Body Mass Index and breast cancer

Research shows that a woman’s weight may affect the outcomes of her breast cancer treatment. The Body Mass Index (BMI) is a number that indicates whether someone is underweight, normal, or overweight. In this article, we present the studies that correlate BMI with breast cancer.

What is the Body Mass Index?

The Body Mass Index (BMI) is a metric that takes into account a person’s weight and height, and corresponds to a number. This number is then used to indicate which BMI “category” this person should be classified into:

- Underweight
- Normal weight
- Overweight
- Obese


How is BMI calculated?

There is no “secret recipe” behind the Body Mass Index formula. Although the foundation for the formula was built in the mid-1800s by Adolphe Quetelet (a Belgian scientist), it was an American scientist (Ancel Keys) who formalised the below formula in 1972:


Body Mass Index =
(weight in kilograms)
divided by
(height in meters, squared)


For example, the BMI for a person who weighs 70 kilos and is 1.70m tall is 70 / 1.702 = 24.2.


What are the ranges for each BMI category?

The World Health Organization has established the following ranges based on an adult’s Body Mass Index:


  • Underweight: less than 18.5
  • Normal: between 18.5 and 25
  • Overweight: between 25 and 30
  • Obese: more than 30


BMI ranges

A good way to visualise the Body Mass Index concept and its calculation is through the following graph:


Keep in mind that the formula works in the same way for both men and women, and for all populations. On the other hand, ranges for children are different, and some national health organisations have established their own ranges to adjust for body types and fat composition.


Body Mass Index and Breast Cancer

There has been plenty of research around BMI and breast cancer.
Below you can find some highlights from large studies involving women diagnosed with breast cancer.

- In research conducted with almost 54,000 breast cancer patients, survival was significantly improved in patients with a Body-Mass-Index below 25 (considered “normal”). In the same research, scientists concluded that the effects of post-surgery therapy seem to be lost more rapidly in obese patients. You can read the full study, if interested, in the 2011 publication in the Journal of Clinical Oncology.

- On the topic of obesity (defined as Body-Mass-Index above 30), a study of almost 15,000 women diagnosed with breast cancer indicated that obese patients have worse prognosis. At the same time, the authors remind us that women who are obese are also at higher risk of cancer in the first place (potentially skewing this data). If interested, you can read the abstract in the journal Breast Cancer Research and Treatment.

- There were similar conclusions made in another study on obesity and breast cancer (with more than 6,000 patients) focused on recurrence and survival. The corresponding research was published in the Annals of Oncology in 2013 and you can read both the abstract and full version.

- In an older research publication, the results of a longitudinal study with more than 5,000 patients were presented. The conclusion there was that weight (and weight gain) were related to higher rates of breast cancer recurrence and mortality. Interestingly, these associations were stronger in women who never smoked. If you would like to read this Journal of Clinical Oncology publication, you can access both the abstract and full version.


What does all this mean?

While the Body Mass Index formula is intended to provide an easy calculation for everyone, each of us is different. Especially when diagnosed with cancer, there are many parameters that must be taken into account before a specific recommendation is made. Even so, there is still plenty of information that is waiting to be discovered, and further research is necessary.

An open, candid discussion with your medical team, and a balanced approach towards food, overall nutrition habits, exercise and wellbeing are still the best recommendations for everyone. 


For more specific information on breast cancer, read our article on triple-negative breast cancer, diet and nutrition.

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