Study: Insulin resistance linked to differences in breast cancer survival between Black and White women

IN-DEPTH REVIEW OF RESEARCH

Study background:

Black women are more likely to die from breast cancer compared to White women regardless of the stage of breast cancer at diagnosis. More Black women have a metabolic condition called insulin resistance than White women. This study looked at whether insulin resistance might contribute to the association between race and breast cancer outcomes. This study also looked to see if proteins on breast cancer cells that respond to insulin differed between breast cancers from Black and White women.

Researchers of this study wanted to know:

Whether insulin resistance may be one of the factors that contributes to the association between race and breast cancer prognosis. 

Populations looked at in this study:

This study looked at 515 women newly diagnosed with invasive breast cancer. Participants were recruited from ten US hospital sites in New York, New Jersey, Connecticut, Maryland and Michigan.

A total of 87 Black women and 428 White women enrolled in the study.  Clinical data recorded for participants included self-reported smoking and alcohol use, diet and physical activity, education, income and insurance.

Study design:

At the study visit, participants had height and weight recorded which was used to calculate body mass index (BMI). Blood pressure was also recorded.

Following an overnight fast (minimum of 8 hours), a blood test was used to measure blood glucose, insulin, C-peptide, total cholesterol as well as high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol and triglycerides.  Insulin resistance was calculated from fasting glucose and fasting insulin levels.

The study used the Nottingham prognostic index (NPI) to determine prognosis. The NPI is a number calculated following surgery for breast cancer. The NPI score is based on the size of the tumor, the number of positive lymph nodes and the grade of the tumor.

The researchers also looked at the levels of insulin receptor and insulin-like receptor in participants tumors.

The researchers used additional statistical modeling test to see if insulin resistance may be one factor that influences poor prognosis in Black women.

Study findings: 

• Race was associated with prognosis. 
  • 28 percent of Black women (24 of 87)  had a poor NPI prognostic score
  • 15 percent of White women (63 of 428) had a poor NPI prognostic score.
• Race was associated with insulin resistance.
  • 40 percent of Black women (35 of 87) had insulin resistance.
  • 20 percent of While women (85 of 428) had insulin resistance.
• Statistical modeling showed that insulin resistance mediated the association between Black race and poor prognosis.
  • Insulin receptor expression was higher in breast tumors from Black women compared to White women.
  • The ratio of insulin receptor to  insulin-like growth factor receptor 1 was higher in breast tumors from Black women compared to White women and was associated with a higher NPI score.

Limitations:

Limitations of this study included the fact that the researchers were not able to determine how long patients had insulin resistance prior to their breast cancer diagnosis. The researchers also had missing data for some patients on other breast cancer risk factors such as reproductive history.

In this study, the obesity as defined by BMI was lower in participants than would be expected for the general US population. This could be due to the fact that study participants largely resided in the east coast states of New York, New Jersey, Connecticut and Maryland. In these states, self-reported obesity by BMI is between 25 and 30 percent compared to 35 and 57 percent for women in the US. Although the exact causes of this difference in obesity are not all known, they likely  reflect differences in social and economic advantage..

Importantly the authors acknowledge that insulin resistance may be one of a number of factors that contribute to the difference between breast cancer prognosis in Black women compare to White women.

Context:

Many complex factors likely contribute to the observation that Black women are more likely to die from breast cancer compared to White women.  Data suggests that Black women are more likely to be treated differently than White women following a breast cancer diagnosis. Other barriers include include access to care, screening and treatment as well as biologic and genetic differences in tumors. In addition, research supports the link between insulin resistance and risk for many types of cancer including breast cancer. This research looked at one of the many factors that may mediate disparities in breast cancer outcomes between Black and White women.

Conclusions:

This research looked at insulin resistance and showed that it may be one factor that affects why Black women are more likely to die of breast cancer than White women. This may be due to the effects of insulin on tumor insulin receptors.  It will be important to design future studies to look at whether lowering insulin levels improves breast cancer survival for Black women.

Posted 7/30/20