A while back, I did a Q&A about excess dietary protein and whether or not it could lead to fat storage. The short answer was that, while the biochemical pathway exists, the likelihood of it ever happening are roughly zero. For some reason, despite my answer being written in what I felt was a clear way, many people seemed confused. Among other silly inferences, people somehow heard me saying that overeating carbs or protein can’t make someone fat. And that was not true. So to clear it up, let me look at the simple issue of how we get fat.
.At a fundamental level, fat storage occurs when caloric intake exceeds caloric output and there is a surplus energy balance. 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, they concluded, obesity was caused by something else. The problem is this, the data set is wrong. This is a fact that we’ve known for 30 years but that the author was somehow unable to unearth in his “5 years of dedicated research”.
Study after study after study over the past 30 years shows people systematically under-report their food intake, by as much as 30-50%. They also over-report their activity by about the same degree. This means that they will say they are eating 1800 calories when they are really eating 2400 or more. They overestimate the calorie expenditure of activity to as much if not a greater degree. They think they are burning 900 calories when it’s closer to 300.
Because when you put those same people in a metabolic ward and control their food intake and activity, the energy balance always holds. Always. It’s only when you take self-reported data at face value that it doesn’t.
And make no mistake I am NOT saying that people are consciously or knowingly lying about their food intake. Most people simply suck at knowing how much they are actually eating. Lean people do it, obese people do it, active people do it.
Hell, even registered dieticians, who should be good at this, do it. Self-reported food intakes are simply awful and wrong (which is a big part of why nutritional epidemiology is crap). If you’re mistaken enough to believe the self-reported values, you reach even more screwed up conclusions about things. Just like Gary Taubes did.
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. That is, to appear healthier than they are, they selectively omit the burger and fries. Honestly, if anybody is lying, it’s these folks. Everybody else is just bad at this.
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, which I discuss in some detail in The Ultimate Diet 2.0. 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
The same nutrient balance concept holds for protein, carbohydrate and alcohol (which I will not discuss 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. Please realize 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?
Individual Nutrient Metabolism
And this is where people got confused in my previous Q&A about excess protein intakes. And where things can get drastically unconfused if you’ll read this reasonably short piece on how nutrients are oxidized or stored in the body. For those who didn’t read the linked article, I’ll summarize the key points below:
- Ingested dietary fat is primarily stored, eating more of it doesn’t impact on fat oxidation to a significant degree.
- Carbs are rarely converted to fat and stored as such.
- When you eat more carbs you burn more carbs and less fat. Eat less carbs and you burn less carbs and more fat.
- Protein is basically never going to be converted to fat and stored as such.
- 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).
Ok. Let’s work through this one at a time.
When you eat dietary fat, it’s primary fate is storage as its intake has very little impact on fat oxidation. A question that invariably comes out of this is whether or not people have to eat fat to burn fat. And the short answer to that question is no. Eating more fat does not increase the body’s use of fat for fuel under most conditions. Additionally, dietary fat intake also does not impact much on the use of protein or carbohydrate for fuel.
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 by and large the conversion of carbohydrates to fat for storage is not a major pathway in humans. However, this doens’t mean that carbohydrate can’t contribute to fat gain. Because when you eat more carbohydrates you burn more carbohydrates and less fat. If fat burning is decreased, more of the fat that you are eating can then be stored as fat. So the effect is indirect and I want to repeat it to make sure it’s clear.
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.
The same holds true for protein which is effectively never converted to and stored as fat. When you 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.
How We Get Fat
I really want to make sure the above is clear before going forwards so let me summarize it even further.
Let’s assume someone is eating at exactly maintenance calories so they are neither gaining nor losing fat. Now they create a 500 calorie per day surplus.
Let’s look at what happens physiologically based on where those extra calories are coming from. That is, mechanistically why all three can end up making you gain fat.
- Excess dietary fat is directly stored as fat.
- Excess dietary carbs increases carb oxidation, impairing fat oxidation so more of your daily fat intake is stored as fat.
- Excess dietary protein increases protein oxidation, impairing fat oxidation so 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 at the end of the day and variations in carbohydrate and fat intake don’t tend to make much of a difference here.
Which also doesn’t mean that the choice of diet doesn’t matter at all. It can depending on the context and I’ve done a comparison of the diets elsewhere on the site.
Why Not Just Eat Zero Dietary Fat?
Hopefully all of the above makes sense. Basically, excess intake of all three nutrients can cause fat gain, just for different reasons. Fat is stored directly and both carbohydrate and protein intake decrease fat oxidation so more fat is stored.
Which raises a fairly obvious question: If carbs and protein are rarely converted to and stored as fat, but can cause fat gain indirectly, wouldn’t a high-carb, high-protein zero fat diet cause zero fat gain?
And the answer is still no. I mentioned above that DNL does not happen in humans under most situations. Severe carbohydrate overfeeding can cause it but that tends to be rare. But one time when DNL is upregulated in humans is when dietary fat intake falls below 10% of total daily calories. Under that condition, carbohydrates can and are converted to fat for storage. You’ll still gain fat.
Because the body is ultimately much smarter than we are. When dietary fat intake is adequate (i.e. 10% of total calories or more), the primate fate of dietary fat is storage and protein and carbohydrates are used for other things. And when dietary fat drops to an extremely low level the body will start converting carbohydrates to fat for storage. It might even start converting protein to fat although I suspect that pathway is still pretty damn inefficient.
What About Alcohol?
And of course people are wondering about alcohol. Well alcohol is complicated and has to wait for another article.
But now you know how we get fat.