CHICAGO — The presence of an impaired metabolism exacerbates the impact of obesity as a risk factor for developing knee osteoarthritis and is associated with reduced physical functioning, Mary Fran Sowers, Ph.D., reported at the 2004 World Congress on Osteoarthritis.
Such findings suggest that “the role of obesity with respect to osteoarthritis and functioning may extend mechanistically beyond that of just simple joint loading,” said Dr. Sowers, an epidemiology professor at the University of Michigan, Ann Arbor.
Current OA treatments should be evaluated for their potential to exacerbate these metabolic derangements, because this exacerbation is likely to diminish treatment efficacy.
“An understanding of the added contribution of the obesity subtypes could be very useful in guiding primary and secondary treatment efforts,” Dr. Sowers added at the meeting, sponsored by the Osteoarthritis Research Society International.
Researchers have identified several obesity subtypes, including individuals who are obese but metabolically healthy. This may occur in about 20% of obese persons and is characterized by large amounts of fat mass but normal insulin levels and favorable cardiovascular risk factor profiles.
Another risk group comprises individuals of normal weight but who have metabolic profiles more typically seen in the obese.
This risk group may account for about 15% of the general population and is characterized by low HDL cholesterol, higher triglyceride levels, and higher levels of inflammatory markers.
A community-based cohort of 775 women aged 43–53 years was evaluated for metabolic obesity, defined on the basis of three body mass index (BMI) cutoff points and the presence of two or more of the following metabolic derangements: diabetes or fasting glucose greater than 125 mg/dL, serum C-reactive protein greater than 2 mg/L, HDL less than 45 mg/dL, triglycerides greater than 200 mg/dL, or a waist-hip ratio greater than 0.81 cm.
The investigators found that 34% of the women were not obese (BMI less than 26 kg/m2) and had no metabolic derangements.
Another 31% of the participants were overweight to obese (BMI 26–34 kg/m2) without a metabolic derangement, and an additional 15% were overweight/obese women who did have a metabolic derangement.
Finally, 12% were very obese (BMI greater than 34 kg/m2) without a metabolic derangement, and 8% were very obese women who did have a metabolic derangement.
Among those without a metabolic derangement, the odds of having knee OA were increased among women who were either overweight/obese (odds ratio 1.9) or very obese (OR 7.0), compared with women who were not obese and had no metabolic derangement.
But when obesity was associated with a metabolic derangement, the risk of knee OA was three times higher in overweight or obese women (OR 3.3) and nine times higher in very obese women (OR 9.0), compared with women who were not obese and had no metabolic derangement.
The impact of metabolic disorders and weight on OA risk was consistent across all four of the physical tests: speed measured during walking on gait mats, grip strength, timed walk, and timed stair climbing, Dr. Sowers said.
There was no loss in leg strength unless women had an impaired metabolism, and then the loss was most pronounced in individuals with the highest BMI.
Dr. Sowers proposed that metabolic disorders and obesity may affect leg strength by altering glycation products in the muscles, by allowing fatty infiltration of muscle tissue and compromising selective muscle fibers, or by causing innervation problems.