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October 21, 2020 | 6 minute read

Weight and Breast Cancer Risk: It’s More Than a Number

Headlines can take you off track when it comes to strategies to reduce breast cancer risk. Claims about a food to eat to protect against breast cancer, or a hazard to avoid, can be eye-catching. But they can leave many women unaware of the protective potential of limiting weight gain seen in strong evidence from the AICR Third Expert Report.

Even small yearly gains in body fat add up. It’s not hard to reach menopause and find yourself 40 or 50 pounds heavier than your young adult weight. And for some women, larger weight gains year after year make it add up even faster. Disturbingly, the scale doesn’t tell it all – loss of muscle over the years can mask gains in body fat.

And it is body fat…not “weight” that is the concern for breast cancer risk.

Body Fat and Breast Cancer Risk: Not All-or-Nothing

According to analysis by AICR’s Expert Panel, combining over 50 studies:

  • Risk of breast cancer increases 12% with each 5-point increase in body mass index (BMI). That’s an increase of about 30 pounds for an average woman. BMI is a way of expressing weight in relationship to height that’s commonly used as an indicator of healthy or excessive weight.
  • Each increase from the bottom of the healthy range to the highest levels of obesity is tied to a greater and greater risk of breast cancer. This is not an all-or-nothing link that simply compares people with the very highest and the very lowest BMI.
  • Waist size matters. Larger waist size also increases risk of post-menopausal breast cancer, even after adjusting for BMI. A larger waist tends to signal more “visceral fat,” which is fat settled deep in the abdomen that is more active in producing compounds that can promote growth of cancer cells.

So what if your BMI is rated healthy? BMI isn’t a perfect indicator of body fat. Some people with a normal BMI have levels of body fat high enough to pose risk. In a study of over 3,400 postmenopausal women all with a normal BMI, those in the top quarter for body fat had double the risk of invasive ER+ breast cancer compared to women in the lowest quarter for body fat. Based on this and other studies, the researchers concluded that normal BMI may be inadequate as a proxy for risk of breast cancer in postmenopausal women.

Why Does Excess Body Fat Increase Breast Cancer Risk?

Fat cells have important functions in the body, but excess body fat increases risk of postmenopausal breast cancer in at least three important ways.

  • Increased estrogen: After menopause, body fat is the main site of estrogen production. More body fat generally means higher levels of circulating estrogen. Overweight and obesity particularly increase risk of postmenopausal estrogen receptor-positive (ER+) breast cancer, which is by far the most common form of the disease. In this form of cancer, breast cancer cells receive growth signals from estrogen.
  • Inflammation: As body fat expands, chronic low-grade inflammation often develops, enhancing production of free radicals that can damage DNA. This can trigger signaling pathways and gene expression that promote cancer development and progression.
  • Insulin resistance: Higher levels of body fat can also cause metabolic changes that make insulin less effective, causing the body to secrete more. High levels of insulin enhance signals for tumor growth.

Step One: Stop or Slow Weight Gain

According to analysis for the AICR Third Expert Report, each 11-pound gain from your weight at age 18 to 20 is associated with a 13% increase in risk of postmenopausal ER+PR+ breast cancer.

Don’t stress and panic over each small gain. But studies are clear that even if eating habits and physical activity habits stay the same, a slowing metabolism means that most adults gain weight. And it’s easy for habits to shift over time in ways that boost weight gain.

If you are experiencing weight gain, consider choices you could make to stop or slow that gain:

  • Make water your primary beverage. Unsweetened tea and coffee are also good choices, but save sugar-sweetened drinks for occasional use only.
  • Start with slightly smaller portions of food (except for vegetables). Pause and tune into hunger signals before auto-pilot sends you back for seconds.
  • Experiment with new vegetables and find new ways to fix old favorites so that it’s fun and delicious to make veggies the largest part of your plate. That’s one of the key strategies to be able to eat more without excess calories.
  • Develop ways to deal with stress and comfort yourself without relying on calorie-laden foods. Grow your ability to tend to emotional needs in ways that don’t always rely on food.

Does Weight Loss Reduce Breast Cancer Risk?

What if you’re already carrying excess body fat? We need more research to understand how intentionally losing some pounds can affect breast cancer risk.

Meanwhile, studies consistently show that losing and maintaining a modest drop in body fat can improve all three of these hazards that excess body fat poses for breast cancer risk:

  • Losing and maintaining a modest drop in body fat can reduce markers of inflammation.
  • Insulin resistance also decreases with modest weight loss, which also reduces risk of type 2 diabetes.
  • Some studies show a drop in estrogen levels with weight loss. Even more consistently, there’s less estrogen available because of increases in binding proteins that tie it up. A modest drop in estrogen available to promote cancer is seen even with less than a 5% weight loss in some cases, with greater drops at weight loss over 10%.

Move More: Look Beyond Weight Loss

Regular physical activity seems to be a key influence in limiting weight gain with age and avoiding regain after weight loss.

However, even with no change in weight, regular physical activity can help avoid unhealthy elevations of insulin and reduce inflammation. And AICR analysis shows that regardless of weight, more physical activity is better for reducing breast cancer risk.

Aim for a Multiplier Effect to Reduce Breast Cancer Risk

Avoiding weight gain – and perhaps changing some habits to allow modest weight loss if you’re ready – is a solid target. But instead of unproven supplements or unsustainable diets, achieve your weight management goals with strategies that have independent potential to reduce breast cancer risk. Focus on a mostly plant-based diet, limit alcohol consumption and make physical activity a regular part of your life.

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