I like reading Mark Sisson’s articles and follow his blog, Mark’s Daily Apple, because he provides an alternative and mostly well-reasoned perspective. For those who don’t know, Mark Sission advocates for a Primal diet, which is loosely based off the Paleo diet, and is author of The Primal Blueprint.

I came across a post of his again from somebody claiming a high fat diet to be the best, titled Top 7 Most Common Reactions to Your High-Fat Diet (and How to Respond). While it does illustrate some good points, and may serve useful to those following (and defending) a high-fat diet, it also presents some fallacies that I’d like to address.

The article begins by addressing the demonization of fat, which for the most part I agree with – fat has unjustly become an enemy to some people, but I also don’t think fat should be considered a friend either.

“Isn’t all that fat gonna glom onto your arteries?”

“My arteries are not pipes. Fat is not solidifying in my blood like it can in the plumbing. Atherosclerosis is a complex process with dozens of factors beyond what’s in your diet, let alone the fat content.”

Atherosclerosis is a complex process and is dramatically oversimplified by many – a major concern that I too share. Fat doesn’t solidify in your blood. I don’t really have any problems with this section as it reasonably addresses a common myth. That’s not to say that intake of certain fats doesn’t contribute harm to arteries, just that the process is more complex than this blanket statement.

“Isn’t all that fat gonna make you fat?”

“No. Eating a high-fat, low-carb diet is the easiest way to inadvertently eat less without sacrificing satiation or satisfaction. It also improves your ability to access stored body fat rather than lean mass, which is helpful for fat loss.”

The idea of fat making you fat is overblown and exaggerated. Fat doesn’t make you fat any more than sugar makes you fat. The biggest culprit to weight gain is the combination of refined carbohydrates and refined fat. Eating refined carbohydrates results in a rapid rise in blood sugar, calling forth insulin to regulate. Fat blocks the storage of glycogen, meaning the refined carbohydrates can’t be stored in the muscles. With all these calories and lots of insulin present, what do you think happens? You get some more fat to wear.

Dr McDougall is well-known for promoting the line, “The fat you eat is the fat you wear,” but I think this line is generally promoted out of context. The macronutrient fat can easily be stored as fat on your body, and when a portion of your fat (e.g. belly fat that’s cut off) is chemically analyzed the source of fat can easily be determined. For example, if you’re overweight and eat lots of fish then this can easily be determined by analyzing your fat – you’ll have fish fat on your body. Or maybe you eat lots of olive oil, so the fat on your body will match the same composition of olive oil. This storage however will only occur from an overabundant intake of calories. If you eat lots of calories and you eat lots of fat, the fat that you eat will literally, chemically speaking, be the fat you wear.

You can become overweight if you eat lots of calories with very little fat too, but obviously if you’re eating very low fat then you’re not going to find much of that fat on your body. If you overconsume carbohydrates then sugars can convert to fat that you wear through a process called de novo lipogenesis – though this is a more difficult and inefficient process.

Simply put: calories make you fat when you eat too much of them.

So technically, the statement, “Fat doesn’t make you fat,” is true. Mark also talks a little about overeating fat resulting in weight gain and how it’s difficult to overeat fat: “It’s difficult to overeat on a high-fat, low-carb diet.” This may be true, but it’s also difficult to overeat on a low-fat, high-carb diet. A large serving of potatoes and vegetables “is far more filling than some crusty bread spread with butter” too.

Carbohydrate is the body’s first source of energy, with fat being a back up and protein being a last resort. When the body is in a state of ketosis – induced through starvation or a low-carb diet – then fat becomes the body’s primary energy source. Hence, as Mark states, “Dietary fat in the presence of large amounts of dietary carbohydrates can make it difficult to access fat for energy, while dietary fat in the presence of low levels of dietary carbohydrates makes it easier to access fat for energy.” That’s not to say that fat cannot be used as energy – the body will turn to fat for energy when needed after carbohydrates are used up. Does any of this really matter? Not really. Eat food and it’ll eventually (if it’s digested) be used as energy or stored as fat. Create a caloric deficit and body fat will be burned for energy.

Fat and carbohydrates can both be easy to overconsume in refined forms. Oil is a refined form of fat, and table sugar (sucrose) is a refined form of carbohydrate. It’s more easy to overconsume these refined foods because they don’t provide as much satiety as whole foods, like starches (high sources of carbohydrates) or nuts (high sources of fat).

In fact, studies have shown that low-carb, high-fat diets not only reduce weight, they also retain or even increase lean mass. That means it’s fat that’s being lost (rather than the nebulous “weight”), which is what we’re ultimately after.

The study referenced here illustrates large differences in protein intakes, which likely was more a contributing factor than the carbohydrate restriction itself. A number of factors can affect muscle loss and losing weight usually entails at least some muscle loss if not only due to reduced need to carry excess fat (unless one is continuously working out while losing weight, hence also creating more muscle). This is only applicable to people looking to lose weight.

“But Dean Ornish/my mom/Walter Willet/the AHA/my doctor said saturated fat will give you heart attacks.”

“The most recent studies have concluded that saturated fat intake likely has no relation to heart disease, contrary to popular opinion.”

This is an absurd conclusion and isn’t a representative of recent studies or decades of previous study. Mark says that he “tend[s] to listen to the science, rather than what [he] think the science is saying,” but unfortunately also cherry picks his studies and has a clear confirmation bias. To come to such a conclusion you’d seemingly have to close your eyes, walk around blindly, block your ears and not acknowledge anything that contradicts your world view. Who cares about scientific evidence when we can faithfully hold onto a few poorly analyzed observational correlative studies?

A 2011 study found that “reducing the intake of CHO with high glycaemic index is more effective in the prevention of CVD than reducing SAFA intake per se.”

Notice how this doesn’t actually back up the claim that “saturated fat intake likely has no relation to heart disease?” The study still illustrated saturated fat intake having a relation to heart disease, contrary to Mark’s conclusion, but also asserted that a low-GI diet is more effective in preventing heart disease. Is this new? Not at all, we already know intake of refined carbohydrates (which are high-GI) contributes to heart disease. The general public has added more foods high in sugar into their diets, and this has lead to even more health problems than were present a few decades ago.

From a 2010 study out of Japan, saturated fat intake “was inversely associated with mortality from total stroke.”

This is an observational study that relied on questionnaires, and it’s talking about strokes. Wait, what? How this relates to Mark’s conclusion, I don’t know. It’s interesting though and warrants further study. Likely there are other dietary and lifestyle factors at play.

From the study: “Assuming that the inverse association between SFA and stroke mortality is causal, it would nevertheless be inappropriate to recommend an increased consumption of SFA-containing products to the general Japanese population, because it might increase population levels of total cholesterol and the risk of IHD.” Again, contrary to Mark’s conclusion.

More from the study: “this study was conducted among Japanese, so extrapolation to other populations should be made with caution … Application of discrepant results to public health practice must be cautious. We believe that this discrepancy could be explained in part by a low distribution of SFA intake among Japanese … It is well known that the SFA intake is far lower in Japan than in Western countries.” Again, there could likely be other dietary and lifestyle factors at play here, so extrapolating these observations to standard Western diets high in saturated fats and sugar is unwarranted. Encouraging a dietary practice that could increase heart disease – already the top killer in the USA – in order to possibly, based on correlative observational data, reduce stroke seems a little crazy to me. Thankfully there are other ways to minimize your risk of stroke without increasing saturated fat intake.

A 2010 meta-analysis found “that there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD.”

This study, sponsored by Unilever (producer of dairy products) and National Cattlemen’s Beef Association, seems to be a crux for the pro-saturated fat advocates. Too bad it has been criticized for being bias and drawing incorrect conclusions. (See: Meta-analysis of effect of saturated fat intake on cardiovascular disease: overadjustment obscures true associations and Clearing Up The Confusion Surrounding Saturated Fat.)

Mark states, “That looks pretty clear cut to me,” but it’s only clear cut when you block out everything that you disagree with.


Also, I’m not aware of Dr Dean Ornish ever claiming that “saturated fat will give you heart attacks,” as he and others in the field know this is a dramatic oversimplification. Saturated fat intake is a risk factor and can be a contributor to heart attacks, not a cause in of itself.

“Isn’t all that cholesterol gonna raise your cholesterol?”

“Dietary cholesterol does not affect total blood cholesterol. In fact, when we do eat cholesterol, our bodies make less of it to keep our blood levels in balance.”

Here Mark contradicts himself in his conclusion as he also states: “Save for a select few who are “hyper-responders,” the vast majority of people can eat cholesterol without it affecting their cholesterol levels.” We can rephrase the conclusion though to: Dietary cholesterol does not dramatically affect total blood cholesterol for many people, and greatly affects some others. In this case it’d be true, as cholesterol only has a limited effect on total blood cholesterol for many people. (As noted by the Japanese study above: “It is well known that a greater intake of [saturated fat] increases the blood total cholesterol concentration.”)

Eating cholesterol isn’t actually that bad – if you’re not super sensitive to dietary cholesterol – and there are worse things you could do. However, there’s no need for dietary cholesterol because our bodies produce enough cholesterol for optimal functioning. There’s no benefit of consuming additional cholesterol. Foods that are high in cholesterol are usually high in saturated fat, and this does affect cholesterol. Many other factors affect cholesterol levels, not just dietary cholesterol and saturated fat.

“Where do you get your energy?”

“Fat is the body’s preferred and most reliable form of energy, which is why we store excess energy as fat on our bodies. Unless you think we accumulate body fat just to make pants fit tighter.”

Carbohydrate is the body’s preferred energy – at least I assume due to it being the first macronutrient utilized by the body. (Alcohol is the first source of energy utilized by the body, but alcohol doesn’t have useful function within the body unlike carbohydrate and fat and so isn’t classed as a macronutrient). However, seeing as the body is a biological organism that holds a conscious being and not itself a conscious being that can succinctly express its preferences, it’s difficult to assert what exactly its preference is. How do we define the most reliable? We have abundant foods available rich in both carbohydrates and fats, so arguably both are pretty darn reliable forms of energy.

This section is what annoys me the most because it illustrates an incorrect understanding of biology and evolution. It appears common within the Paleo and Fruitarian movements to believe in the perfection of nature – if only we all went back to our roots we’d all be healthy and happy! Except that nature is perfectly imperfect. It’s disconcerting because both movements are largely based on theories of evolution that are largely incorrect. Biology doesn’t owe you reason or purpose, evolution is merely a process of mutations – some that work, providing benefit, and many that don’t.

We don’t store fat for any specific reason but due to biological processes. One could rationalize reasons, and there may be advantages (or disadvantages) to a biological function, but you can’t expect biology to neatly provide you a package of reasons. Life just is what it is, imperfections and all. When you start throwing around assertions that X occurs due to reason Y rather than X results through process Y, you leave the scientific realm and enter religious territory.

It’s true that we can use stored body fat in times of starvation, but an obese individual will probably die from starvation before they get skinny. We don’t hibernate during the winter like bears. Granted, lots of body fat may be helpful in a famine, but is a famine a reasonable concern in today’s society? Maybe you think it is and think that the benefits of excess body fat outweighs the negatives, who knows.

Fat is a good source of energy, especially if you’re in a state of starvation or purposeful ketosis. Carbohydrate too is a good source of energy. We need calories for energy, and it’s healthful to have several sources of energy. There are serious health concerns for long term states of ketosis and its safety has not yet been proven. Carbohydrates can be safely and healthfully used as a predominant energy source.

“But isn’t fat totally free of nutrients? How do you get your vitamins if you’re eating all that fat?”

“Certain fats, like egg yolks, palm oil, extra virgin olive oil, cod liver oil, and grass-fed butter, are some of the most nutritious foods in existence. And without fat in your meals, you often won’t absorb all the nutrients that are present in other foods like leafy greens, since many of them require fat for full absorption.”

Some fats are nutritious. The refined fats mentioned in this section tend to be high in a few nutrients and can hardly be called nutritious overall. There are refined carbohydrates that are high in a few nutrients too, like blackstrap molasses – this is high in calcium. Obviously it wouldn’t be advisable to eat large amounts of blackstrap molasses, nor would it be advisable to eat large amounts of refined fats like extra virgin olive oil.

The most nutrient dense foods, by far, are green leafy vegetables – and these foods are high in carbohydrates. Nuts and seeds – high in fat – can also be considered nutritious. However, most oils are virtually absent of nutrition or high in just a small number of nutrients, so for the most part are just empty calories – good if you need the extra energy, but not so good if you need healthful nutrition.

Adding fats, even in the form of nuts and seeds, to vegetables can increase nutrient absorption, hence it’s a practice I encourage. However, you don’t need much fat in order to maximize benefits, a small handful of nuts or seeds thrown into a salad is all that’s needed.

For more information on our dietary guidelines, see: The Rainbow Plate.

“Doesn’t the brain run on carbs, not fat?”

“While it’s true that the brain requires some glucose for energy, using fat-derived ketones as well can make the brain run more efficiently and reduce its glucose requirements. On top of that, your body can probably create more glucose than your brain even requires.”

The brain does require glucose to function. It’s also true that the body can create glucose; whether or not the body is able to create enough glucose for optimal functioning on a carbohydrate-restricted diet, I don’t know. In the short term, restricting carbohydrates will likely result in you becoming grumpy, but the body adapts in the longer term after ketosis is induced and ketones become the main source of energy. (See: Do Low Carbohydrate Diets Make You Dumber?)

Summing Up

Mark’s article may be helpful for those people justifying and defending their controversial high-fat diets. There’s a lot of social stigma around diets, and high-fat and low-fat advocates alike face similar problems regarding social acceptance. Ultimately though these responses for justifications for following a high-fat diet leaves a wanting because they don’t provide adequate reasoning.

I consider a high-fat diet to be an experiment, and a potentially dangerous experiment at that. The disadvantages far outweigh the possible benefits and there aren’t any apparent benefits that cannot be otherwise more healthfully obtained.

  • Updated 3 years ago
Robert Roose

Robert is a nutrition advocate focused on optimal health and longevity.