The Fat-Sugar Metabolism Debate

Back in the 50's when the heart health debates were raging, given clear connection between body fat and heart disease, scientists were divided between whether dietary fat or dietary sugar contributed to body fat. At the time, the dietary fat camp won. Today, however, thanks to modern biochemistry, we know that it's not quite that simple. To highlight this, let me give you a simplified summary of the biochemistry of metabolism when energy is needed (fasted state):

With permission from my biochemistry professor

Now that you've been thoroughly bewildered, I want to focus on two key aspects of metabolism: energy use and energy storage. (No, I won't share any more biochem slides).

Glucose, the most prevalent form of sugar used, is converted to pyruvate in all cells, which, in most cells is converted to Acetyl-CoA, which is sent through the Krebs cycle to generate more energy.

Other sugars are converted to glucose or a subsequent product on the glycolysis pathway

Fats are delivered as fatty acids directly to the muscles, which are converted to Acetyl-CoA and are slotted into the Krebs cycle. In the liver, they're converted to ketone bodies for use in the muscle and brain or packaged into VLDL for distribution as fatty acids.

So, when energy is needed, both forms are used, but I want to direct you to something in the bottom left corner entitled "Carb flame". A funny quirk about the Krebs cycle: with just the right amount of carbohydrate, the Krebs cycle skips a couple steps and becomes a super-efficient fat-burning powerhouse.

So, then, what happens when you have more energy than you need? It goes to storage. All sugars, except Fructose, are initially converted to glycogen for storage. When the glycogen stores are filled up, they are then converted to fat. Fructose, however, is not able to be converted to glycogen in the liver, which is the main processing centre, so it goes straight to fat. Yes, Fructose can be converted to glycogen in the muscle, and certain other tissues, but the volume processed is significantly less.

I want to note, quickly, that Fructose is the sweet sugar. It's found in high-fructose corn syrup and is also part of cane sugar (which includes white/table sugar, brown sugar, and molasses/treacle).

Dietary fat storage is much simpler. It's stored as fat.

Hmm... I seem to be losing my case aren't I. Well, let's consider the factors controlling whether your body is using or storing energy: your hormones.

Insulin and Glucagon are the two key players here. Now, as I promised not to inject any more biochem slides, you'll have to trust me, okay?

Glucagon triggers the body to release storage forms of energy because the body needs energy now. Remember, when Glucose is gone, you need Glucagon. Insulin triggers the body to store energy when there is excess. As such, Insulin is responsible for activating and increasing the number of proteins that convert sugars to glycogen and Fructose, fatty acids, and excess sugars to fat.

Here's the kicker: Glucose drives insulin secretion. What's more interesting, though, is that glucose taken orally (eaten) causes insulin to spike more than insulin taken by injection. Now, yes, free fatty acids also contribute to insulin production, but to a lower degree and I must note that majority of fatty acids in the body are not free-circulating.

So, what's the verdict? Both dietary fat and sugar are metabolised to produce energy when needed. Both dietary fat and sugar are stored when not needed. It would appear, then, that both are beneficial, in the right amounts. Too much dietary fat will contribute to body fat. Too much dietary sugar will also contribute to body fat. Unlike dietary fat, though, excess dietary sugar will contribute to elevated insulin levels.

Oh, if I had the space to begin discussing the effects of chronic high insulin, but, in short form, high insulin levels can lead to high blood pressure, type 2 diabetes mellitus, coronary artery disease, cardiovascular disease, obesity, polycystic ovarian syndrome, and a couple other chronic health conditions.

Again, sugar, like fat, is not bad in moderation, but, in excess, it is a cause of many of the health problems that plague us today in America.