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The condition known as obesity is defined as the case in which a person has an unhealthy body weight.  In other words, they are “fat.”  The Body Mass Index (BMI) is a common way to determine if a person is obese by comparing their height to their weight.  The equation that is used is as follows:

body mass index calculation

The more a person weighs in comparison to how tall they are, they fatter they are.  At least that’s the idea.  But it isn’t perfect, especially among active people.  Weightlifters for instance, will often rank as “severely obese” on the BMI scale because they carry a lot of weight for their height.  But of course, this does not mean they are unhealthy; in fact, quite the contrary.  Overall though, using the BMI is at least a good starting point.

Below is a chart of BMI’s but you can also go here to use a calculator to determine your BMI:

BMI in kg/m2 Weight Category
Below 18.5 Underweight
18.5 to 24.9 Normal
25.0 to 29.9 Overweight
30.0 to 39.9 Obese
40.0 or higher Severely obese

Researchers and health professionals often use the BMI because it is simple and easy.  If you know someone’s height and weight, you can pretty accurately determine if they are overweight, unless of course they are an athlete.  To be more accurate, other ways of determining “fatness” are frequently utilized.  For instance, comparing a patient’s fatness on various parts of their body such as is done waist to hip ratio measurements.

Too much weight results in disease; this we know

Obese and even just overweight folks are more likely to all sorts of diseases, including heart disease, high blood pressure, high cholesterol, diabetes, heart attack and stroke.  But don’t think that’s even close to the entire list.  And for those that qualify as severely obese, the risk is even higher.  Of course, all of these lead to death, which by the way increases with weight1,2.

According to a survey conducted by the CDC, 70% of adults age 20+ were overweight or obese.  Over one-third of these were obese.3  These numbers come from 2011-2014, so its almost certainly higher now.  Let’s compare this to the same survey in 1988-1994 in which just over half of this demographic was overweight or obese.

Childhood obesity is ‘big’ problem too

Childhood obesity has skyrocketed right along with adults.3  From 2011-2014 nine percent of kids ages 2-5 were obese or overweight.  17% of kids 6-11 and 20% of 12-19-year-olds made the cut.  Yet these figures were only 7%, 11% and 10%, respectively in 1988-1994.  All of these age groups together increased 17% from the 1988-1994 survey to the 2011-2014 survey.4

Obesity among racial groups

Obesity is also more or less common among racial groups.  It turns out that from 2011-2012, non-Hispanic blacks had an overweight/obesity rate of a staggering 47.8%.  Next were Hispanics with 42%.  Non-Hispanic whites and non-Hispanics were found to be 33.4% and 10.9%, respectively.  Children showed similar trends with 21.9% of Hispanics, 19.5% of non-Hispanic blacks, 14.7% of non-Hispanic whites and 8.6% of non-Hispanic Asians were overweight or obese.

Obesity and cancer

There are quite a few large studies that suggest, at the very least, that there is a strong link between obesity and cancer.  At best, obesity causes cancer.  In any case, the evidence repeatedly suggests that body fat and cancer risk are linked in at least a number of cancers, including:6

Endometrial cancer7,8

  • Esophageal cancer9
  • Gastric cardiac cancer10
  • Liver cancer11,12
  • Kidney cancer14
  • Multiple myeloma15
  • Meningioma16,
  • Pancreatic cancer17
  • Colorectal cancer18
  • Gallbladder cancer19,20
  • Breast cancer21,22,23
  • Ovarian cancer24
  • Thyroid cancer25

Obesity and cancer are certainly related but how?

While studies give us reliable evidence linking cancer risk to obesity, they don’t tell us how this happens.  However, a few ideas for how obesity could cause cancer have been suggested by scientists.

  1. Most people that are obese also experience increased inflammation throughout their body. Inflammation is already known to often result in damage to DNA, which of course can manifest as cancer.26,27,28
  2. Fat produces estrogen. More fat, more estrogen.  High estrogen, higher cancer risk, especially in women.
  3. Two hormones called insulin and IGF-1 are often higher in the obese. This is what leads to diabetes and diabetes can lead to cancer.29
  4. Adipokines, a class of hormones, are produced by fat cells. These hormones tells cells to grow and cancer cells get the message too, but you don’t want them growing!
  5. Fat tissue affects other hormones that regulate cell growth too.
  6. Changes occur in breast tissue over time and there is always the possibility of cancer occurring. In obese women, there is a lot more fat in the breasts and therefore a higher cancer risk.30
  7. Fat affects the immune system and its ability to function properly. Oxidative stress, which leads to cancer, cannot properly be combated by an obese immune system.31

How often do obese people actually get cancer

Estimates suggest that in 2012, some 28,000 (3.5%) cancer cases in men and 72,000 (9.5%) in women were a direct result of obesity.32  This varied among cancer types but was as high as 54% for female gallbladder cancer.  In other words, more than half of the female gallbladder cancer cases were the result of nothing more than the women being obese!  The U.S. in particular has the highest rate of obesity-caused cancers in several categories.33

If obesity and cancer are so strongly related, will my risk go down if I diet?

As I mentioned before, studies cannot always tell us “for sure” that x causes y, but when many of them seem to be telling us the same thing, the evidence becomes more compelling.  For instance, a number of studies have shown that people that gain weight slower over time than average, also have lower than average risks of colon, kidney, breast, endometrial and ovarian cancers.34

Other studies have directly considered cancer risk among weight losers.  Some, but not all, have found that weight loss seems to be associated with a lower risk of cancer.  However, many of these studies were just looking at patients that lost weight but not if it was intentional or not.  In short, its hard to say that losing weight on purpose was the cause.

There are some good studies however on bariatric surgery (‘stomach staple’) patients.  Most of these seem to suggest that obese individuals that receive bariatric surgery are at a much lower cancer risk.35  The Women’s Health Initiative study however found that women who were already obese, didn’t experience a change in breast cancer risk if their weight changed (up or down).  But women of normal weight who gained 5%+ of bodyweight experienced an increased risk.36

The bottom line regarding obesity and cancer

The evidence is pretty strong that obesity increases one’s cancer risk.  That isn’t to say that if you are obese, you will definitely get it, but you are more likely to.  Even if you don’t get cancer, you are almost guaranteed to experience disease and discomfort that a ‘normal weight you’ would not.  But if you maintain a ‘normal’ bodyweight you are far more likely to live a long, happier, cancer free life.