By Lyle McDonald

Carbohydrate and Fat Controversies: Part 1

Although there are still many Protein Controversies (usually regarding kidney health, bone health, etc.), nowhere in the dietary world is there quite as much controversy as over carbohydrate versus fat intakes.

In this article, I want to look at carbohydrate and fat intake in terms of the various arguments and debates that tend to surround them.

The main controversy here revolves around what amounts of carbohydrates and/or fat are ideal, healthy, recommended, etc. and that’s what I’ll focus on. I’m not going to deal with body composition explicitly in this article, I’ll save that for another day.

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’.

Various attempts to promote so-called ‘high-fat’ or ‘low-carb’ diets have usually been shot down as fads although there is increasing research evidence that, at least for some individuals (usually those with insulin resistance) higher fat intakes and lowered carbohydrates may be both beneficial and preferred.

However, for the most part, I’d say that mainstream dietitians are still on the carbs = good, fat = bad bandwagon with higher fat/lower carbohydrate diets being relegated to the diet ‘fringe’.

Both groups can bring impressive (or at least impressive looking) data to the table but, as usual, extreme stances are invariably incorrect and the truth lies somewhere in the middle; this particularly debate is no different.

The third group (and the one I put myself in) recognizes that whether or not carbohydrates or fats are ‘good’ or ‘bad’ depends on the context. The source of the carb or fat, the rest of the diet, the goal of the individual, genetics, activity, etc. all factor into this issue. So while it may be convenient to give simplistic recommendations of the ‘X is bad, Y is good’ variety, simple in this case tends to be incorrect.

Perhaps the most succinct way of describing what I’m going to detail is that there are no good or bad foods only good or bad diets. That is, within the context of one type of diet or individual situation, a specific food may be excellent; under other conditions it may be a poor choice.

What does the Body Require?

So that some of my comments will make sense, I need to cover a smidgen of nutrient physiology, mainly having to do with the issue of carbohydrate ‘requirements’ (a topic I cover in detail in How Many Carbohydrates Do You Need).

As I think I’ve managed to work into every book I’ve ever written, there is no strict physiological requirement for carbohydrates (this factoid is often used by the low-carb diet groups as part of the rationale for their dietary approach).

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 while it can’t use fatty acids directly, it can use a fatty acid derived fuel in the form of ketone bodies. After roughly three weeks of adaptation to using ketones, 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.

I’ve seen papers where researchers point out that the body requires no carbohydrates which then go on to say that a proper diet should contain at least 50% carbohydrates. It doesn’t make much sense.

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 (this pathway isn’t utilized massively in humans, although a few conditions will make it relevant).

So, outside of the small essential fatty acid requirement, one could make an argument for there being no physiological requirement for fats either.

What does the body then require on a day to day basis if there is no real requirement for either carbohydrates or fats? Well, outside of the basics like water and air, roughly eight amino acids are essential to get from the diet, there’s the small essential fatty acids requirements and of course vitamins and minerals. Everything else, strictly speaking is optional.

I would note that, to avoid starving to death, sufficient calories will be required. Since it’s generally unrealistic to consume your entire daily caloric requirement from protein, that means that carbs, fats, or a combination of the two, will generally be needed to supply sufficient energy to the body.

But, as noted above, most tissues in the body show a great deal of flexibility, using carbs when they are available and fats when carbs aren’t available. Note also that the body has its own store of fuel, primarily in the form of body fat that is mobilized when sufficient amounts of other nutrients aren’t available.

So Why Do Most Argue that Carbs are Good and Fats are 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.

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.

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 stigma against 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.

I’ll address this issue in more detail in Carbohydrate and Fat Controversies: Part 2