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Nearly all of the evidence linking obesity to cancer risk comes from large cohort studies, a type of observational study. However, data from observational studies can be difficult to interpret and cannot definitively establish that obesity causes cancer.
Despite the limitations of the study designs, there is consistent evidence that higher amounts of body fat are associated with increased risks of a number of cancers, including:
Endometrial cancer: Women who are over-weight are 2-4 times as likely as healthy-weight women to develop endometrial cancer.
Oesophageal adenocarcinoma: Twice as likely as healthy-weight people to develop oesophageal adenocarcinoma, and people who are extremely obese are more than four times as likely.
Gastric cardia cancer: Twice as likely as healthy-weight people to develop cancer in the upper part of the stomach.
Liver cancer: Twice as likely as healthy-weight people to develop liver cancer.
Kidney cancer: Twice as likely as normal-weight people to develop renal cell cancer. The association of renal cell cancer with obesity is independent of its association with high blood pressure
Multiple myeloma: Have a slight (10% to 20%) increase in the risk of developing multiple myeloma.
Meningioma: Increased by about 50% in people who are obese and about 20% in people who are overweight.
Pancreatic cancer:5 times as likely to develop pancreatic cancer as healthy-weight people.
Colorectal cancer: A higher BMI is associated with increased risks of colon and rectal cancers in both men and in women.
Gallbladder cancer: Compared with healthy-weight people, people who are overweight have a slight (about 20%) increase in risk of gallbladder cancer, and people who are obese have a 60% increase in risk of gallbladder cancer.
Breast cancer: In postmenopausal women, a higher BMI is associated with a modest increase in risk of breast cancer.
Ovarian cancer: Higher BMI is associated with a slight increase in the risk of ovarian cancer.
Thyroid cancer: Higher BMI (specifically, a 5-unit increase in BMI) is associated with a slight (10%) increase in the risk of thyroid cancer.
Many observational studies have provided consistent evidence that people who have lower weight gain during adulthood have lower risks of colon cancer, kidney cancer, and for postmenopausal women—breast, endometrial, and ovarian cancers.
Fewer studies have examined possible associations between weight loss and cancer risk. Some of these have found decreased risks of breast, endometrial, colon, and prostate cancers among people who have lost weight. However, most of these studies were not able to evaluate whether the weight loss was intentional or unintentional (and possibly related to underlying health problems).
Stronger evidence for a relationship between weight loss and cancer risk comes from studies of people who have undergone bariatric surgery (surgery performed on the stomach or intestines to induce weight loss). Obese people who have bariatric surgery appear to have lower risks of obesity-related cancers than obese people who do not have bariatric surgery.
Most of the evidence about obesity in cancer survivors comes from people who were diagnosed with breast, prostate, or colorectal cancer. Research indicates that obesity may worsen several aspects of cancer survival, including quality of life, cancer recurrence, cancer progression, and prognosis (survival).