Cancer and Obesity: Not Such a Linear Relationship

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Obesity has been established as a strong risk factor for the development of cancer. African Americans and Hispanics are particularly at risk, and their access to health care is often poor. How do racial and ethnic disparities in the development of obesity as well as access to care intersect to affect cancer survival? 

The answers are complex, and it’s not just about body composition, according to Elizabeth M. Cespedes Feliciano, ScD, of the Kaiser Permanente Division of Research, Oakland, California. Dr. Feliciano and her colleagues explored these associations using Kaiser Permanente’s large, diverse patient population. She discussed multiethnic differences in body mass index (BMI), body composition, and survival in breast and colorectal cancer, at the 10th Annual American Association for Cancer Research (AACR) Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved, held in Atlanta.1 

BMI may be useful at the population level, for surveillance, but with a growing population of older, ethnically diverse cancer survivors, we are going to need more precise measures of muscle and fat mass if we are going to individualize supportive interventions.
— Elizabeth M. Cespedes Feliciano, ScD

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“There is a disproportionate burden of obesity among black Americans in the United States,” she said. This is especially true for adult black women, of whom 57% are labeled obese, compared with 33% of white women, based on 2012 data. 

“This has given rise to the hypothesis that these high obesity rates might lead not only to an increased risk of breast cancer, but could explain some of the disparities that we see in cancer survival, comparing black and white women,” she said. 

Unfortunately, this is not easy to prove, according to Dr. -Feliciano. Across racial groups, it seems that higher BMI at diagnosis does not consistently predict survival after a breast cancer diagnosis. 

“Understanding the size of the envelope does not always tell you what’s inside it,” she said. “Some nuances are there, but overall, body composition appears to associate with survival, whereas BMI does not in all cases. BMI is not going to explain the racial/ethnic disparities in survival.”

Deciphering the Impact of BMI

Different categories of BMI were evaluated in the California Breast Cancer Survivorship Consortium. While there is a U-shaped relationship between BMI and mortality in white cancer patients—with mortality almost doubling at the extremes of BMI, ie, underweight and morbidly obese—mortality is not increased for African Americans within any BMI category.

The same study examined alternative measures of obesity, including waist-to-hip ratio, and found a different pattern of association. Here, an elevated mortality risk was observed among black women in the higher quartiles, more than twice that of non-Hispanic whites and twice that of black women in lower quartiles. No elevation in risk was observed among non-Hispanic white women.

Such data prompted Dr. Feliciano and her colleagues to address some questions that may inform these associations: 

  • Do differences in body composition (amount and distribution of muscle and fat mass) play a role?
  • Does body composition at a given age and BMI differ by race among cancer patients?
  • Do measures of body composition impact survival among cancer patients where BMI does not?
  • Do these associations differ by race or cancer site?

Imaging Muscle and Fat Mass

Computed tomography (CT) scans, which are used to assess body composition, have been informative in this research. “You see that at the same exact BMI, there can be very different levels of skeletal muscle mass,” she said, displaying CTs that showed two patients, both with a BMI of 40 kg/m2; one had a skeletal muscle index that met the criteria for sarcopenia (35 cm2/m2), whereas the other had a “robust abdominal muscle mass” (58 cm2/m2).3 

Conversely, at the same skeletal muscle index (29 cm2/m2), different patients can have very different amounts of adipose tissue. In scans of patients who were both sarcopenic, BMI was 40 kg/m2 in one patient but 15 kg/m2 in the other. Clearly, BMI can vary widely among patients with the same muscularity. “We really need to move beyond BMI to these more precise measures of muscle and fat,” she concluded. 

SCANS Studies Dig Deeper

This approach has been used in the Sarcopenia, Cancer, And Near-Term Survival (SCANS) studies in breast cancer (B-SCANS) and colorectal cancer (C-SCANS). These studies have enrolled large cohorts of patients with nonmetastatic breast cancer (n = 3,281) and colorectal cancer (n = 3,262), mostly from Kaiser Permanente. 

Researchers looked for an association between BMI at diagnosis and risk of death from any cause among non-Hispanic white and non-Hispanic black breast cancer patients. Adjusted for confounders, the following were the key findings:

  • No association with mortality after breast cancer 
  • For BMI of 25 to 30 kg/m2: a nonsignificant inverse association for overweight black women but no elevation in mortality risk for whites
  • For BMI > 30 kg/m2: a nonsignificant incremental increase in risk for black women and no trend for white women.

“BMI here is not doing its job of associating with survival,” she commented. “We tried looking at imaging data in these women to understand these findings.”

By measures of visceral fat, however, a different pattern emerged according to race. With increasing BMI, black women had lower levels of visceral fat than white women of the same BMI, though at higher BMI levels, black patients had on average “a little more subcutaneous fat,” she said. 

“That’s important to keep in mind. BMI is not representing the same thing in these populations,” she pointed out.

As for skeletal muscle, at every BMI category, black breast cancer patients had significantly more muscle. A similar pattern was seen for both men and women with colorectal cancer and also for noncancer populations—“differing patterns of adipose tissue distribution and muscularity by race and ethnicity,” she said. 

Body Composition and Survival

Dr. Feliciano’s team then explored the association between visceral adiposity in the breast cancer patients and the risk for death. On average, black women in the B-SCANS study had less visceral fat, but with increasing adiposity, a “robust” increased risk of death after breast cancer emerged. “So while these [black] women may have less visceral fat, visceral fat is definitely emerging as an independent risk factor for death,” she said. “In the white patients, we did not see that.” 


For more on the effects of obesity on cancer survival, see Cespedes Feliciano EM, et al: Metabolic dysfunction, obesity, and survival among patients with early-stage colorectal cancer. J Clin Oncol 34:3664-3671, 2017.

Interestingly, in both black and white breast cancer patients, increased muscularity was associated with a decreasing risk of death after breast cancer and also in colorectal cancer. “Importantly, while these differences in body composition help us understand and interpret the associations of BMI with mortality, they are not explaining racial disparities,” she said. 

Current findings are “underscoring the need to move beyond BMI,” she concluded. “BMI may be useful at the population level, for surveillance, but with a growing population of older, ethnically diverse cancer survivors, we are going to need more precise measures of muscle and fat mass if we are going to individualize supportive interventions.” 

Meanwhile, clinicians can tell patients that body composition can be modified through exercise, and this has many quality-of-life benefits and potentially even survival benefits. ■

DISCLOSURE: Dr. Feliciano reported no conflicts of interest.


1. Cespedes EM: 2017 AACR Conference on the Science of Health Disparities in Racial/Ethnic Minorities and the Medically Underserved. Presented September 28, 2017.

2. Kwan ML, et al: Am J Epidemiol 179:95-111, 2013.

3. Martin L, et al: J Clin Oncol 31:1539-1547, 2013.