Calories in vs. calories out. You’ve heard it a thousand times, maybe rolled your eyes at it, maybe dismissed it because some influencer told you their diet “breaks the rules of thermodynamics.”

Here’s the thing: no diet breaks the rules of thermodynamics. And I have $3,000 and 60 days of data to prove it.

The Experiment

I’m a registered dietitian. I also react to fitness content — and one clip from Sam Selleck stopped me cold. His take: if you eat 2,000 calories of Skittles and Cinnamon Toast Crunch, you’ll lose weight. Eat 3,000 calories of chicken and rice, you gain weight. The diet doesn’t matter. Calories do.

He’s right. But most people hear that and still don’t believe it, because the diet industry has spent decades selling you on the idea that certain foods are magic and others are poison.

So I decided to test it on myself — properly. Not a casual experiment. A 60-day, lab-tested, DEXA-scanned, blood-work-documented experiment that I’m currently writing up as a case study for publication.

Here’s what I did:

30 days of whole foods: balanced macros, variety of plants and proteins, standard healthy eating approach.

30 days of carnivore: exclusively animal-based foods — meat, eggs, cheese, cream.

Both phases were locked to the same caloric intake (2,000–2,100 calories per day) and the same protein target (170–180 grams per day). The nature of the foods was completely different. Everything else was controlled.

What the Numbers Showed

On whole foods, my body fat went from 16.3% to 16.1% — a 0.2% drop. My weight went from 179 lbs to 172 lbs.

On carnivore, starting from 15.8% body fat, I dropped to 15.3% — a 0.3% drop. Weight went from 179 lbs to 169 lbs.

The difference between those two phases? 0.1%. Essentially nothing.

When calories and protein were identical, it didn’t matter whether I was eating leafy greens or ribeyes. Fat loss tracked the same. The diet was irrelevant. The calories were the point.

Why People Still Fail on “Good” Diets

Here’s where it gets more complicated — and where I part ways a little with the pure “calories only” crowd.

Calories are the mechanism of weight loss. That’s not up for debate. But maintaining a calorie deficit is where most people fall apart, and that’s a real, legitimate challenge.

Genetics can make some people chronically hungrier than others. Medical conditions like diabetes make calorie control harder. Emotional eating derails even the best intentions. Low activity levels make a meaningful deficit difficult to create. Poor food quality keeps you hungry even when you’re technically “eating enough.”

This is why diets that seem to “work better” often do. It’s not that carnivore or keto or paleo magically rewrites your metabolism. It’s that restricting entire food categories creates natural guardrails. During my carnivore phase, I had maybe 10 foods available to me. The fat kept me fuller longer. I wasn’t thinking about food constantly. The deficit stayed in place — not because of the diet, but because the diet made the deficit easier to maintain.

GLP-1 medications like Ozempic and Tirzepatide work by the same logic at a much stronger level. They suppress appetite so aggressively that people eat less without trying. Less food, calorie deficit, weight loss. Still thermodynamics.

The Better Question to Ask

Most people approach weight loss by asking “what diet should I do?” That’s the wrong question.

The right question is: how do I manage my hunger well enough that staying in a calorie deficit actually becomes possible for me?

A major randomized controlled trial backs this up. Participants eating widely different macronutrient breakdowns lost similar amounts of weight when total calorie intake was controlled. Yes, higher protein and lower fat diets show slight advantages in some controlled feeding studies. But those advantages are small compared to what consistent adherence to a calorie target produces over time.

Long term, it comes down to which approach you can actually stick with.

What This Means for You

You don’t need to eat 2,000 calories of donuts to prove a point. And you don’t need carnivore or keto or any other restrictive framework to lose weight.

What you need is an approach that:

Creates a calorie deficit you can realistically maintain

Includes mostly whole, minimally processed foods (they manage hunger far better than processed ones)

Fits into your actual life — not a version of your life where everything goes perfectly

Doesn’t require you to eliminate entire food groups permanently

Thermodynamics doesn’t care about your feelings. But sustainability does — and sustainability is the only thing that makes thermodynamics work in your favor over the long run.

Want to see the full breakdown of the 60-day experiment — blood work, DEXA scan results, and everything else? Check out the full video series on my channel.