 50 years old — were published online September 13 in Diabetologia.

"We tried to capture…marijuana use in young adulthood, when you would assume it would be the highest" and then look for the development of prediabetes or diabetes, Michael P Banks, PhD, from the University of Minnesota School of Public Health in Minneapolis, told Medscape Medical News.

"It is unclear how marijuana use could place an individual at increased risk for prediabetes, yet not diabetes," Dr Bancks and colleagues concede. Possibly other confounders play a role, or marijuana may have a greater effect on blood glucose at a prediabetes level, they suggest.

More research is needed to clarify the effects of marijuana use on diabetes risk, they add.

Does Early Marijuana Use Affect Midlife Blood Glucose?

Previous research has suggested that greater marijuana use is associated with consuming more calories but also having lower values for body mass index (BMI), waist circumference, fasting insulin, and insulin resistance, Dr Bancks and colleagues write.

They examined data from the CARDIA population to determine the association between self-reported marijuana use and prediabetes and diabetes. Based on American Diabetes Association criteria, prediabetes was defined as fasting glucose 5.6 to 6.9 mmol/L, 2-hour oral-glucose-tolerance-test (OGTT) glucose 7.8 to 11.0 mmol/L, or HbA 1c 5.7% to 6.4%; type 2 diabetes was defined as fasting glucose ≥7.0 mmol/L, 2-hour OGTT glucose ≥11.1 mmol/L, or HbA 1c ≥6.5%.

CARDIA is an observational study that enrolled healthy people aged 18 to 30 living in four US cities (Birmingham, Alabama; Chicago; Minneapolis; and Oakland, California) and then followed for 25 years. The current analysis included 3151 participants in CARDIA who were free from diabetes at year 7 (1992–1993) and 3034 participants in year 25 (2010–2011), 18 years later.

At baseline in the current study, the participants had a mean age of around 32 years old. Based on their replies to a questionnaire, 625 participants had never used marijuana, 738 participants had used it one to nine times, 774 participants had used it 10 to 99 times, and 1014 participants had used it at least 100 times (defined as heavy marijuana users).

Although smoking is the most common way of using marijuana, the mode of marijuana use was not specified in responses, and it is "unclear if the effect of marijuana differs by mode of use," the authors observe.

Compared with the participants who had never used marijuana, those who were heavy users were more likely to be male (62% vs 41%), current smokers (38.6% vs 7.7%), and drink more than one alcoholic drink a day (30.6% vs 4.2%).

Many heavy marijuana users reported using illicit drugs more than 10 times (crack cocaine 34.1%, other cocaine 40.2%, heroin 5.5%, and amphetamines 29.7%), whereas 0% to 0.5% of people who never used marijuana reported taking these illicit drugs that often.

On the other hand, compared with participants who never used marijuana, heavy users had a lower average BMI (26.4 vs 27.4) and reported being more physically active and eating a healthier diet. Both groups had a similar waist circumference (84.9 and 84.5 cm).

At an assessment 18 years later, when the participants had a mean age of 50, compared with those who never used marijuana, those who were current users had a 66% increased risk of having prediabetes (hazard ratio [HR], 1.66; 95% CI, 1.15 – 2.38) and those who were heavy users had a 38% nonsignificant increased risk of prediabetes (HR, 1.38; 95% CI, 0.96 – 1.97), after adjustment for age, sex, race, smoking, alcohol, education, study center, systolic blood pressure, C-reactive protein, physical activity, use of illicit drugs, BMI, BMI2, and waist circumference.

There was no association between marijuana use and diabetes, however.

Individuals who reported a high lifetime use of marijuana when they were young adults were more likely to develop prediabetes by the time they were 50 years old (HR, 1.40; 95% CI, 1.13 – 1.72, P < .01), but they were not more likely to develop diabetes (HR, 1.16; 95% CI, 0.77 – 1.74, P = .32), after adjustment for the same confounders.

Thus more studies are needed, and "future studies should aim to objectively measure the mode and quantity of marijuana use in relation to prospective metabolic health," the researchers conclude.

Dr Bancks received a National Institutes of Health (NIH) training grant to conduct this study. CARDIA is conducted and supported by the National Heart, Lung, and Blood Institute (NHLBI) in collaboration with the University of Alabama at Birmingham, Northwestern University, the University of Minnesota, the Kaiser Foundation Research Institute, and Johns Hopkins University School of Medicine and is also partially supported by the Intramural Research Program of the National Institute on Aging (NIA) and an intra-agency agreement between the NIA and NHLBI. The authors report no relevant financial relationships.

Diabetologia 2015. Published online September 13, 2015. Article