It is estimated that in 2022, 1 in 8 people worldwide and 1 in 4 people in the UK, were living with obesity.
Overweight and obesity happen when your food intake is higher than your body needs for daily living and your amount of physical activity. Obesity is now considered a complex chronic disease.
There are many reasons why you might put on weight; inherited genes, lifestyle, and what you eat can all play a part.
The Body Mass Index (BMI) is the most common tool used to estimate body fatness.
BMI is calculated by dividing your body weight in kilograms by your height in meters squared.
BMI score is categorised into underweight, healthy weight, overweight, obese and severely obese.
Check your BMI with this NHS BMI calculator.
Table 1: BMI score categorisation based on ethnic background
| European background | Asian, Chinese, Middle Eastern, Black African or African-Caribbean background* | ||
| BMI Score (kg/m2) | Category | BMI Score (kg/m2) | Category |
| >18.5 | Underweight | >18.5 | Underweight |
| 18.5 – 24.9 | Normal weight | 18.5 – 22.9 | Normal weight |
| 25 – 29.9 | Overweight | 23 – 27.4 | Overweight |
| 30 – 34.9 | Obesity class 1 | 27.5 – 32.4 | Obesity class 1 |
| 35 – 39.9 | Obesity class 2 | 32.5 – 37.4 | Obesity class 2 |
| 40 or above | Obesity class 3 | 37.5 or above | Obesity class 3 |
* Research shows people from some Black, Asian and minority ethnic groups are more prone to central adiposity and have an increased cardiometabolic health risk at lower BMI thresholds.
Being overweight or obese can help to promote the growth of several cancers, including breast cancer.
In women, there is a link between excess weight and breast cancer risk, but this depends on menopausal status. In postmenopausal women, being overweight or obese increases the risk of breast cancer by up to 30%. In premenopausal women, being overweight or obese does not appear to be associated with an increase in breast cancer risk, however, they are associated with a variety of other health risks, including cardiovascular diseases, diabetes, and other cancers.
In men, excess weight is also linked to an increased risk of male breast cancer.
Being overweight or obese has complex effects on your body and your cells. It can affect your cell’s ability to grow and repair, your immune system, and, importantly for breast cancer risk, your hormone system.
Fat (adipose) cells can produce the sex hormone oestrogen. This is especially relevant for postmenopausal women as fat becomes the main source of oestrogen after menopause. Having higher oestrogen levels can increase breast cancer risk.
In addition to oestrogen, fat cells can release a hormone called leptin. Studies have shown that higher leptin levels can increase the risk of developing breast cancer.
Weight gain and obesity can lead to long-term (chronic) inflammation within our bodies. This means that the immune system can be active when it shouldn’t be. Obesity-related inflammation can also encourage insulin resistance, which means that there may be too much sugar in the blood, leading to several health implications.
Weight loss, if maintained, can help reduce the risk of breast cancer for postmenopausal women.
More research is needed to understand which types of weight loss interventions are the most beneficial.
However, avoiding weight gain may be a better breast cancer prevention tool than weight loss.
For more information on Weight and Breast Cancer, read our full science review.
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