The "Begin Rust" book

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I've been giving this topic a lot of thought over the past few weeks. I decided to coalesce my thoughts into this post. I'll start off with a pretty significant caveat. Each theory I'm putting down below is almost certainly espoused by somebody. But there are too many different people with different theories to in any meaningful way say "vegans believe X," for instance. Instead, consider this a high level lay of the land for these theories, and realize that there are nuances that many people will make.

In other words, this isn't even my opinion. What I'm putting below is almost a caricature of a bunch of different opinions. So take it with a grain of salt. I'll end off with my own, current-but-evolving opinion.

OK, with that out of the way, let's state a clear question: why do people become obese? There are multiple variations of this question that play in too:

  • Why are people obese more often today than, say, in the 1970s, at least in the western world?
  • Why are more people suffering from metabolic disorders like diabetes, chronic conditions like heart disease and cancer, and so on, at seemingly higher rates than previously?

Calories in, calories out

This theory of obesity is arguably the simplest. At the very least, it's the one we all tend to learn from an early age, so it's most familiar. The theory is pretty simple. We consume a certain number of calories, that's called calories in (CI). That's all the food you put into your mouth. Then you have calories out (CO), which is all of the energy you expend. This can be through normal processes your body goes through to keep you alive, via explicit exercise, or even fidgeting while you're sitting at your desk reading this blog post. In theory, if you could live inside some kind of a measurement chamber, calories out would be the total amount of heat you produce through everything you do.

So why do people become obese? Because CI > CO. If you eat more than you use, your body stores the extra calories. Then you gain weight. And if you do that for long enough, constantly staying in a caloric surplus, you become overweight, and then obese.

How much does a calorie weigh?

The first problem I want to point out about CICO is a pedantic one. This way of framing the theory makes it sound like calories weigh something. And now that I have more calories in my body, I weigh more. But (ignoring some physicist who's about to yell at me) energy doesn't actually weigh anything! A calorie doesn't have mass. When we talk about "gaining weight" from "calories," what we're actually talking about is storing additional fat (or glycogen), which can later be burned to release energy.

Alright, fair enough, that seems like a little bit of a distinction without a difference. But it does have an important outcome. You never have extra "energy" circulating around your body. You have specific kinds of matter. It may be surplus glucose or triglycerides in your blood. It may be overly filled glycogen stores in your liver. It may be too many visceral fat cells. Or overloaded fat cells. Or subcutaneous fat. And all of these behave differently.

OK, with that tangent addressed, let's get more substantive.

CICO isn't a theory

As I've defined CICO above, it seems like a coherent theory with strong predictive power. In my experience, most of the disagreements and confusion come from the fact that that's not true. For example, let me state some strawman outcomes from the framing of CICO above:

  1. If I measure my calories out for a week, then eat less than that, I will lose weight
  2. If I'm weight stable, and I start eating more food, I will gain weight
  3. If I'm weight stable, and I continue eating the same amount of food, I will stay the same weight

Those statements may seem valid, but they're definitely not. First off, there's too much ambiguity in other variables. In other words, I didn't state ceteris paribus, or "other things equal." For example, in (3), if I continue eating the same amount of food, but increase my exercise to burn an extra 1,000 calories a day, most people would expect me to lose weight.

But now let's throw in a ceteris paribus. Other things equal. What do we mean by other things? Let's say I've clocked in eating 2,500 calories a day for the past month, and my weight has been stable. I then reduce my caloric intake to 2,000 calories a day. I continue all normal daily activities, with zero modifications. Ceteris paribus, right? We've now reduced calories in, kept calories out locked, and therefore CICO predicts weight loss.

Except that isn't true either! Keep in mind that calories out isn't simply "the total amount of exercise you do." It's based on metabolic processes, and fidgeting, and heat generation. My previous paragraph made an implicit, and perhaps wrong, assumption: I can change calories in and, by controlling all other variables in my life, leave calories out unchanged.

And here we come to the primary objection to the CICO model: calories in and calories out are not independent variables. And in fact, almost everyone agrees to this fact. If you massively decrease the amount of food you eat, you'll ultimately enter starvation mode and expend less energy. If you increase your food consumption, your body will seek methods of burning off—instead of storing—extra energy.

I don't remember where I first read this, but CICO is better described as a definition than as a theory. It is not predictive of outcomes. Instead, it defines three terms and a relationship among them. Specifically: calories in - calories out = energy delta. If you consume 2,500 calories, and burn 3,000 calories, your energy delta is defined as -500 calories. If those calories are taken from storied body fat, you'll lose approximately 56 grams of fat.

However, as soon as you move beyond CICO as a definition or a tautology into the realm of diet advice, then you begin to enter a true realm of disagreement.

Quick recap, then the theories

Alright, so that's a lot of theoretical nonsense, right? Let's recap, and then explore what some of the various theories are around becoming obese.

  • Obesity comes from gaining weight as fat
  • You gain fat by storing consumed calories (energy) as body fat
  • The amount of energy you store is defined as calories in - calories out
  • If you could reduce calories in without affecting calories out, you would lose weight (or gain weight more slowly)
  • However, as mentioned above and detailed below, these two variables may be intricately linked
  • And therefore, changing one without changing the other may be easier said than done

OK, let's move on!

Eat less, move more

Also known as: sloth and gluttony. I think for most people, this slogan is essentially synonymous with CICO. And this is where an unfair sleight of hand has been pulled. As I've said, most people agree with CICO as a definition. But the "eat less, move more" as a prescription is where they begin to differ.

Eat less, move more has been the basis of common medical advice around obesity for a long time. Consume less calories, fight off the cravings, exercise more, overcome the lethargy. And you will lose weight. There are a number of objections to this:

  • Statistically, it seems to not be true. This advice has been pushed for decades, and obesity has gotten worse. Arguably this is due to non-compliance (because of sloth and gluttony). The counter to that argument is that it doesn't matter. For whatever reason, this advice hasn't produced the desired outcome at a societal level, and therefore it doesn't work. (And a counter to that would be: sure, it doesn't work for everyone, but it might work for me, because I am less slothful and less gluttonous.)
  • This slogan fails to explain why obesity has gotten worse. Have human beings suddenly become more lazy and gluttonous? Maybe. That's a big claim to make. As we'll see, there are theories that are mostly in line with the "eat less, move more" philosophy that give some explanations.
  • Taken to an extreme, "eat less, move more" leads to "a calorie is a calorie." It doesn't matter if I'm eating 400 calories of white bread, 400 calories of butter, or 400 calories of high fructose corn syrup. It's all the same to the body. This is patently, and demonstrably, false. You can test it yourself, if you have the will power. Spend a week doing nothing but guzzling 2,000 calories a day of heavy whipping cream and sticks of butter. See what happens on the scale. Then spend a week eating nothing but 2,000 calories a day of broccoli. What happened on the scale that week? If you believe your body fat stores will respond to those situations differently, then you probably don't believe all calories are created equal.

Why we eat more?

What if the choice of the food we eat impacts how much food we eat? If you were bothered by my last bullet point, you're probably already headed in that direction. With great ease, I could consume 4,000 calories a day of my favorite brand and flavor of ice cream. That combination of sugar and fat, with those delicious and unique flavors, and the incredible density of energy in that ice cream, would make those calories disappear in an instant.

4,000 calories of broccoli is another story. And I like broccoli. It's just difficult to imagine consuming that much of it. Broccoli is energy sparse. It has high water content. It has fiber. And it just doesn't have the same appeal as a bowl of ice cream.

So let's move beyond "a calorie is a calorie." No, not all calories are created equal! Different food sources encourage different eating behaviors. Some kinds of food cause satiety. Some kinds of food encourage ravenous hunger.

Stephan Guyenet almost certainly falls into this category with The Hungry Brain, which I've read, enjoyed, and still follow some advice from. That book analyzes hormonal signals that may trigger our brain to want to overeat. There's a lot to the theory, but to massively simplify it: our brain was built to seek carbs and fats as scarce energy resources. And in nature, you'll find them. But modern processed food slams these together into things like donuts that completely overpower our natural satiety mechanisms.

Ted Naiman and William Shewfelt take a similar kind of approach in The P:E Diet. I'm not even sure if the two camps disagree on things, or if the former is more theoretical with the latter being more instructive. I'd love to hear a discussion between Guyenet and Naiman. Essentially, in The P:E Diet, the problem is that our modern diet has insufficient protein. (Fiber and micronutrients get thrown in too, but if you follow Naiman on Twitter, protein definitely takes center stage.)

So to sum up: It's still about Calories In/Calories out. We need to reduce Calories In (and increase calories out, though that's secondary). But instead of using sheer will power to overcome gluttony, we should make food choices that encourage satiety to make it easier on ourselves.


There are lots of arguments about the carbohydrate-insulin theory of obesity. I'm not talking about that (yet). Instead, I'm talking about something much simpler. Take a person who burns 2,000 calories a day. Give them a diet of 2,000 calories a day. Confirm over a number of weeks that they do not gain any weight. (If necessary, adjust caloric intake.) Awesome.

Now begin injecting them with insulin every time they eat. Do not allow them to eat more food or exercise less. They're going to gain weight. How is that possible? Isn't that a violation of CICO?

No, not at all. A complex series of hormonal responses will occur inside the subject's body. Insulin will encourage energy storage. Less total available energy in the bloodstream will encourage other body systems to reduce metabolism. Or some other series of events that will ultimately lead to reduced energy expenditure. Calories out go down, therefore energy delta increases, therefore weight gain.

There's likely some disagreement on the details I provided above. I did a quick search, and didn't find any specific studies demonstrating the isocaloric effect I just described. (If someone has a study on it, please send it to me, I'll update the post.) Maybe injecting insulin encourages people to eat more. But it's a well known phenomenon among diabetics that insulin injections lead to weight gain.

Still don't believe me? OK, take a type 1 diabetic, who produces next to no insulin. Without insulin injections, they will hit diabetic ketoacidosis and burn through massive amounts of energy stores.

Point being: weight gain and loss cannot be exclusively explained by food consumption and exercise. Hormonal factors do have an impact, at least in extreme circumstances.

Many theories of obesity within the low-carb world revolve around the idea of regular insulin secretion in the body leading to insulin resistance, leading to increased insulin production, leading to increased obesity. They would explain the insulin as leading to increased caloric intake and decreased expenditure. The root cause of this may be fructose, or carbs in general, or some other mechanism.

According to these theories, the best way to combat obesity would be to reduce or remove foods that trigger insulin production. This will allow the body to become insulin sensitive again. And you'll no longer be battling against a hormone trying to make you fat. Approaches to this would include:

  • Intermittent or multiday fasting
  • Low carb diets, or more extreme: ketogenic or zero-carb diets

Fat versus sugar burner

This approach overlaps the previous one significantly, but comes with a different mechanism. The theory goes that our cells have two primary fuel sources: fat and glucose. Glucose is the default fuel source for various reasons. If it's available, it will "crowd out" the fat. And burning fat is slower, requiring more mitochondria. If you constantly have a stream of fat and glucose entering your cells, your cells will prefer burning the glucose, slowly burn the fat, not produce enough mitochondria, and end up storing extra fat.

Instead, if you remove the glucose source, your cells will be forced to produce more mitochondria to meet the energy demands based on fat alone. This will turn you into fat burning mode, and your body will quite happily burn up stored body fat. You'll feel less cravings for between-meal snacks, because you're easily getting by on your energy reserves. And you may even increase your overall metabolism, leading to increased calories out.

The same approaches from the previous section apply here.

Linoleic acid

This theory is relatively newer, at least to my reading. And it's complicated enough in its mechanisms that, if you're interested, you should definitely go read up on it at Fire in a Bottle. As a very high level, imprecise summary:

  • Linoleic acid, an omega-6 fatty acid, is an essential nutrient (we die without it)
  • However, we don't need much of it, and historically have received small amounts of it
  • Since the American Civil War, industrial seed oils (cottonseed, corn, soybean, etc) have been increasing in usage, with a massive uptick in the 1970s
  • These oils are unnaturally high (for our diet) in linoleic acid
  • In large concentrations in our bodies, linoleic acid causes metabolic disfunction

The Croissant Diet follows this theory. I will say up front that I have not been successful with my attempts at this, but that may have been my own error. The Croissant Diet provides the very alluring promise of getting to each starch, sugar, and refined saturated fats (e.g., butter), all while fixing metabolic disfunction and losing weight.

Food quality

The final one is really a catch-all basket. "Food quality" is a broad term. Lots of people have lots of definitions of what makes food "good" and "bad." For example:

  • Vegans say that animal food is bad and plant food is good
  • Carnivores say that plant food is bad and animal food is good
  • Paleo diet fans say that food invented in the past ~10,000 years is bad, older food is good

And so on. Each of these theories espouse some claims of what makes the bad food bad, and why it leads to various diseases. Typically, to some extent, the claims will end up in one of the above buckets, at least as far as obesity is concerned. For example, in Primal lore, overconsuming starch that you haven't "earned" (through high intensity workouts) will lead to fat accumulation, likely through the action of insulin. Many vegan advocates will point out the high proportion of saturated fat in animal products to explain weight gain.

This section may sound like I'm dismissive of these claims. I'm not. I think there's a lot to the idea of food quality. It's just that the term is so broad, and the mechanisms so varied, that it's hard to nail anything down.

My thoughts

Up until now, my point was to give you the lay of the land. Hopefully now, the next time you see a debate going on between "low carb plant based" versus "carnivore" versus "high saturated fat," you'll have some concept of what the underlying mechanisms are they're arguing about. Or maybe not; there may be some brand new mechanism people have come up with!

For myself, I base my decisions on three things:

  • Personal experience: what's worked, what hasn't. This is definitely an n=1 kind of experiment, but since I'm advocating for myself and just sharing that information, I feel comfortable with it.
  • Scientific studies, especially statistical analyses of changes over time in diet versus health outcomes. And take all of it with a huge grain of salt, much of the scientific literature is contradictory and, in my opinion, coming from predetermined conclusions.
  • An overall belief that "ancestral diets," whatever that may mean, are likely best. I believe we evolved towards certain kinds of eating, and second-guessing that may be difficult. I'm not quite at the point of believing that we can't improve our diet through science and technology at all. I just believe it's difficult to surpass millions of years of evolution, and we don't have the full story yet.

I like root cause analyses. I would love to say "this one thing caused obesity." I've tried acting that way previously, and my own diet outcomes make me think it's wrong.

So I believe there's a mixture of all these things. I believe overeating is a major cause of obesity. I've seen it in myself. Even when I stick to "healthy" (by some definition) foods, I have a natural tendency to overeat. Simply eliminating sugar and seed oils, and even carbs, isn't enough to help me lose weight. I've needed to use techniques to help me control my hunger as well.

I also don't believe it's all about the calories. I think there's ample evidence that different kinds of food cause different behavior and outcomes in the body.

In the past, I've had the most success at weight loss with massive calorie restricted plant based diets, extreme keto, calorie counting, multiday fasting, and carnivore. For weight maintenance, things have been much easier. Almost any diet that gets me off of sugar and seed oils, and more generally off of processed foods, lets me stay weight neutral.

And that's really the final and most easily repeated advice: if you're going to do only one thing, stop eating processed foods. It's basically the one piece of advice everyone seems to agree on.

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