Obesity and Cancer Risk

Each year, over 684,000 Americans are diagnosed with cancers linked to obesity, according to the Centers for Disease Control and Prevention (CDC). This troubling trend shows an increase in these cancers, especially among younger people, even as cancers not related to excess weight, like lung and skin cancers, are on the decline.

Is obesity becoming the new smoking? Not quite. Unlike the clear-cut relationship between smoking and cancer, the link between obesity and cancer is more complex. While about 42% of cancers, including colorectal and postmenopausal breast cancers, are associated with obesity, only about 8% of cancer cases are directly attributed to excess body weight. Many people develop these diseases regardless of their weight.

Despite strong evidence linking excess body fat to cancer, it's unclear exactly when weight gain begins to influence cancer risk. Does gaining weight later in life pose a different risk than being overweight from a young age? Additionally, could losing weight at some point in adulthood reduce the risk? These questions remain unanswered.

A Consistent but Complicated Relationship

With obesity affecting about 42% of US adults and 20% of children and teenagers, many studies have explored how excess weight might influence cancer rates. Most evidence comes from large studies that can't definitively prove cause and effect but do show consistent associations. Higher body mass index (BMI), especially in the obese range, is linked to a higher risk of several cancers.

A comprehensive report by the International Agency for Research on Cancer (IARC) in 2016 reviewed over 1,000 studies and found that more than a dozen cancers, including some of the most common and deadly, are linked to excess body weight. This list includes esophageal adenocarcinoma and endometrial cancer, which have the highest risk, as well as kidney, liver, stomach, pancreatic, colorectal, postmenopausal breast, gallbladder, ovarian, and thyroid cancers, plus multiple myeloma and meningioma.

Obesity is often associated with factors like poor diet, lack of exercise, and metabolic conditions such as diabetes, which also contribute to cancer risk. Therefore, high BMI likely affects cancer risk directly and indirectly.

How Excess Fat Contributes to Cancer

Preclinical research highlights several ways excess body fat might contribute to cancer. One broad mechanism is chronic inflammation, as excess fat tissue raises levels of substances like tumor necrosis factor alpha and interleukin 6, which fuel inflammation. Excess fat also leads to high insulin levels, which can promote tumor growth.

The mechanisms vary by cancer type. For hormonally driven cancers like breast and endometrial cancer, excess body fat can alter hormone levels in ways that promote tumor growth. For instance, fat tissue can convert androgens into estrogens, feeding estrogen-dependent tumors. This might explain why excess weight is linked to postmenopausal, but not premenopausal, breast cancer.

The Strength of the Association

While many cancers are linked to excess weight, the strength of these associations varies. For example, severe obesity increases the risk of endometrial cancer sevenfold and esophageal adenocarcinoma by nearly five times compared to people with normal BMI. For other cancers like ovarian, colorectal, kidney, and stomach cancers, the risk increase ranges from 10% to 80%.

A 2018 study by the American Cancer Society estimated that excess weight accounts for 7.8% of all cancer cases in the US, second only to smoking. The impact of excess weight is more pronounced in certain cancers, accounting for 60% of endometrial cancers and about one-third of esophageal, kidney, and liver cancers. Even at the lower end, such as 11% of breast cancers, the population impact is significant, especially in groups with higher obesity rates.

When Do Extra Pounds Matter?

When in life does excess weight begin to affect cancer risk? Evidence suggests that weight gain at any age increases cancer risk. A meta-analysis found that weight gain after age 18 is associated with higher postmenopausal breast cancer risk. A 2023 study linked sustained overweight or obesity from age 20 to middle age with increased risks for colorectal and other gastrointestinal cancers after age 55.

The rise in early-onset cancers (diagnosed before age 50), particularly gastrointestinal cancers, raises the question of whether obesity among young people is partly to blame. Data from the Nurses' Health Study II indicated that women with obesity had double the risk for early-onset colorectal cancer compared to those with a normal BMI.

Does Weight Loss Help?

Logically, if high BMI increases cancer risk, losing weight should reduce that risk. However, evidence supporting this is limited and primarily observational. Studies on weight loss after bariatric surgery show promising results, with a lower incidence of obesity-related cancers compared to non-surgical groups.

For more modest weight loss, some evidence suggests benefits for postmenopausal breast and endometrial cancers. A 2020 analysis found that women over 50 who lost as little as 4.4-10 pounds and maintained it for a decade had a lower breast cancer risk. Greater weight loss showed even more significant risk reduction.

In summary, while obesity is a significant risk factor for many cancers, the relationship is complex and influenced by various factors. Continued research is needed to fully understand how and when excess weight impacts cancer risk and how weight loss can mitigate this risk.