How We Get Fat

 Energy Intake Exceeds Energy Output

At a fundamental level, fat storage occurs when caloric intake exceeds caloric output.  Now, I know that a lot of people claim that basic thermodynamics don’t hold for humans. Simply, they are wrong.  Invariably, the studies used to support this position are based on a faulty data set: to whit, they are drawing poor conclusions about what people SAY that they are eating.

For example, one popular book bases one of its many incorrect theses on a 1980 report suggesting that the obese ate the same number of calories as the lean.  Ergo, obesity was caused by something else.  The problem is this, the data set is wrong.  A fact we’ve known for nearly 30 years but that the author was somehow unable to become aware of in his ’5 years of dedicated research’.

Study after study after study over the past 30 years shows that the obese systematically under-report their food intake (by up to 30-50%) and over-report their activity (by about the same).  So when they say they are only eating 1800 calories per day, they may be eating 2400-3600 calories per day.  And their activity isn’t nearly what they think.

And when you put those same folks in controlled metabolic ward conditions and control their food intake and/or activity output…voila, the energy balance equation holds.  It’s only when you believe the (incorrect) self-reported data that it doesn’t.

And make no mistake I am NOT saying that the obese are lying about their intake, not consciously anyhow.  Most people simply suck at knowing how much they are actually eating.  Leave them to self-report it and they almost always screw it up.   If you’re mistaken enough to believe the self-reported values, you reach even more screwed up conclusions about things.

In that vein, I have found that the chronically underweight “I can’t gain weight no matter what I do” are invariably vastly over-estimating what they are eating.  That is, they are eating far less than they think.  Other studies show that ‘health conscious people’ tend to under-report their true ‘junk food’ and dietary fat intake; to appear more healthy they conveniently forget or leave out that trip to the burger joint.

Put differently, this isn’t something that only occurs in the obese (so spare me accusations of ‘hating the obese’ or some nonsense).  Am I clear or are people going to misinterpret me some more in the comments and claim I said that fat people lie about their food intake?  Because I’m not saying anything of the sort.  Make no mistake, I’m sure some do lie about it; most are just clueless about how much they are actually eating.

Now let me make it clear that there is obviously a lot more going on here, hormones and all manners of other stuff impact on the energy balance equation.  For example, chronically elevated cortisol does a lot of nasty things in terms of reducing metabolic rate (reducing the energy out side of the equation) as well as negatively impacting on calorie partitioning (where calories go when you eat them).  But for the most part, a lot of that is outside of our control.  It’s relevant but you can’t do much with most of it.  So I’ll focus on calories.

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 Nutrient Intake, Oxidation and Storage 

The primary storage of fat in the body is in fat cells, duh.  Most of that is found in what is called subcutaneous fat, which is found under the skin.  There is also fat stored around the gut area called visceral fat (this surrounds the organs).  Fat can also be stored in ‘bad’ places like the liver and pancreas under certain conditions; this is called ectopic fat storage.

I’m going to focus here on subcutaneous fat.  There, whether or not fat is stored or removed comes down to a concept called fat balance,    You can think of fat balance as the fat specific equivalent of energy balance.  That is

Net Change in Fat Stores = Fat Stored – Fat Burned

I’d note that the same nutrient balance holds for protein, carbohydrates and alcohol (which I’m not going to talk about today).  That is, the net effect on bodily stores, whether protein or carbohydrate stores in the body increases, decreases or stays the same comes down to the balance of protein/carb stored vs. protein or carbs/burned.

So at a fundamental level, fat gain occurs when fat storage exceeds fat burning (technically oxidation).  And fat loss occurs when fat oxidation exceeds fat storage.  I’d note that both processes take place in some amounts throughout the day, controlled by a host of processes I’m not going to talk about.  Just recognize that what happens over time in terms of your fat stores comes down to the relationship between those two processes: fat storage – fat oxidation.

So what determines fat oxidation and fat storage rates?

 Back to Nutrient Intake, Oxidation and Storage

  1. Carbs are rarely converted to fat and stored as such

  2. When you eat more carbs you burn more carbs and less fat; eat less carbs and you burn less carbs and more fat

  3. Protein is basically never going to be converted to fat and stored as such

  4. When you eat more protein, you burn more protein (and by extension, less carbs and less fat); eat less protein and you burn less protein (and by extension, more carbs and more fat)

  5. Ingested dietary fat is primarily stored, eating more of it doesn’t impact on fat oxidation to a significant degree

Let’s work through this backwards.  When you eat dietary fat, it’s primary fate is storage as its intake has very little impact on fat oxidation (and don’t ask me a bunch of questions about “But people say you have to eat fat to burn fat?” in the comments.  That idea is fundamentally wrong but would take an entire article to address).  It also doesn’t impact greatly on the oxidation of the protein or carbohydrates.

Carbohydrates are rarely converted to fat (a process called de novo lipogenesis) under normal dietary conditions. There are exceptions when this occurs.  One is with massive chronic overfeeding of carbs.  I’m talking 700-900 grams of carbs per day for multiple days.  Under those conditions, carbs max out glycogen stores, are in excess of total daily energy requirements and you see the conversion of carbohydrate to fat for storage.  But this is not a normal dietary situation for most people.

A few very stupid studies have shown that glucose INFUSION at levels of 1.5 total daily energy expenditure can cause DNL to occur but this is equally non-physiological.  There is also some evidence that DNL may be increased in individuals with hyperinsulinemia (often secondary to obesity).  There’s one final exception that I’ll use to finish this piece.

But when you eat more carbs, you burn more carbs and burn less fat.  And that’s why even if carbs aren’t directly converted to fat and stored as such, excess carbs can STILL MAKE YOU FAT.  Basically, by inhibiting fat oxidation, excess carbs cause you to store all the fat you’re eating without burning any of it off.  Did you get that?  Let me repeat it again.

Carbs don’t make you fat via direct conversion and storage to fat; but excess carbs can still make you fat by blunting out the normal daily fat oxidation so that all of the fat you’re eating is stored.  Which is why a 500 cal surplus of fat and a 500 cal surplus of carbs can both make you fat; they just do it for different reasons through different mechanisms.  The 500 calories of excess fat is simply stored; the excess 500 calories of carbs ensure that all the fat you’re eating is stored because carb oxidation goes up and fat oxidation goes down.  Got it?  If not, re-read this paragraph until it sinks in.

Oh yeah, the same holds for protein. Protein isn’t going to be converted to and stored as fat.  But eat excess protein and the body will burn more protein for energy (and less carbs and fat).  Which means that the other nutrients have to get stored.  Which means that excess protein can still make you fat, just not by direct conversion.  Rather, it does it by ensuring that the fat you’re eating gets stored.

Of course protein also has the highest thermic effect, more of the incoming calories are burned off.  So excess protein tends to have the least odds of making you fat under any conditions; but excess protein can make you fat.  Just not by direct conversion to fat; rather it’s indirectly by decreasing the oxidation of other nutrients.

Ok, is the above clear enough? Because I can’t really explain it any simpler but will try one last time using bullet points and an example.  Let’s assume someone is eating at exactly maintenance calories.  Neither gaining nor losing fat.  Here’s what happens with excess calories.  Assume that all three conditions represent identical increases in caloric intake, just from each of the different macros.  Here’s what happens mechanistically and why all three still make you fat:

  1. Excess dietary fat is directly stored as fat

  2. Excess dietary carbs increases carb oxidation, impairing fat oxidation; more of your daily fat intake is stored as fat

  3. Excess dietary protein increases protein oxidation, impairing fat oxidation; more of your daily fat intake is stored as fat

Got it?  All three situations make you fat, just through different mechanisms.  Fat is directly stored and carbs and protein cause you to store the fat you’re eating by decreasing fat oxidation.

And I’d note again, since someone will invariably misread this that that doesn’t mean that a low-carb and/or low-protein diet is therefore superior for fat loss.  I’m not saying that and don’t think that I am.  Because in such a situation, while you may be burning more fat, you’re also eating more dietary fat.  So net fat balance can be unchanged despite the dicking around with macronutrient content.  It still comes down to the deficit.

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The Obvious Question: Why Not Just Eat Zero Dietary Fat?

And now I’ll answer the question that I know every person who has read (and hopefully understood) the above is asking: so if carbs and protein are rarely converted to and stored as fat, and make you fat by decreasing fat oxidation and causing all ingested dietary fat to get stored as fat, can’t I eat as much as I want of protein and carbs so long as my dietary fat intake is zero?

And the answer is still no.  Remember how I teased you above with one other exception, when carbs are converted to fat for storage?    That exception is when dietary fat is below about 10% of total daily calories.  Under that condition, the body ramps up de novo lipogenesis.  So you still get fat.

Because the body is usually smarter than we are.  Under conditions where dietary fat intake is ‘adequate’ (meaning 10% of total calories or more), the primary fate of that fat is storage and protein and carbs are used for other things.  And when dietary fat is too low, the body will start converting ingested carbs (and probably protein, though it would still be rare) to fat for storage.

Oh yeah, the other question you’re going to ask in the comments “What about alcohol?”  That’s going to require a full article so be patient.  I know that’s another thing lacking on the Internet but so be it. But I can briefly tell you that alcohol is seen as a poison to you body because it is a poison and that means your liver will shut down normal functions to deal with this invader. Taking your liver “offline” will disrupt metabolic processes that help maintain weight. Also chronic alcohol use will damage your liver.

And I really hope that clears things up.  If it doesn’t, re-read this piece until it is.

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