The Truth About High Fat Diets

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For some strange reason, there has been a pendulum swing from low fat to high fat diets over the last decade. As high fat diets have become popular, there have been many claims made about the use of high fat diets and why they might be the best tool for fat loss. Let me enumerate them here in a short list:

  1. Eating fat makes you burn fat
  2. High fat diets make you burn more calories
  3. A ketogenic state makes you burn the most fat and offers a metabolic advantage
  4. Eating more fat makes your lose more fat since you are using fat for fuel
  5. Carbohydrates are stored more easily than fat.
  6. You eat less when you eat high fat meals
  7. Fat is more satiating than carbohydrates
  8. Eating fat before a meal makes you eat less.

Now as science is a methodology to test hypotheses, and all of these are hypotheses and they have been tested, the data is out there. Let us see what we can learn from the data.

What is a High Fat Diet?

When you follow a high-fat diet, like keto, you’re filling up on fats, which could make up about 60% to 80% of what you eat every day. Instead of getting your energy from carbs, your body starts using fat for fuel—a switch called ketosis.

Some people say this helps you lose weight and gives you more energy. But the truth is, our bodies are pretty complicated. The latest research shows that just because you eat a lot of fat doesn’t mean you’ll lose more weight or feel fuller longer, especially when you compare it to meals with more protein or carbs.

What we thought we knew about high-fat diets is getting a second look, and it turns out there’s more to the story than we first believed.

Does Eating Fat Make You Burn More Fat

The short answer is yes, your body adapts quite quickly and robustly to the types of foods you consume. If you eat a low-carb, high-fat diet your body will start using more fat for fuel (1,2,3,4). If you eat a low-fat, high-carb diet, your body will start using more carbohydrates for fuel. This is incredibly well documented in the literature.  Here are just a few data points and studies showing this is true (1).

Here is the other interesting thing, it appears that it isn’t the “high fat” that makes you burn more fat, it is the lack of carbohydrates. When you consume high fat and high carbs you don’t push the RER as much as if you just restrict carbohydrates.

You also see the same effect if you eat low carb and low fat or if you are fasting (5,6,7). Carbohydrates really appear to be the main “fuel selector” (more on these details in a later article). This really should fundamentally change how you view high fat diets and their mechanisms.

As you consume less carbohydrates and more fat your body WILL start oxidizing more fat for fuel. That is really incontrovertible. However, we need to be very smart with how we interpret this. Using more fat for fuel does not equal more weight loss. This has also been demonstrated in several studies, including the following.

Clearly, more fat oxidation does not equal more weight/fat loss.  More on this topic to follow.

Does a High Fat Diet Make You Burn More Calories

It has been claimed that eating fat makes your body expend more calories due to something about “efficiency” or some weird perturbation of biochemistry.

Honestly, these claims never made any sense to me from a basic physics and chemistry stand point and the fact if you wanted to lose weight you would want to be less “efficient”, but I will entertain the idea nonetheless and see what the data says.

To test this we simply have to look at studies that compare energy expenditure between high-fat and low-fat diets (1,8,9). These have been done dozens of times and there is plenty of data to pour through.

The data are pretty clear at this point that consuming a higher percentage of your overall food intake from dietary fat does not convey any magical metabolic effects that increase overall energy expenditure. In short, no, a high fat diet does not make you burn more calories.

Here are some data to demonstrate the effect of increasing fat intake as a percentage of diet and showing the effect on energy expenditure: nada (10).

Does Ketosis Offer a Metabolic Advantage?

Once we see the data that a high fat diet does not offer any metabolic advantage the next logical question is, does pushing that to the extreme and entering a state of ketosis offer any advantage? If there were any dietary perturbations that one would think would offer a metabolic advantage it would be a ketogenic state as it essentially requires an additional “step” in human metabolism.

The data exists for this question and the answer is quite compellingly no (8,11). In several different populations studied in very rigorous study designs, there is no metabolic advantage one can observe. Here is some data from Kevin Hall’s study showing a ketogenic state does not offer a metabolic advantage nor a benefit to fat loss in a calorie-controlled, protein clamped state, even when ketones are elevated.

Now it IS possible that this effect is so tiny we can’t see it, but then what would be the point, pragmatically speaking.

Does a High-Fat Diet Help You Lose Weight?

The data above show that eating fat does make you oxidize more fat, but does that actually translate into greater overall fat loss since it doesn’t change total energy expenditure?

Well, there is a plethora of data to tease this out. Mechanistic studies show us that high fat diets do not result in more fat loss.

No mechanistic evidence exists to support this idea (See the two studies shown above). The case is pretty much closed on that front.

Now that doesn’t mean in the real world high fat diets might have some other magical property that results in more weight loss and fat loss. This has also been tested, ad nauseam in trials that look at the diets in more real-world settings among diverse populations. I mean seriously, here are just a few studies, along with their data (12, 13, 14, 15).    

Here is a meta-analysis that we discussed in an earlier post (15). 

A roughly 2 pound benefit over 12 months, that is almost meaningless when we are talking about the goal of diets for weight loss, especially when you hold the data juxtapose to data showing adherence effects weight loss at much more drastic scales (pun intended).

Below you can see how high adherence results in 10-20 pounds more lost over 12 months, where as diet composition had zero effect on weight loss.

The data is just crystal clear, there is no meaningful benefit to high fat diets for fat loss. We can probably stop spending our dwindling science dollars funding them**.

Are Carbohydrates Stored More Easily Than Fat?

The claim is often made that when consumed in excess carbohydrates convert to fat and are stored through De Novo Lipogenesis. Yes, that is a big, cool, fancy sounding word, but it doesn’t really mean all that much in humans.

A lot of people cite rodent studies showing that excessive carbohydrate intake results in the creation of lots of fat. Rodents have VERY different liver metabolism than humans and the human capacity for De Novo Lipogensis is a fraction of rodents.

In fact, one of my favorite papers on the subject is titled, “No common energy currency: de novo lipogenesis as the road less traveled”. In this seminal editorial, Hellerstein opens with the lines, “Bees make wax (lipid) from honey (carbohydrate). Pigs fatten on a grain diet. Indeed, all organisms, from bacteria to mammals, have the enzymes of de novo lipogenesis.

The physiologic function of de novo lipogenesis has therefore seemed obvious to biochemists: the de novo lipogenesis pathway links carbohydrates and fats, the 2 most important forms of chemical energy for most organisms.

Because storage of energy as lipid is much more efficient than storage as carbohydrate, the presumption has been that animals use de novo lipogenesis as a metabolic safety valve for storage of carbohydrate energy present in excess of carbohydrate oxidative needs (ie, carbohydrate energy surplus). On the basis of this presumed role, inhibitors of de novo lipogenesis [such as (–)hydroxycitrate, an inhibitor of ATP citrate (pro-S)-lyase] have received attention as potential therapeutic agents for obesity and hyperlipidemia.

Most experimental data in humans, however, contradict this view of the function of de novo lipogenesis. Initial studies in which indirect calorimetry was used showed little or no net de novo lipogenesis after short-term carbohydrate overfeeding (16). Subsequent isotopic studies confirmed the absence of quantitatively significant flux through hepatic de novo lipogenesis under most conditions of carbohydrate energy surplus (17,18).”

Additionally, in that same editorial he contextualizes one of the best carbohydrate overfeeding studies where they examined how much fat is “created” from consuming large amounts of carbohydrate, “Additionally, McDevitt et al  (19) report that, in all settings, the total de novo lipogenesis flux represented a small fraction of both the surplus carbohydrate energy ingested and the total fat stored in the body.

The authors calculated that between 3 and 8 g fat/d was produced through de novo lipogenesis compared with 360–390 g carbohydrate ingested/d and 60–75 g body fat stored/d. Thus, the addition of excess carbohydrate energy to a mixed diet so that total energy intake exceeded total energy expenditure (TEE) increased body fat stores, but not by conversion of the carbohydrate to fat. Instead, the oxidation of dietary fat was suppressed and fat storage thereby increased.”

When it comes to de novo lipogenesis in humans the data tells us it is largely much ado about nothing. Here are some data showing de novo lipogenesis under eucaloric and hyper caloric settings in lean and obese people. However even despite the higher rates of conversion seen in both the McDevitt study above, and this data, it is abundantly clear that that fat is then oxidize and only a TINY percent contributes to overall energy balance.

Storage “Efficiency”

Another argument about high fat diets is that fat is burned when consumed so it is not stored as efficiently as carbohydrate. This claim also makes zero sense when you think about it. Dietary fat, when consumed, is either utilized for energy or stored directly in its “native” state as a fatty acid molecule.

Dietary carbohydrates are either utilized for fuel, stored as muscle glycogen, or converted into fatty acids and then stored. There are more “checkpoints” and “conversions” that must occur, and each biochemical conversion requires energy to do so.

In theory, storing carbohydrates should be less efficient than storing fat.  Here are the relevant biochemistry pathways just to show your the giant cluster that is DNL.*

This is exactly what the data tells us (Table found in 10, adapted from Blaxter 1989).

Dietary fat is stored as body fat with roughly 96% efficiency. Dietary carbohydrate is stored as body fat with roughly 80% efficiency, while it is stored as muscle glycogen with roughly 95% efficiency. It is quite clear that dietary fat is stored far more efficiently as body fat than carbohydrate.

We can say with about 95.8% confidence that diets high in fat intake are more efficient at storing food calories as fat than diets high in carbohydrate intake.

Does A High Fat Diet Make You Less Hungry?

In studies that do not strictly clamp calories, you see a dose-dependent relationship between the percentage of dietary fat intake and overall caloric intake. Essentially, as you increase the percentage of your diet from dietary fat your total calorie intake goes up, not down.

This is the opposite of the hypothesis that high fat diets are naturally more satiating and you eat less without counting calories (Figures taken from 10, adapted from Lissner et al. 1987 and Stubbs and Prentice, 1993; Stubbs et al., 1995a, b). 

In some really cool studies where they “covertly” changed the percentage of dietary fat intake in free-living situations and in metabolic ward studies this also holds true, meaning it isn’t some purely  “psychological” effect of knowing you have higher fat content in your food.

In fact, when people consumed 20 or 40% of their daily energy from fat they actually consumed less calories per day in free-living situations but consumed more calories once they hit the 60% mark. 

Now there is some data to show the opposite, but I think the cumulative data show it isn’t as simple as, “high fat diets make you eat less”.

Does Eating Fat Before a Meal Make You Eat Less?

When I was younger Men’s Health was my bible. Not kidding, that was what got me into the whole nutrition game in the first place. I’ve even emailed their editors trying to convince them to let me write for them, still no luck (MH editors, if you read this and are feeling charitable email me [email protected]. Seriously, one of my life long dreams is to see an article of mine in there. Make a brother’s dream come true!). 

I remember very vividly an article, well one of their little sidebar articles, stating that eating a serving of fat 15 minutes before a meal can make you feel full and eat less during the meal so you can lose more weight. 

You know I tried that little trick. I don’t remember exactly what happened that night but I still have the idea seared into my mind. This is something that has been a cornerstone of nutrition circles for decades, but is it true?

There are several studies where they give people dietary fat or dietary carbohydrates prior to a meal and see if this makes them eat less food at that meal and throughout the rest of the day.

When you look at that data (see below) it doesn’t appear that fat intake prior to a meal lowers the intake at that meal and may actually contribute to higher calorie intake the rest of the day (sorry Men’s Health, we might need to update that one).

Is Fat more Satiating Than Carbohydrate?

I saved the best for last!

If I had a nickel for every time I heard someone say that fat is more satiating than carbohydrate I would be building an Iron Man suit with my billions (what else do you do with a billion dollars?).  Now, to be fair I have also uttered this a lot in my life, for years it was dogma, a known fact. Fortunately, there is data we can use to change our minds on topics.

To start, when you look at the satiety index fat intake does not appear to be positively correlated with satiety, it is actually negatively correlated as assessed by Holt et al. in 1992 (20). Opposite of what most people might think, the humble white potato, which has virtually no fat and is all starch, has the greatest “satiety score”. This also matches well with much of the high carbohydrate vs high fat meal data shown throughout this article. 

Another study actually examined the individual macronutrients and it appears that fat may actually be the least satiating macronutrient in terms of its impact on satiety over the course of roughly 3 hours.

The Wrap Up

So let us sum up those initial statements and evaluate their “trueness” and “falseness”

  1. Eating fat makes you burn fat: True
  2. High fat diets make you burn more calories: False
  3. A ketogenic state makes you burn the most fat and offers a metabolic advantage: False
  4. Eating more fat makes you lose more fat since you are using fat for fuel: False
  5. Carbohydrates are stored more easily than fat: False
  6. You eat less when you eat high fat meals: False
  7. Fat is more satiating than carbohydrates: False
  8. Eating fat before a meal makes you eat less: False
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