In nutrition, it’s rare to find a complete consensus on anything. Even with decades of data there continue to be controversies surrounding dietary protein intakes, generally revolving around kidney or bone health, among others. But even those pale in comparison to some of the current arguments or controversies over carbohydrates and fat. While there are many, the main ones I will examine here will have to do with the health related controversies. Body composition issues will have to wait for a separate article.
Two (or Three) Dietary Camps
Generally, folks fall into one of two camps regarding whether they think carbohydrates or fats are good or bad. For a couple of decades now, the mainstream of dietary advice has been more or less stuck in the mindset of “fat is evil” and “carbohydrate is good” This trend started with the early work of Ancel Keys on dietary fat and heart disease . This continues to spur debate although many of the criticisms of his work are based on a misunderstanding of it.
For decades, various attempts to promote “high-fat” or “l0w-carbohydrate diets” have been shot down as fads, unsustainable of simply unhealthy. At the same time, research continues to emerge showing that such diets may have health benefits at least for some individuals, usually those suffering from some degree of insulin resistance. In those situations, lowering total carbohydrate intake, which will necessitate an increase in dietary fat may be beneficial.
That said, I’d still tend to say that mainstream dietitians and nutrition “experts” tend to fall into the “carbs are good and fat is bad” bandwagon with lower carbohydrate diets still being considered somewhat more of a “fringe” approach. Those fringes tend to get dismissed although it’s often for the wrong reasons. Many on that fringe go past advocacy to zealotry and I find that many are criticizing the messenger more than the message.
But I don’t care about zealots, I care about science. And while both the high-carb/low-fat and low-carb/high-fat groups can both bring impressive amounts of (carefully selected) research to the table, I find that extreme stances are invariably incorrect. The truth usually lies somewhere in the middle and I feel the same about this debate.
Because there is a third dietary “camp” the one I find myself in. This group recognizing that talking about any of this in absolutes is nonsensical. Whether carbohydrates or fats are “good” or “bad” depends on the context. The sources of carbohydrates or fat being eaten, the rest of the diet, the goal of the individual, their genetics, activity levels, insulin sensitivity, etc. all play a role in determining what might be best for them.
So what might be best for a lean athlete with high activity levels and good insulin sensitivity might be absolutely wrong for someone who was inactive, insulin resistant and carrying excess body fat. What might be best for the second individual might be equally wrong for the first. It all depends on context.
The Body’s Nutritional Requirements
For the rest of the article to make sense, I need to examine a bit of nutrient physiology, mainly addressing the issue of the body’s dietary carbohydrate requirements.
Simply put, as I have put in almost all of my books, the body has no strict nutritional requirement for dietary carbohydrates. That is, they are not an essential nutrient. This fact is actually one of the arguments put forth by low-carbohydrate diet groups as a rationale for why their diet is superior.
Most tissues in the body can readily use fatty acids for fuel just as easily as glucose. There are a few tissues such as the renal medulla, red blood cells and one or two other that can only use glucose. However, those cells essentially make their own glucose by recycling lactate (produced from glucose metabolism) back into glucose.
The brain is in its own weird category. Under most conditions, it relies exclusively on glucose (and people still incorrectly state that “the brain can only use glucose for fuel”). But while the brain cannot use fatty acids directly, it can use a fat derived fuel called ketone bodies. After roughly 3 weeks without carbohydrates (or in total starvation) the brain may only need 25 grams/day of glucose or so, which can be made by the body (in the liver and kidney) from sources such as lactate, pyruvate, amino acids and glycerol.
Even the American Dietetic Association bible, the RDA Handbook, states that there is no requirement for dietary carbohydrates. Any decent nutrition or physiology book will state the same. Despite this basic biological fact, many researchers and diet authorities still insist that the majority (50-60% or more) of the human diet should come from carbohydrates. It seems a bit inconsistent.
At the same time, outside of a small essential fatty acid requirement (a few grams per day from the fish oils, EPA/DHA), fats aren’t truly required by the body either. All of the tissues I mentioned above will use glucose if you provide it (the heart is an exception, almost exclusively relying on fatty acids for fuel) and the body can make fatty acids out of other sources if need be through a process called De Novo Lipogenesis (DNL). I’d note that under normal conditions DNL does not contribute a significant amount of fat to the body in humans.
Which means that outside of that small EFA requirement, the body doesn’t really require dietary fat either.
What Nutrients Does the Body Require?
If there is no dietary requirement for carbohydrates and only a small one for the EFA’s, what does the body actually need for survival. There are the real basics like air and water which generally aren’t a problem. There are essential amino acids which come in dietary protein, nitrogen (which also come from dietary protein), vitamins and minerals. Everything else we eat is technically optional.
I would point out that to avoid starving to death, sufficient calories (the energy provided from the metabolism of food) is required. Since it’s unrealistic approaching impossible to get a total day’s energy requirements from dietary protein, this means that some amount of carbohydrates and/or fats will have to be eaten.
But as noted, the body show’s a great deal of metabolic flexibility in what it can or cannot use for fuel. If it is available, glucose tends to be preferred by most tissues if it is available although they can switch to fatty acids when carbs aren’t. And unlike the body’s carbohydrate store which are fairly limited, the body has a fairly large store of energy in bodyfat
Are Carbohydrates Good and Fats Bad?
Despite the fact that there is no physiological requirement for carbohydrates in the human diet, the most common dietary recommendation in modern times is generally to reduce fat intake and increase carbohydrate intake. I’m going to address the issue starting from that standpoint.
A good question might be why is this stance taken. While I can’t read the minds of these folks (and I hate to contribute to grain lobby USDA conspiracy theories), I think the reasons is actually fairly simple: we have to eat something to meet our calorie requirements.
There’s usually a limit to how much protein can be reasonably consumed (and most authorities seem to be against ‘high’ protein intakes as well) so that means that the rest of the diet (in terms of energy) must come from either carbohydrate or fat. AS I mentioned, in the 70’s, the stigma against dietary fat started to develop and it all pretty much went from there. Fat was implicated as the cause of heart disease, stroke, obesity, you name it and excessive fat intake was blamed.
Since people have to eat something and because of the general negatives surrounding a high fat intake (some of which is warranted, some of which isn’t), policy makers recommend a high-carbohydrate intake by default.
The bigger question is whether or not this is a scientifically defensible position.
Examining the Debate
As noted, the usual argument goes that high-fat diets cause high-cholesterol, heart disease, cancer, obesity and the rest, as evidenced by the high incidence of those disease in modern diets (which are typically high in fat). But that’s a questionable conclusion to draw.
The modern diet is also high in carbohydrates with most of those carbohydrates being highly refined, low-fiber foods. In contrast, the modern diet is often low in fruits and vegetables which is a double whammy. Fat intake is often quite high and there tends to be an overconsumption of both saturated fats and trans-fatty acids with an insufficient amount of healthy fats such as the fish oils present.
This intake is typically coupled with low levels of activity, a high body fat percentage and lots of stress. Overconsumption of alcohol and/or smoking are also present. Basically, there tend to be lifestyle patterns that cluster together where people that do one “unhealthy” thing do other “unhealthy” things. Trying to separate out one factor from the rest is a fool’s game. It is one reason of many why nutritional epidemiology, which often tries to determine a singular factor causing something, is such garbage. Everything in life is multi-factorial and health is no different.
But that means that pinning the blame on any singular factor, including fat intake is a mistake. Equally, expecting a reduction in dietary fat intake to magically fix the problem is not only mistaken but disingenuous. Even here you run into a big issue, when people reduce the intake of one nutrient they have to increase another.
One thing that happened in the 80’s is that the reduction in dietary fat was accompanied not only by an increase in carbohydrate intake but an increase in refined carbohydrate intake which caused its own set of problems. Specifically the improvements in cholesterol levels were offset by an increase in Triglyceride (TG) levels, an independent risk factor for heart disease. This isn’t the case when dietary is replaced with whole grains but that’s not what happened in the real world by and large.
As well, the quality of fat probably made a bigger difference than the total intake to begin with. Invariably replacing saturated fat with monounsaturated fat (i.e. Olive oil) or polyunsaturated fatty acid (PUFA) improved blood lipid levels and reduced cardiovascular risk without a reduction in total fat intake. Mind you, some of the true fringe loonies are convinced that PUFA is the real problem in the modern diet and saturated fat is totally healthy even if the data completely contradicts that point.
Similar comments can be made in terms of obesity and fat gain. There is no doubt that fat is more calorically dense than carbohydrates at 9 cal/gram vs. 4 cal/gram. So for the same volume of food eaten, dietary fat will contribute twice as many calories. Dietary fat also doesn’t send a short-term fullness signal which can lead to passive overconsumption. Simply, foods high in dietary fat tend to taste good, have good mouth feel and it’s fairly easy to overeat them. Of course, it’s even easier to overeat foods high in sugar and fat so even here blaming fat per se may be missing the point.
Along with other issues, it’s easy to see how “high-fat” diets can be blamed for causing obesity. But it’s never that quite that simple and many other factors have changed in the modern environment that contribute to increasing levels of obesity beyond just the dietary fat content.
One is the increasing intake of refined carbohydrates (and note again that high sugar/high fat foods are supremely easy to overconsume). Another is reduced daily activity requirements. Portion sizes continue to increase and there are many other potential contributors to the problem beyond just dietary fat.
I’m not saying that you can dismiss an excessive dietary fat intake as part of the problem.
I’m simply saying it’s not the ONLY factor.
Even in a dieting context, it’s clear that the fat composition of the diet makes very little difference. Lower and higher fat diets all work equally well (meaning not very well at all) regardless of composition.
Certainly a high-fat intake can cause people to eat more in the modern world but it’s rare for anyone to eat pure dietary fat. The issue is invariably with high-sugar/high-fat foods. And even that is usually part of an overall lifestyle. Blaming either fat or carbs is missing the point. It’s the combination.
Back to Dietary Fat and Health
Getting back to health issue, the simple fact is that not all studies link a higher fat intake to an increased risk of disease. Perhaps the most well developed data come from studies of the Mediterrannean diet which may contain as much as 37% dietary fat calories. This would be considered a moderate or even high-fat diet by most authorities and yet the diet is invariably associated with lower cardiovascular risk.
There are many reasons for this not the least of which is that the majority of the fat is from monounsaturated fat with only 9% coming from saturated sources. But the diet is also high in fruits and vegetables with many other variables at work. At least one of those is the overall lifestyle in the countries who adopt it, a factor that is often forgotten. I bring it up only to show that a high-fat intake isn’t a problem per se. It’s all about context. The rest of the diet, the environment, it all plays a role.
An additional data point would be the Alaskan Inuit, a population known for eating a very “high-fat” intake while showing a low level of cardiovascular disease. This was first observed in the 1970’s, at a time when most of the dietary fat intake in this group came from the fat found in fish. This discovery actually led to interest in the fish oils. However, that same group shows increasing rates of western disease when they adopt a western diet with higher intakes of saturated fats and refined carbohydrates.
Similarly, there are cultures such as Asians who consume a very high-carbohydrate diet without many of the health risk or obesity problems seen in the Western world. At least this is true if they are following a traditional diet. As with the Inuit, as soon as such culture adopt a western diet high in refined carbohydrates and dietary fat, they start to develop the same problems as everyone else.
In a different context, we might consider Kenyan distance runners who may consume up to 70%+ of their total calories as carbohydrate while remaining lean and apparently healthy. However, this has to be tempered by the fact that their primary carbohydrate sources are vegetables, Ugali (made from corn/maize) and kidney beans. It’s all unrefined. And that is being consumed at high amounts on a base of both leanness and extremely high levels activity, running 20-30 hours per week.
The point of all of this being that it isn’t the carbohydrate or dietary fat intake per se that is an issue from a health standpoint. It’s about context. In one context a very high-carbohydrate intake may be completely unproblematic. In another it may cause enormous health issues. The same holds for dietary fat. The total amount being consumed, the sources of each, activity levels, etc. all come together.
Gaining or Losing Weight
A factor that is often unconsidered in terms of whether a given nutrient is good or bad is whether or not the person is losing or gaining weight/fat. For example, low-carbohydrate/high-fat diets that cause weight and fat loss invariably improve a variety of health parameters. This is especially true in the case of insulin resistance. However, those same diets may cause those same lipid parameters to get worse if weight is gained.
Similarly, the negative impact of a high-fat diet can be offset by aerobic activity. Presumably the activity is ensuring that incoming dietary fats are being burned for energy rather than being stored where they can cause problems.
Studies in diabetics are finding that higher mono-unsaturated fat intakes (and lowered carbohydrate) intakes may be healthier than the converse. This, of course, only holds if calories are strictly monitored and controlled to avoid weight gain. When weight is gained, from nearly any dietary approach, blood sugar control in diabetics worsens.
Issue with Extremely Low-Fat Diets
It’s important to also consider the flip side of the anti-fat dogma. Because reducing dietary fat to extremely low levels can cause its own set of problems. One issue is adherence. Very low fat diets tend to be tasteless and lack mouth feel and this can impact adherence. At least some studies show that moderate dietary fat intakes provide better long-term adherence. In the big picture, that trumps any short term results (as I mentioned, high-fat diets often result in overeating due to passive overconsumption).
At extremely low-fat intakes, fat soluble vitamin absorption may be impaired although it takes the most extreme of low-fat diets to make this a big issue. Even in health terms, when total dietary fat goes down, both the “good” and “bad” fractions tend to decrease so overall health risk may not be improved. This is accompanied by what I mentioned above, if people replace dietary fat with unrefined carbohydrates, as they commonly do, TG levels go up which can raise heart disease risk.
Now, there is no doubt that calorie intakes go down when dietary fat is reduced to low levels. However, in the long-term this can backfire for various reasons. One is that extremely low-fat diets tend to leave people hungry all the time. Even moderate amounts of fat per meal slow gastric emptying, keeping food in the gut longer. It also stabilizes blood sugar. Bot of these tend to keep people fuller in the long term.
Additionally, as I mentioned, people have to eat something. When dietary fat is reduced, carbohydrate intake goes up. And while researchers seem to assume that people will increase their intake of unrefined, high-fiber carbohydrates. But here researchers need to get out of the lab and look in the real world. Because that’s rarely what happens. Rather, people replace a lowered dietary fat intake with higher intakes of refined carbohydrates.
In the 198o’s, when the message was misheard that carbohydrate couldn’t make you fat (which they absolutely can) people started overeating. And food companies rapidly rushed low-fat but high calorie foods to market. Often they had more calories than the higher fat version. And people overate them.
Are Carbohydrates Bad?
Which leads us back to the other group who maintain that carbohydrates rather than dietary fat are the real problem in the modern world. They will blame the health problems of the modern world on carbohydrate intake. Modern health problems such as heart disease are often blamed on increasing carbohydrate intakes and there is the always amusing argument for the insulin hypothesis of obesity. The idea being that it is the increase in insulin from carbohydrates, rather than calories or dietary fat, that cause fat gain.
Which isn’t to say that carbohydrates can’t be a problem. But as with the above discussion, it depends on context. And you can basically reverse what I wrote. Is the modern diet high in carbohydrates? It can be. Is it high in refined carbohydrates? In many western cultures, yes. Is this highly refined carbohydrate intake a source of many issues in terms of health and weight gain? Absolutely. Does that make carbohydrates per se bad? No. Even a high refined carbohydrate intake could be completely fine in the context of an extremely high activity level. It’s simply that most people don’t have an extremely high activity level.
Even looking at the obesity issue, in the same way some groups blame dietary fat per se, others blame carbohydrates. I mentioned the insulin hypothesis above but it’s often been argued that the obesity epidemic was caused by the recommendation of low-fat diets in the 1980’s. As with most extreme arguments this is scientifically indefensible.
First and foremost most people didn’t make it to low-fat intakes to begin with. Rather, during the 1980’s, everything was changing. Among them was an increase in total fat, carbohydrate and calorie intake. That was along with vast societal and environmental changes. People were eating more of everything, along with a changing world that required less activity. And that combination drove the increase in obesity.
Some have gone, attempting to pin the blame on a single carbohydrate, High-Fructose Corn Syrup (HFCS). Found in sugary soda and other refined foods, it has been argued cause fat gain in some way independent of its sheer calorie intake. But this is equally nonsensical. HFCS has the same essential composition as table sugar and isn’t inherently any more fattening than any other source of calories.
Rather, the issue was more to do with the fact that most HFCS was being consumed in liquid form such as sugary sodas. These are treated completely differently in the body than solid carbohydrates. As well, ask yourself the following question: do you know many people who drink a lot of sugary soda who don’t have an overall problematic lifestyle in terms of their food intake and activity level. HFCS was never the sole issue. Quite in fact, in recent years, HFCS intake has gone down as obesity continues to increase. Clearly it was never more than part of a bigger issue.
Reaching a Point
The point of all of the above isn’t to argue for either a high-carbohydrate or low-carbohydrate or high-fat or low-fat diet as being superior. First and foremost, the optimal diet depends on context. At best a given diet might be optimal for a given individual under a specific set of conditions. There are no absolutes.
Beyond that, any aspects of either carbohydrate or dietary fat that is “good” or “bad” depends on a variety of factors. The amount being eaten, the source of the nutrient, the activity of the individual, their insulin sensitivity, etc. all play a role. A high-carbohydrate diet may be totally appropriate for someone with high activity levels. It might even be appropriate for someone who is less active if they eat primarily unrefined carbohydrates.
Since almost nobody actually does that, reducing total carbohydrate intake for less active individuals with insulin resistance or the metabolic syndrome makes more sense. That requires raising dietary fat intake. Is this inherent a problem? Well that depends. It’s fairly clear that the Mediterranean diet with it’s nearly 40% dietary fat intake is very healthy but only because most of it comes from Olive oil. The rest of the diet contributes.
Even a diet higher in saturated fats might be perfectly appropriate when someone is actively losing fat. That same saturated fat intake might become problematic at weight maintenance or when they are gaining weight?
Does this make sense? Neither dietary far not dietary carbohydrate can or should be demonized in an absolute sense. And, frankly, most of the “controversy” goes away when you start looking at the context of the situation. What is good in one situation may be bad, neutral or disastrous in another and vice versa.
Now, I want to make it very clear that I’m not trying to make this either a pro-fat or anti-carbohydrate article or trying to make a low-carbohydrate diet the default choice for anybody. My point is simply that the idea that ‘fat is bad’ and ‘carbs are good’ (or the opposite) is too simplistic to be meaningful.
Not all fat is bad and not all carbs are good. The source, the composition of the rest of the diet, the total amounts you’re eating of each, your activity level and other variables all factor in. Whether you’re talking about health risk or obesity, you can’t simply pin the blame on one factor or the other.
So, under conditions of high caloric intake, with a high intake of refined carbohydrates (meaning chronically high insulin levels), poor quality fat choices (too much saturated fat and/or too little unsaturated fats), little activity, minimal fruit and vegetable intake, etc. a high-fat intake is probably very detrimental from a health standpoint. Sadly, this describes a fairly typical diet in the modern world (especially the US).
In contrast, with reduced or even controlled caloric intake (such that bodyweight goes down or is maintained) and most of the fat coming from unsaturated sources (note: excessive polyunsaturated fats has its own set of problems), a high fruit and vegetable intake, reasonable activity levels, keeping body fat levels down, etc. higher fat intakes may be no problem at all. In some situations, an increased fat intake (again, from healthy sources within the context of activity and a high fruit and vegetable intake) may be beneficial compared to the alternatives (e.g. increasing carbohydrate intake).
- The Energy Density of Foods
- Comparing the Diets
- Insulin Sensitivity and Fat Loss
- Examining Some Dietary Protein Controversies
- Do Ketogenic Diets Have a Metabolic Advantage?