Marci Evans, RD - Calories In Calories Out is BS

Calories In = Calories Out is BS

I posted an image on social media a few weeks ago stating “Calories In = Calories Out is Bullsh*t” and it was met with a massive response.

I had a sense it’d strike a chord and it sure did. One of the dominant themes I heard from students and professionals was, “It is BS, BUT it’s what I was taught in school!” And even more disheartening, I got this message from students, interns, and recent grads. This left me both depressed and eager to do a little bit of myth busting here.

First, a rant.

The mantra “calories in = calories out” is unhelpful because it assumes our bodies can be worked over like a math calculation. It can’t. I’ll never forget my first year as a dietitian. I was busy preparing a workshop and the book I had bought to help me prepare listed examples like this:

  • Remove 1 Tb of butter/day and lose 10 pounds in 1 year
  • Swap 1 oz of chocolate for 2 TB jelly beans and lose 5 pounds in 1 year

These kinds of dieting messages are useless because it assumes you eat the same exact number of calories every day and that subtracting a specific number of calories will lead to a consistent caloric deficit. That’s not how it works when you’re human. This message is also harmful because it’s misleading. It gives the false notion that if you count your calories precisely (spoiler: you can’t) and exercise enough willpower (spoiler: willpower dooms us all), you can simply write up a math calculation to get yourself to your chosen weight. None of this is possible and here’s why.​​​​​​​

The Math is Incomplete

Of course our body weight is impacted by what we eat and how our body uses that fuel. But it’s also influenced by a host of other complex mechanisms that we have very little control over, thanks to the genetics passed along from our parents. Most importantly, attempts to decrease intake and increase output creates a massive change in our physiology that undermines efforts to down-regulate our weight. This includes but is not limited to:

  1. Basal metabolic rate: This accounts for about 70% of our metabolic activity and decreases as weight is lost.
  2. Appetite hormones like leptin and ghrelin: Turns out that, as a person loses weight, the body shifts the production of these hormones to encourage increased intake.
  3. Changes in the reward system in the brain: For you neuroscience geeks, it’s particularly related to the orbital frontal cortex, which is related to the reward pathways in our brain making sure we seek out more food and don’t die of famine.

Consequently, when we try to alter calories in, calories out, there are a host of other “numerics” that step in to complicate the equation. However, an important note is that the way and the degree to how these responses happen in you body is outside of your control and largely dictated by genetics.

What I Am Not Saying

I am not saying it is impossible to increase or decrease your weight by changes to what you eat and how you move. I AM saying that manipulating one’s weight is not as simplistic as calories in = calories out and reducing our experience to such an incomplete equation is both false and harmful. It also fails to explain why the vast majority of people regain lost weight. This is largely due to the mechanisms I listed above.

“Re-Thinking Thin”

The book, “Rethinking Thin” by Gina Kolata, changed my view on weight regulation when I read it over a decade ago. She cited research done by “obesity” researchers back in the 80’s. In brief, participants were fed calorie-controlled diets and were only allowed to move in highly controlled ways. The researchers assumed that this would prove that “obese” individuals simply had a willpower issue. Nope. The results were shocking. Even though calorie levels were the exact same for the participants, they lost, gained, and maintained massively different weights. The light bulbs flicked on and they realized something else must have been happening in the equation of weight regulation. And this explains why humans can consume very similar amounts of food and look differently! We are designed for diversity – height, hair color, skin color, and weight.

If you are looking for an even deeper dive into this topic, I can highly recommend the research of Dr. Michael Rosenbaum, whose talk on “The Metabolic Changes that Occur with Weight Loss,” provided me with up-to-date science when I heard him speak at an eating disorders conference a few years ago. I’ll provide citations below. Ironically, it’s the work of “obesity researchers” that continues to inform my non-weight centered approach to health and well-being.


References to Consider:

My Message Isn’t Sexy

I am acutely aware of how NOT sexy my message is. We can’t control our bodies in ways we’d like to believe – at least not without consequence.

But by engaging in balanced, sustainable, kind, and pleasurable exercise and eating, our body size will sort itself out. Anything beyond that is likely to drive you up a wall.