Obesity and cancer: what weight has to do with the disease

A bigger link than it seems

Many people associate cancer with smoking or genetics. Few know that excess weight also plays an important role:

• In developed countries, about 13% of new cancer cases are linked to excess weight.
• More than 30 types of cancer have now been associated with obesity.
• In Brazil, several of the most common cancers are linked to it, including cancers of the breast, bowel, liver, ovary and endometrium (the lining of the uterus).
• The more excess weight, the higher the risk. For some tumors, the risk can rise by up to 56% for each step up in body mass index (BMI).

Why body fat raises the risk

Body fat is not just an energy store. It works like a gland: it produces hormones and other substances that affect the whole body. When there is too much of it:

• The body is inflamed all the time, at a low level we can't feel. This inflammation makes errors in the cells' DNA more likely.
• Insulin stays high. Besides controlling blood sugar, insulin stimulates cells to multiply. This is true even for people who don't yet have diabetes.
• Hormones change, such as estrogen, which matters especially for breast and uterine cancers.

It's not just the number on the scale

BMI, which divides weight by height squared, is useful, but it doesn't distinguish muscle from fat. That is why waist size also matters: it shows belly fat, the kind most closely linked to risk. A simple rule: your waist should measure less than half your height.

It is also possible to have a "normal" weight and little muscle. Muscle strength is protective: it helps insulin work better and helps you tolerate treatment better.

What about people who already have cancer?

Excess weight also matters during treatment:

• Some tests become more difficult, but adapted equipment and techniques exist. Mammography, for example, is still the most important test and can deliver excellent quality with the right adjustments.
• Surgery carries a higher risk of infection, slow wound healing and blood clots.

You deserve care without judgment. Weight is not a personal failing or a lack of willpower: it is a disease with many causes. If you have ever felt disrespected during a medical visit, that should not have happened. Look for a team that makes you feel welcome.

What really helps

• Small losses count. Losing 5% to 10% of your weight already improves insulin, inflammation and blood pressure. You don't need to reach an "ideal" weight to benefit.
• Get moving. Aim for at least 150 minutes a week of moderate activity, or 75 minutes of vigorous activity. Any extra movement helps, and the benefits appear even before the weight comes down.
• Eat better, not just less. At least 5 servings of fruit and vegetables a day, less fat and fewer ultra-processed foods. Ideally, follow a plan tailored to you by a dietitian.
• Obesity medicines, such as those in the semaglutide and tirzepatide family, have shown promising results. They are expensive, can cause nausea and other digestive side effects, and need to be taken long term, because the weight tends to come back when you stop.
• Bariatric surgery is a very effective option for people with severe obesity, with large and lasting weight loss.
• Look after your mind too. Anxiety and low mood are common and make it harder to look after your body. Psychological support makes a difference.

If you are in treatment, don't start a diet on your own. During chemotherapy, losing muscle can be harmful. Talk to your team to find the right time and the right approach.

For information only. It does not replace an individual medical assessment.