Smoking May Affect MS Risk Differently Based on Race, Ethnicity

Smoking status has differential effects on the risk of developing multiple sclerosis (MS) among Black and White individuals, according to study findings published in Multiple Sclerosis and Related Disorders.

Over the past 6 decades, the incidence of MS among White individuals has remained stable, while incidence of MS in Black individuals has increased. Smoking is a well-known risk factor for developing MS; however, this evidence-based research has been primarily conducted in White populations, leaving a gap in the research that supports the same association between smoking and MS risk across other races and ethnicities.

Researchers at Harvard T.H Chan School of Public Health in Boston and the Uniformed Services University of the Health Sciences in Bethesda, Maryland theorized that increased smoking among Black populations explained the increase in MS among this racial group. To assess this, they collaborated to conduct a nested, case-control study of 157 Black and 23 White US military personnel among more than 10 million active-duty who had serum samples stored at the Department of Defense Serum Repository. These 180 individuals subsequently developed MS confirmed by a neurologist during follow-up.

The researchers measured serum cotinine concentrations to indicate recent tobacco exposure and compared the 180 individuals who developed MS with age-, sex-, and race-matched control participants. Of the 185 Black control participants, 24 (13%) had cotinine concentrations higher than 25 ng/mL, while 23 of the 157 (15%) Black individuals with MS demonstrated concentrations over 25 ng/mL. In contrast, 14 of 46 (30%) White control participants and 8 of 23 (35%) White individuals with MS demonstrated cotinine concentrations over 25 ng/mL.

Accounting for loss of participants during follow-up of a minimum of at least 2 years, the number of Black participants who smoke in the control group increased to 35 out of 168 (21%) and 32 out of 145 (22%) Black individuals with MS. Contrastingly, the number of White participants who smoke decreased in the control group (7 of 28; 25%) and increased among those with MS (7 of 14; 50%).

The researchers did not find an association between pre-symptomatic smoking status and MS risk among Black individuals (rate ratio [RR], 1.08; 95% CI, 0.63-1.85; P =.78).

These results remained consistent after adjusting for 25-hydroxyvitamin D measurements (RR, 1.09; 95% CI, 0.63-1.90; P =.75), lowering the cotinine threshold to 13 ng/mL (RR, 0.93; 95% CI, 0.55-1.57; P =.79), and restricting baseline cotinine measurements to samples collected 2 years (RR, 1.47; 95% CI , 0.73-2.98; P =.29) and 5 years (RR, 1.2; 95% CI, 0.48-2.98; P =.70) prior to onset of MS to minimize confounding by any preclinical disease activity (reverse causation).

Interestingly, smoking was not significantly associated with MS risk among the White cohort (RR, 1.85; 95% CI, 0.56-6.16; P =.31) either; however, the point estimate was similar to that observed in other studies. This finding indicated that a differential effect of smoking on MS risk existed between the 2 racial groups.

“Smoking was not associated with MS risk in Black people,” the researchers concluded. “Given the consistent association between smoking and MS risk in predominantly White populations, this may suggest that the association between smoking and MS varies by race/ethnicity.”

Study limitations included lack of sufficient power due to the moderate sample size, potential residual or unmeasured confounders, and the limited number of White individuals with cotinine measurements, which impacted the ability to assess for interactions between smoking and race.

Additionally, use of cotinine measurement only reflects recent tobacco smoking and may potentially lead to the misclassification of smoking status since it overlooks individuals who stopped smoking or those who only smoke occasionally.

Disclosures: Several study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see original source for full list of disclosures.