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What Causes Weight Gain?

TW/CW: Almost all of the links included in this post contain fatphobic/weight-centric messaging and/or images, the “o” words, specific calorie/weight numbers, etc. Unfortunately, most weight science is still being conducted with the assumption that higher weights are a problem/disease.

Several weeks ago I received an interesting email from a reader:

“Something I’ve been wondering about in all the reading I’ve been doing about weight loss is about weight gain.


It seems clear that long term weight loss is maybe not quite impossible, but nearly.  I’ve read lots about how your body wants to keep itself at a predetermined weight.


So….why didn’t my body keep me at some previous predetermined weight?  Like, whenever I was done growing or something.  I mean, how does a person gain weight?  


I look at my uncle – he’s always been tall and thin.  Only since he hit middle age did he start to develop a “paunch” (extra flesh in his lower belly)


Some of us gain steadily over our lifetime.  Many start to grow in girth when middle age hits and our metabolism slows.


I know this is really long winded.  I guess the question is how do we gain weight?  I mean, not a how-to guide, but how does it happen?  If there is some secret set point that your body wants to be, where does that come from?  When and how is it determined?


If a person wants to prevent weight gain, is that possible?  Does it have to happen in childhood?  Can an adult prevent weight gain?  Is it all totally arbitrary?”

This is such a good question, because despite our society’s obsession with weight, most of us don’t actually have a good understanding of the mechanisms behind it. In fact, an article published in 2014 on where the fat goes when someone loses weight was still making headlines this year because the answer is surprising to many, including health professionals. (If you’re avoiding the links because you’re worried about being triggered, the answer is that it mostly gets breathed out as carbon dioxide, and the rest is excreted as water.)

Weight is the Sum of Many Parts

Part of the reason why this question can be difficult to answer is because there are lots of different components that make up our weight. While the dominant fatphobic narrative would lead us to believe that we are simply fleshy bags of fat, our bones, muscles, organs, fluid status, partially digested food, etc. are all part of the number on the scale. It’s normal for weight to fluctuate by several pounds within a day, mostly due to fluid shifts.

A Short, Unsatisfactory Answer

I’m assuming though, that the question is about fat gain, and so I did some reading and… you’re not going to like the answer.

We gain weight/fat when energy intake consistently exceeds energy expenditure, or calories in versus calories out.

I know, I know. You’re thinking, “What, really? There’s no way it could be that simple! Doesn’t that go against everything HAES®? What about that complicated diagram that people keep sharing?”

Shiftn Obesity System Influence Diagram

OK, let’s pause here. Despite the simplicity of the phrase, it doesn’t mean that eating X calories will make a person be Y weight. You’ve probably seen this phenomenon in your day-to-day life—we all seem to know someone who can eat large amounts of food and stay relatively the same weight, while others seem to gain weight even with a small amount of food. To complicate things even further, when we take in fewer calories than what our bodies need, additional mechanisms kick in that slow our metabolism and increase our appetite, as our body’s way of keeping us from starving to death.

Calories are Difficult to Measure

One of the reasons why calorie-tracking apps like MyFitnessPal make me cringe is because it perpetuates many false assumptions about calories and metabolism. First, when you sign up, it gives you a calorie recommendation based on your weight, height, age, sex, and weight goal. Not only is this problematic because there is so much more that determines our metabolism, but it also creates the illusion that our metabolism is consistent from day-to-day, when in fact it is constantly shifting based on new inputs and environments.

Next, you enter in all the foods you eat and activity you do. Many people like MyFitnessPal because of its large database of foods; however, most of it is user-generated, based on nutrition labels. In Canada and the US, companies are allowed a 20% margin of error when it comes to calories on the Nutrition Facts table. In reality, the margin of error might be even greater than that because the FDA in the US and Canadian Food Inspection Agency in Canada simply don’t have the resources to be the Nutrition Facts table police.

On top of that, studies consistently show that we tend to underreport the amount of calories that we eat, and overreport the amount of calories we burn. Not because we’re pathological liars, but simply because we don’t obsessively measure every morsel of food that passes through our lips. (That’s a sign of an eating disorder.)

This is just part of the reason why the relationship between calories and weight is not straightforward. But even in experimental settings, where calories in and out are measured obsessively, people don’t gain or lose weight at the same rate. Additionally, after weight gain or loss, we see greater increases or decreases in metabolism, respectively, than is expected.

It’s (Mostly) In Your Genes

Our weight is actually mostly dictated by genetic factors—some estimates put it at up to 70%. This doesn’t mean that our genes decide that we’re going to be at a certain weight range and tries to hold us there; rather, the genes code for different factors that could influence our weight, like the hormones that affect our hunger and fullness cues, how much energy we use to perform different tasks, our metabolism, and how effectively our body uses different fuel sources (i.e. carbs versus fat), just to name a few. Another way that I like to think about it is that our genes determine how our bodies react to different inputs that can affect our weight, which can partly explain why the calories in/calories out relationship is not straightforward.

Interestingly, while we tend to “blame” our genes for giving us a “low metabolism”, most of the genes that have been identified as having an effect on weight have to do with appetite regulation (“calories in”) rather than calories out.

Where It Gets Interesting

Of course, genetics alone can’t explain the fact that average body weight has been increasing more than previous decades since the 70s and 80s—genetic adaptation simply doesn’t happen that quickly. So, non-genetic factors like food availability, lifestyle, and environment absolutely play a role. However, instead of looking at population-based “solutions” like large-scale changes in the food supply or living environments, most weight management efforts put the onus on the individual, and this may be producing the opposite of the intended result.

As I mentioned above, when we take in less energy than what our body uses, additional mechanisms are activated that decrease our metabolism and increase our appetite. While it was often thought that this was a response to weight loss, it’s been observed that these mechanisms can be activated within 24 hours of a calorie deficit.

The most dramatic example of this is the “Biggest Loser study“, where when they measured the metabolism of contestants six years after they appeared on the Biggest Loser, it was still lower than expected for their body composition, even when the contestant had regained all of the weight they had lost on the show. These adaptations, coupled with observation of chronic dieters, have led some researchers to conclude that dieting, or any attempt at weight loss, is actually correlated with weight gain. Given that a significant number of people have tried to lose weight multiple times—a recent survey in the UK found that the average woman will try 130 diets in her lifetime—it begs the question, how has our society’s obsession with weight loss actually contributed to weight gain?

How has our society's obsession with weight loss actually contributed to weight gain? Click To Tweet

Does It Really Matter?

We now know that weight gain happens when we consistently take in more calories than our body uses, but most of us don’t have the tools to accurately measure how many calories our bodies are using, nor how many calories we’re taking in on any given day. So, where does that leave us?

I think this question of why we gain weight only matters when we view higher weights and weight gain as a “problem,” and fat acceptance/Health At Every Size® is about framing weight as neutral, or at worst, a non-modifiable risk factor. While from a population perspective, higher weights are correlated with some negative health outcomes, in individuals, BMI is an incorrect predictor of health status about 50% of the time

While it was interesting for me to explore some of the possible mechanisms behind weight gain, I think it’s more important to explore why this question is being asked in the first place—whether it’s about health, weight stigma, or simply curiosity, the answer won’t be found in trying to control or manipulate our weight.

What are your questions or concerns about weight? Please share your thoughts and insights in the comments below, or book a FREE 20-minute Appetizer Call with me and let’s chat!

2 Comments

  • Ruth Leach on Nov 20, 2018 Reply

    It is worth noting too, VERY much so, that weight gain also happens when a person is taking in an inadequate supply of energy – literally in a calorie deficit, because of the way that the metabolism down regulates in the face of restriction, and also because of the way that it does the same thing in someone who has a history of restrictive eating.

    By the time cyclic dieters, or people with long term EDs (not confusing the two, just pointing out that there are many similarities in the physiological impact of restrictive eating) get into middle age, they can find that they are gaining weight on what could be termed “subclinical starvation”. Dr. Emily Cooper’s (of Seattle Performance Medicine) research in this area is compelling reading. She works with athletes who gain weight in a deficit, and helps them to increase their intake to properly fuel their activity, which often results in weight loss.

    Gwyneth Olwyn has also written extensively about this over the past 10 years, within the context of ED recovery and fattism, weight bias, and thincentrism. https://edinstitute.org/paper/2015/1/21/part-i-systematic-review-of-weight-gain-correlates-in-literature

    • Vincci Tsui on Nov 20, 2018 Reply

      Thank you so much for sharing this research, Ruth! I was hesitant to share the concept of “starvation mode” because in my own experience, I haven’t seen weight gain as a result of a calorie deficit—only weight maintenance. But I’ve definitely seen situations where an increase in intake has led to weight loss.

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