Breast cancer health centre

Breast cancer and weight changes

Cancer Research UK says about 75% of women diagnosed with breast cancer put on weight during their treatment. This includes women who were a healthy weight before treatment.

Experts say there is evidence that managing weight and taking exercise may reduce the risk of the breast cancer returning.

What causes weight gain with breast cancer?

Many things contribute to weight gain. One reason for weight gain can be premature menopause brought on by chemotherapy. Menopause makes it easier for you to gain weight. That's because it causes your metabolism to slow down. There is also a change in body composition. Your body gains more body fat and loses lean muscle.

Normal weight gain involves a gain in both fatty and lean tissue. However the weight gain brought on by chemotherapy involves only fat. The loss of lean body mass and the gain of body fat are typical of the normal ageing process. A woman having chemotherapy ages the equivalent of 10 years over the course of just one year.

Some research suggests that weight gain is also related to lack of exercise. The drop in the level of physical activity may be due to the fatigue, nausea and pain that come with breast cancer treatment.

Weight gain may also be related to intense food cravings. Some women have cravings that typically involve sweets and carbohydrates during chemotherapy. These foods can cause weight gain, especially when they're not eaten in moderation.

Do other breast cancer medications cause weight gain?

Women undergoing other forms of breast cancer treatment gain an average of nearly a kilogram (2lb) a year. Women taking steroid medications during cancer treatment may also experience weight gain. However the weight gain associated with steroids usually becomes noticeable only after weeks of continuous use.

Steroids are hormonal substances that cause an increase in fatty tissue. The usual result is a fullness of the neck or face and a big tummy. Another side effect of steroid medications is loss of muscle mass. That loss of muscle makes weight gain more apparent.

Hormone therapy is another treatment that can cause weight gain. Hormone therapy decreases the amount of oestrogen and progesterone in females and testosterone in males. The treatment tends to cause an increase in body fat mass. At the same time, there's a decrease in muscle mass and a change in the way food is metabolised.

Some women taking tamoxifen find that they gain weight. This is possibly due to fluid retention.

Interestingly, weight gain is not typical in women who have undergone surgery alone. The same is true of women who have had surgery followed by radiotherapy.

Can breast cancer treatment cause weight loss?

Yes. Weight loss is also common among breast cancer patients. Typically weight loss is due to the extreme nausea that is a side effect of chemotherapy. Loss of appetite is another possible side effect that can cause weight loss.