Researchers looked at 10 studies involving over 1,000 women to understand how breast cancer treatment affects body composition—basically, how much fat and muscle women have. They found that women consistently gained fat mass (between 3-9%) after treatment, regardless of whether they received chemotherapy, hormone therapy, or both. The findings were less clear about muscle changes. This matters because understanding these body changes can help doctors and patients plan better nutrition and exercise strategies during and after cancer treatment to improve overall health and quality of life.

The Quick Take

  • What they studied: How breast cancer treatment affects the amount of fat and muscle in women’s bodies
  • Who participated: Over 1,000 women (across 10 different studies) who were 18 or older and had early-stage breast cancer (stages 0-3). Studies were published between 2018 and 2024.
  • Key finding: Women gained between 3.3% and 9.2% more body fat after breast cancer treatment. Two studies also found that women gained dangerous belly fat (visceral fat). Muscle changes were less consistent across studies.
  • What it means for you: If you’re a woman with early breast cancer, you may gain fat during or after treatment—this is a common side effect. This information can help you and your doctor plan exercise and nutrition strategies to manage weight and stay healthy. However, more research is needed to fully understand these changes.

The Research Details

This was a systematic review, which means researchers searched five major medical databases (PubMed, CINAHL, Embase, Web of Science, and Cochrane Library) for all studies about body composition changes in breast cancer patients. They started with 734 studies, narrowed it down to 29 that looked promising, and finally included 10 high-quality studies with a total of 1,062 women.

To be included, studies had to measure body composition using advanced imaging methods like DXA scans (similar to X-rays), MRI, or CT scans. The researchers excluded studies that only used simpler measurement methods like bioelectrical impedance because those are less accurate.

The included studies were all published recently (2018-2024), which means the findings reflect current treatment practices. The women in these studies had early-stage breast cancer and received various treatments including chemotherapy, hormone therapy (pills that block estrogen), or combinations of both.

A systematic review is valuable because it combines information from multiple studies to see the bigger picture. Instead of relying on one study that might have unusual results, researchers can identify patterns across many studies. This approach is especially important for understanding side effects of cancer treatment because individual studies might have small numbers of patients or different measurement methods.

This review is fairly reliable because: (1) researchers used a strict process to select studies, (2) they only included studies using accurate measurement methods, and (3) they looked at recent research reflecting current treatment. However, the studies included were quite different from each other (different treatments, different patient groups, different measurement timing), which makes it harder to draw firm conclusions. The authors themselves noted this limitation and called for more consistent research in the future.

What the Results Show

The most consistent finding was that women gained fat mass after breast cancer treatment. Across all 10 studies, fat gain ranged from 3.3% to 9.2%—meaning if a woman had 100 pounds of fat before treatment, she might have 103-109 pounds after. This fat gain happened regardless of what type of treatment women received: chemotherapy, hormone therapy, or a combination.

Two studies specifically looked at visceral fat, which is the dangerous fat that surrounds organs in the belly. Both of these studies found statistically significant increases in visceral fat, meaning the increase was real and not due to chance. This is concerning because visceral fat is linked to heart disease and diabetes.

In contrast, results about lean body mass (muscle and bone) were mixed. Some studies found women lost muscle, others found no change, and some found slight gains. This inconsistency suggests that muscle changes depend on factors the studies didn’t fully explore, like how much exercise women did during treatment.

The review noted that the timing of measurements varied across studies—some measured body composition right after treatment ended, while others measured months or years later. This variation makes it harder to know exactly when fat gain happens. Additionally, different types of treatment (chemotherapy versus hormone therapy) didn’t seem to cause different amounts of fat gain, suggesting that cancer treatment itself, not just one specific drug, affects body composition.

This systematic review confirms what smaller studies have suggested: breast cancer treatment commonly leads to weight gain and fat accumulation. The finding that visceral fat increases is particularly important because previous research has linked this type of fat to worse health outcomes in cancer survivors. The inconsistent findings about muscle loss align with other research showing that muscle changes depend heavily on individual factors like exercise habits and nutrition during treatment.

The main limitation is that the 10 included studies were quite different from each other—they used different treatments, measured body composition at different times, and studied women of different ages and backgrounds. This ‘heterogeneity’ makes it hard to say definitively what will happen to any individual woman. Additionally, most studies were small and didn’t track women for very long periods. The review also couldn’t determine which specific treatments cause the most fat gain because studies mixed different treatment types. Finally, the studies didn’t consistently measure or report on factors that might explain fat gain, like changes in physical activity or diet during treatment.

The Bottom Line

Women with early breast cancer should expect possible fat gain during or after treatment (moderate confidence based on consistent findings). Work with your healthcare team to plan nutrition and exercise strategies before, during, and after treatment to minimize unhealthy weight gain. Focus especially on maintaining muscle through resistance exercise and adequate protein intake. Monitor your weight and body composition regularly with your doctor. If you gain significant fat, discuss this with your oncologist or a registered dietitian who specializes in cancer care.

This research is most relevant for: (1) women recently diagnosed with early-stage breast cancer, (2) women currently undergoing breast cancer treatment, (3) breast cancer survivors, (4) doctors and nurses treating breast cancer patients, and (5) registered dietitians working with cancer patients. Women with advanced breast cancer should discuss these findings with their oncologist, as treatment approaches may differ. Men with breast cancer may have different body composition responses and should consult their doctors.

Fat gain typically occurs during treatment or in the months immediately after treatment ends. Most studies measured changes within 1-2 years of treatment completion. However, some women may continue to gain fat for longer periods. Improvements from exercise and nutrition changes may take 3-6 months to become noticeable, so patience is important.

Want to Apply This Research?

  • Track weekly weight and waist circumference measurements starting before treatment begins (if possible) or at treatment start. Record these in your app weekly. Also note exercise minutes per week and general diet quality to identify patterns between lifestyle and weight changes.
  • Set a specific goal like ’exercise 150 minutes per week’ or ’eat protein at every meal’ and log daily progress in the app. Use the app to schedule reminders for exercise sessions and nutrition check-ins. If weight gain exceeds 5% of baseline, trigger an alert to discuss with your healthcare team.
  • Create a dashboard showing weight trend, waist circumference trend, and exercise/nutrition adherence over time. Compare these metrics monthly to your baseline (pre-treatment if available). Share reports with your oncologist or dietitian at regular appointments to adjust strategies as needed.

This research summary is for educational purposes only and should not replace professional medical advice. Breast cancer treatment decisions and management of side effects should always be made in consultation with your oncology team. Individual responses to cancer treatment vary significantly. If you are a breast cancer patient or survivor concerned about body composition changes, weight gain, or exercise safety during treatment, please discuss these findings with your oncologist, primary care doctor, or a registered dietitian specializing in oncology. This systematic review identifies trends across multiple studies but notes that more research is needed for definitive conclusions.