You may gain weight during and after treatment for breast cancer. Excess weight may also raise your risk for breast cancer recurrence following treatment.

When receiving a breast cancer diagnosis, many people have a long list of questions about why, how, and what’s next.

One thing they may not be thinking about is whether they will gain weight as a result of having breast cancer.

Research suggests that many women do gain weight during and after treatment. Weight gain may also increase the risk of cancer recurrence. This article examines why researchers believe this occurs.

Language matters

You’ll notice that the language used to share stats and other data points in this article is pretty binary, fluctuating between the use of “female” and “women.” Although we typically avoid language like this, specificity is key when reporting on research participants and findings.

The studies and surveys referenced in this article did not report data on, or include, participants who were transgender, nonbinary, gender nonconforming, genderqueer, agender, or genderless.

Researchers believe it’s common for people with breast cancer to gain some weight after receiving a diagnosis of breast cancer. According to a 2017 study, most people gain weight during and after treatment, and the reasons may be “multifactorial.”

For example, chemotherapy may affect a woman’s senses of taste and smell, which may influence their eating patterns.

Some women may also experience fatigue during treatment that limits their motivation and ability to exercise regularly or be more physically active. Chemotherapy may also affect a person’s metabolic rate.

Additionally, some treatments can send women into menopause, a time when some people tend to put on a few extra pounds.

How much weight do women typically gain?

The specific amount of weight gain may vary. For example, a 2021 study found that most women experienced an average weight gain of 1.2 kg (2.64 lb) during treatment for early-stage breast cancer.

Research from 2019 found that 35% of participants receiving adjuvant therapy for breast cancer gained approximately 2 or more kg (4.4 lb) after 2 years.

And a 2022 study of breast cancer survivors found that weight increased over time by approximately .79 kg (1.74 lb) at 1 year to 1.23 kg (2.71 lb) at 3 years.

Weight gain can have a negative impact on breast cancer outcomes.

If you have obesity or are overweight at the time of diagnosis, your prognosis can be poorer, according to a 2017 study.

In addition to its effects on prognosis, overweight and obesity can also affect your quality of life and increase your chances of developing additional conditions such as type 2 diabetes and cardiovascular disease.

According to the Centers for Disease Control and Prevention (CDC), obesity is typically defined as having a body mass index (BMI) of 30 or higher, while BMI for someone with overweight is 25–30.

Additionally, someone’s risk of breast cancer-related death also increases if they gain weight during or after breast cancer treatment, per the same study.

However, research is ongoing. A 2023 study found differences in the breast cancer cells of women with a BMI over 30 and breast cancer cells in women with lower BMIs. The women with higher BMIs had breast cancer cells with more inflammation and different mutations.

While more research is needed, researchers suggest that the information may be useful in influencing future research on targeted treatment strategies.

Your doctor can talk with you about strategies to reduce or minimize weight gain during and after treatment. That may include some lifestyle changes, such as changes to your diet or the addition of a regular exercise routine.

Is BMI an accurate measurement of overweight and obesity?

Though BMI can be a useful starting point, it should not be the only measurement of your health. BMI considers only a person’s weight and height as a measure of health rather than the entire individual. It doesn’t factor in your muscle mass, bone density, overall body type, race, or sex.

Higher body weight is associated with a higher risk of a number of different kinds of cancer. Research suggests that obesity is a well-established risk factor for breast cancer.

Fat cells, or adipose tissue, produce excess amounts of the hormone estrogen. When you have obesity or are overweight, you have an even greater number of fat cells. That fat tissue produces even more estrogen, which can fuel the growth of hormone-receptor-positive breast cancers, as well as endometrial, ovarian, and other cancers.

Also, the extra fat cells may cause or exacerbate chronic low-grade inflammation, which is linked to a greater risk of breast cancer recurrence, according to Breastcancer.org.

Women who have obesity or are overweight are also more likely to have higher levels of the hormone insulin. The American Cancer Society cautions that some cancers, including breast cancers, have been linked to higher insulin levels.

Other potential risk factors for breast cancer include:

According to the American Cancer Society, women who do not have children or breastfeed are also believed to have a slightly increased risk of developing breast cancer.

Similarly, using certain methods of birth control, such as oral contraceptives and hormonal birth control methods, such as implants and intrauterine devices (IUDs), may carry a slightly higher risk.

Some kinds of hormone therapy for menopausal symptoms may also carry an increased risk. If this is a treatment option for you, you should discuss the pros and cons with your doctor when making a decision.

There’s no guarantee that you will gain weight if you begin treatment for breast cancer. But many people do.

If you’re concerned about the potential effects of extra weight on your physical and mental health, talk with your cancer care team. They may be able to recommend lifestyle modifications that work with your treatment plan and help you minimize potential weight gain during breast cancer treatment.