Body fat, not BMI, tied to foot pain

An increase in fat mass index (FMI), but not body mass index (BMI), was associated with both prevalent and future foot pain, a large community-based study showed.

Among a subset of participants, ages 50 and up, from the North West Adelaide Health Study (NWAHS), foot pain was present in 20.2% of the cohort between 2004 and 2006 (stage 2) while 36.4% of the same group had foot pain between 2008 and 2010 (stage 3) of the study. Following multivariable modeling, the odds of having foot pain at stage 2 increased by 8% for each FMI unit at an odds ratio (OR) of 1.08 (95% CI 1.04-1.12), reported Tom Walsh, MHlthSci, of Flinders University in Bedford Park, Australia and colleagues.

The odds of having foot pain at stage 3 of the study increased by 6% for each FMI unit at stage 2 (OR 1.06, 95% CI 1.02-1.11), they wrote in Arthritis Care & Research.

The results suggest that body fat may contribute more to foot pain than body weight, they said.

However, the authors noted that physical activity was protective of foot pain. Also, serum adipokines, tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) were not associated with foot pain at any stage of the study.

“To our knowledge, this is the largest investigation of the association of foot pain, fat mass and adipokines,” Walsh’s group wrote. “This study … adds further to the concept of metabolically, rather than mechanically, derived foot pain.”

The NWAHS is a representative cohort study of randomly selected adults from the north-west region of Adelaide.

The study was started in 1999 to 2003 and stage 2 was conducted from 2004-2006. Stage 3 was done from 2008-2010.

During all three stages, data were collected using a Computer Assisted Telephone Interview (CATI), a self-completed questionnaire, and a clinical assessment.

Data collected as part of Stage 2 were used to examined the factors associated with prevalent foot pain. These covariates from Stage 2 were then used to determine predictors of future foot pain in Stage 3, the authors explained.

Some 1,462 participants were the focus of the current analysis.

The mean BMI for the cohort was 28.37 kg/m2 while the mean FMI was 10.18 kg/m2.

“Both FMI and BMI were significantly different between those with and without foot pain,” the investigators observed.

For example, the mean rank for those with prevalent foot pain was 913.56 based on FMI as a continuous variable versus 750.23 for those who reported no foot pain (P<0.001).

For BMI as a continuous variable, the mean rank for those with prevalent foot pain was 1,082.03 versus 930.36 among those who had reported no foot pain (P<0.001).

In stage 3, the mean rank for foot pain was 523.67 based on FMI as a continuous variable compared with 469.81 for those without future foot pain (P=0.008). Also, the mean rank for future foot pain based on BMI as a continuous variable was 602.58 versus 559.01 (P=0.046).

However, when fat mass was added to the multivariable model, BMI lost its significance.

After multivariable analysis, FMI, depression, poor general health, diabetes, osteoarthritis, arthritis of unknown type, and rheumatoid arthritis remained significantly associated with prevalent foot pain.

Only FMI (P<0.005), arthritis of unknown type (P<0.001) and depression (P<0.035) were significantly associated with future foot pain on multivariable analysis.

The authors did find that physical activity was found to be protective of foot pain (OR of 0.76, 95% CI 0.58-0.99, P=0.048).

Study limitations include the question investigators used to define foot pain, which could have excluded people with nondisabling foot pain and led to an underestimate of the prevalence of foot pain in the study sample.

The authors also did not examine the feet either clinically or radiographically so they could not report on foot structure or function.

Finally, socioeconomic status was not included in the model and that could mediate the effect of foot pain, the said.

And the authors were only able to report on a limited number of adipokines, TNF-α and IL-6, excluding other cytokines which may have been relevant to the findings.

“The clinical implications of our findings are that FMI but not BMI is associated with both prevalent and the development of foot pain after multivariable analysis in adults age ≥50 years,” they stated. “Given the association of FMI with future foot pain, patients with increased fat are at risk of developing foot pain and should be counselled as such, particularly given increased fat mass is modifiable and should not be considered as a chronic condition.”

The North West Adelaide Health Study was funded by the University of Adelaide, the South Australian Department of Health, and the Premier’s Science and Research fund.

Walsh disclosed funding from a Nursing and Allied Health Scholarship and Support Scheme. One co-author disclosed funding from the National Health and Medical Research Council.

Walsh and co-authors disclosed no relevant relationships with industry.

Reference: Walsh T, et al “The association of fat mass and adipokines with foot pain in a community cohort” Arthritis Care Res 2015; DOI: 10.1002/acr.22719.