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Another excellent reason to lose weight if you are overweight or obese: losing weight (through diet or through combined diet and exercise) significantly lowers levels of proteins in the blood that help cancerous tumors grow. In other words, reducing weight could turn out to be a cancer prevention method in overweight and obese persons. Exercise alone did not lower the levels of these cancer-associated proteins.

The study enrolled 439 overweight or obese women (aged 50 to 75 years old) from the Seattle area who were randomly placed into one of four groups for 12 months: exercise only, diet only, exercise plus diet, or no change to health habits. Researchers measured three proteins in blood samples - VEGF, PAI-1 and PEDF – that flow through the body and help in the formation of new blood vessels, a process called angiogenesis. Angiogenesis can occur during such processes as wound healing, but it also occurs during the growth of tumors. Since the three measured proteins are involved in nurturing the growth and survival of tumor cells, this is a great reason to lose weight - to lower their levels in the blood. From Science Daily:

Losing weight lowered levels of proteins associated with tumor growth

Overweight or obese women who lost weight through diet or a combination of diet and exercise also significantly lowered levels of proteins in the blood that help certain tumors grow, according to a Fred Hutchinson Cancer Research Center study published July 14 in Cancer Research, a journal of the American Association for Cancer Research.

The study: Measured three proteins that are known to enhance tumor-related angiogenesis -- the formation of blood vessels that feed tumors and enable them to grow. Was intended to see how cancer-promoting proteins changed when overweight, sedentary, postmenopausal women lost weight through diet or diet and exercise over the course of a year. Enrolled 439 healthy women (they did not have cancer), placing each participant in one of four study arms: 1) Calorie- and fat-restricted diet. 2) Aerobic exercise five days a week. 3) Combined diet and exercise. 4) Control (no intervention).

Found that women in the diet arm and the diet and exercise arm lost more weight and had significantly lower levels of angiogenesis-related proteins, compared with women in the exercise-only arm and the control arm.

This study shows that weight loss may be a safe and effective way to improve the "angiogenic profile" of healthy individuals, meaning they would have lower blood levels of cancer-promoting proteins. Although the researchers cannot say for certain that this would impact the growth of tumors, they believe there could be an association between reduced protein levels and a less favorable environment for tumor growth.

Again, more research finding that being overweight or obese is associated with an increased risk of breast cancer - specifically higher risk of invasive breast cancer in postmenopausal women. They found that the heavier the woman, the higher the risk, but the risk did not vary with hormone therapy use or race and ethnicity. From Medical Xpress:

Obesity associated with increased breast cancer risk in postmenopausal women

An analysis of extended follow-up data from the Women's Health Initiative clinical trials suggests that postmenopausal women who were overweight and obese had an increased risk of invasive breast cancer compared to women of normal weight, according to an article published online by JAMA Oncology. Obesity is a major public health problem in the United States and obesity has been associated with breast cancer risk in observational studies, systematic reviews and meta-analyses.

The Women's Health Initiative (WHI) protocol measured height and weight, baseline and annual or biennial mammograms, and breast cancer in 67,142 postmenopausal women enrolled from 1993 to 1998 with a median of 13 years of follow-up. There were 3,388 invasive breast cancers. Analysis by the authors found:

  • Women who were overweight (body mass index [BMI] 25 to < 30); obese, grade 1 (BMI 30 to < 35); and obese, grade 2 plus 3 (BMI > 35) had an increased risk of invasive breast cancer compared to women of normal weight (BMI < 25)
  • The risk was greatest for women with a BMI greater than 35; those women had a 58 percent increased risk of invasive breast cancer compared with women of normal weight (BMI < 25)....
  • Obesity was associated with markers of poor prognosis; women with a BMI greater than 35 were more likely to have large tumors, evidence of lymph node involvement and poorly differentiated tumors
  • Women with a baseline BMI of less than 25 who gained more than 5 percent of body weight during the follow-up period had an increased risk of breast cancer....

Here it is, a list of 17 cancers linked to being overweight or obese. From Science Daily:

Overweight and obesity linked to 10 common cancers, over 12,000 cases every year in UK

A higher body mass index (BMI) increases the risk of developing 10 of the most common cancers, the largest study of its kind on BMI and cancer, involving more than 5 million adults in the UK, shows. Each 5 kg/m² increase in BMI was clearly linked with higher risk of cancers of the uterus (62% increase), gallbladder (31%), kidney (25%), cervix (10%), thyroid (9%), and leukemia (9%). Higher BMI also increased the overall risk of liver, colon, ovarian, and breast cancers.

Using data from general practitioner records in the UK's Clinical Practice Research Datalink (CPRD), the researchers identified 5·24 million individuals aged 16 and older who were cancer-free and had been followed for an average of 7·5 years. The risk of developing 22 of the most common cancers, which represent 90% of the cancers diagnosed in the UK, was measured according to BMI after adjusting for individual factors such as age, sex, smoking status, and socioeconomic status. A total of 166 955 people developed one of the 22 cancers studied over the follow-up period. BMI was associated with 17 out of the 22 specific types of cancer examined.

Each 5 kg/m² increase in BMI was clearly linked with higher risk of cancers of the uterus (62% increase), gallbladder (31%), kidney (25%), cervix (10%), thyroid (9%), and leukemia (9%). Higher BMI also increased the overall risk of liver (19% increase), colon (10%), ovarian (9%), and breast cancers (5%), but the effects on these cancers varied by underlying BMI and by individual-level factors such as sex and menopausal status. Even within normal BMI ranges, higher BMI was associated with increased risk of some cancers.

There was some evidence that those with high BMI were at a slightly reduced risk of prostate cancer and premenopausal breast cancer. Based on the results, the researchers estimate that excess weight could account for 41% of uterine and 10% or more of gallbladder, kidney, liver, and colon cancers in the UK.