How much does weight actually affect your health?
Fact or Fiction: Is obesity actually an epidemic? Part 3
The weight loss industry and popular media have declared that obesity is extremely dangerous for your health. If you don’t have a bachelor’s degree in nutrition or health sciences, fact checking that messaging can be difficult. That’s where I come in – your guide through current obesity research (backed by an HBSc in Human Biology and Nutritional Sciences, and a Doctorate in Naturopathic Medicine). So without further ado, let’s take a deep dive into the research!
Does weight affect health?
In the scientific literature, we see that weight extremes (both high and low) can have negative impacts to health. But we need more details – how much does weight affect health? And what weights are considered extreme (both on the high and low ends)? To answer these questions we need to break “health” down into the two main facets that are studied in the research: length of life (mortality) and quality of life.
How much does weight affect mortality?
A UK study of over 3 million people used body mass index (or BMI) to study that exact question (1). BMI is a measure of weight in kg divided by height in m (squared). The BMI system puts people in to categories (underweight, normal, overweight, or obese) based on their height and weight measurements. While this system is far from perfect (in clinical practice it’s actually far from useful), it is typically used in research.
Researchers found that obesity (a BMI 30 or over) on average was associated with a 4.2 year reduction in life expectancy for men and a 3.5 year reduction in life expectancy for women, compared to normal weight and overweight individuals (1). They found that this reduction in life had a dose dependant effect – people who had a BMI of 35 had a longer estimated life expectancy than those with a BMI of 40 or greater. The same study found that being underweight (having a BMI below 18.5) is associated with a reduction in life expectancy of 4.3 years, in both men and women (1).
As we saw in the last instalment of this series, the effects of obesity on mortality could be an effect of weight, or could be a reflection of weight bias in the healthcare system (which can also lead to poorer health outcomes) (2).
How much does weight affect quality of life?
We know that health isn’t just how long you live, it’s also how enjoyable those years are. So how much does weight affect the quality of your years?
Having an underweight BMI increases one’s risk for infertility, osteoporosis, and heart arrhythmias. It is also common to have symptoms of fatigue, muscle weakness, difficulty concentrating, mood disturbances, low libido, and digestive dysfunction with low BMI.
On the other end of the spectrum, research has consistently found that obese individuals report lower quality of life on subjective questionnaires (3). This may be because elevated BMI is an independent risk factor for type 2 diabetes, cardiovascular disease, and sleep apnea (4), leading physical discomfort.
However, the physical risk factors of obesity aren’t the only thing that affect quality of life at that weight. It’s important to note that mental health scores were significantly poorer in higher weight categories as well, a result of fat shaming and weight stigma that can have serious consequences on self-confidence and self-worth (3). A lower quality of life score isn’t a reflection of the independent effects of weight, it also reflects how overweight people are treated in our healthcare system and our society at large.
So what weights are considered extremely low or extremely high?
We’ve seen that the high and low tails of BMI are associated with worse health outcomes, but at what BMIs do we see this happening? On the high end we see most negative associations with weight occurring at BMIs above 35. On the low end, BMIs below 18.5 are associated with poorer health outcomes. (Interestingly, in the UK study mentioned above it was found that the BMI associated with the lowest mortality was actually 25, a BMI that is categorized as overweight) (1).
It's all well and good to talk about the numbers, but what do those BMI’s actually look like? The bodies that women are told are healthiest – models with visible abs, flat tummies, and thigh gaps– actually have BMIs that are dangerously low. The average model measures up at 5ft 9in and 110lbs, putting them at a BMI of 16.2, which is categorized as dangerously underweight. If we know that a model’s weight carries the same mortality risk as someone who is obese, why then do we find models on the cover of health magazines being promoted as fitness idols while anyone overweight is put on the evening news with the caption “obesity epidemic”. The extreme response from media shows that the epidemic we have is with fat-phobia, not with fatness itself.
We’ve seen that weight does have a measurable effect health, but this effect is not simply due to biological mechanisms of weight, it is complicated by anti-fat stigma as well. Weight is also obviously not the only thing that affects health – but more on that in a future blog post.
In the next instalment we’ll be looking into how much weight loss impacts health (if at all).
References
1. Bhaskaran K, dos-Santos-Silva I, Leon DA, Douglas IJ, Smeeth L. Association of BMI with overall and cause-specific mortality: a population-based cohort study of 3·6 million adults in the UK. Lancet Diabetes Endocrinol. 2018 Dec;6(12):944–53.
2. Rubino F, Puhl RM, Cummings DE, Eckel RH, Ryan DH, Mechanick JI, et al. Joint international consensus statement for ending stigma of obesity. Nat Med. 2020 Apr;26(4):485–97.
3. Kolotkin RL, Andersen JR. A systematic review of reviews: exploring the relationship between obesity, weight loss and health-related quality of life: Quality of life systematic review of reviews. Clin Obes. 2017 Oct;7(5):273–89.
4. The GBD 2015 Obesity Collaborators. Health Effects of Overweight and Obesity in 195 Countries over 25 Years. N Engl J Med. 2017 Jul 6;377(1):13–27.