Fructose

rough sketch of a fructose molecule

Fructose myths seem to be everywhere these days. Fat was fallaciously made out to be “bad” for us in the 1980s (with disastrous results), today carbohydrates are often villainized in the media, and fructose in particular. I want to offer what I’ve learned hopefully to clear some of this up, but we do have to start with a little background.

Carbohydrate is one of the four kinds of macronutrients the body can use to fuel metabolism. The other three are fat, protein, and alcohol (although alcohol is a special-case). There are four groups of carbohydrates, which are also called saccharides; monosaccharides, disaccharides, oligosaccharides, and polysaccharides. To simplify things we’ll look at the groups most abundant in our diet, disaccharides and polysaccharides, which are more commonly known as sugars and starches.

Sugars and starches are compounds made up of units of monosaccharides linked together. Monosaccharides are the smallest possible units of carbohydrates, from which all other carbohydrates are built.

sugars

  • Sugars (disaccharides) are two monosaccharide molecules (di-saccharide = two-sugars) linked together.
  • Starches (polysaccharides) are longer chains of monosaccharide molecules (poly-saccharide = many-sugars) linked together.

There are a variety of monosaccharides, but by far the majority in most people’s diet are galactoseglucose, and fructose.

Any carbohydrate you eat must be broken down into its monosaccharide components before it can be absorbed by the small intestine and enter the blood for use by the body. This is particularly apparent when those who are lactose intolerant eat dairy. Most dairy is high in the disaccaride lactose which must be broken-down by the enzyme lactase into its monosaccharide components (one galactose + one glucose) before it can be absorbed. But if your body doesn’t make much lactase, much of lactose in the dairy you eat passes through the digestive track undigested and unabsorbed, which can cause significant digestive discomfort.

To recap before we move on:

  • All carbohydrates are made up of different numbers & arrangements of simple sugars called monosaccharides.
  • Carbs must be broken down into their monosaccharides components before they can be absorbed by the body.
  • Almost all the carbs we eat are made up of three monosaccharides; galactose, glucose, and fructose.

After monosaccharides enter our blood stream our bodies convert them all into glucose to fuel metabolic activity. The body usually stores this carbohydrate energy in the form of glycogen, which are big molecules that can be thought of as “bundles” of many glucose molecules linked together (a single glycogen molecule can contain 30,000 glucose units!). The galactose & glucose to glycogen conversion is similar, but the conversion of fructose is different.

The big difference between glucose and fructose is that while glucose can be metabolized nearly anywhere in the body and is strongly coupled to insulin, fructose is metabolized in the liver, and largely outside the influence of insulin.

The liver prefers to convert fructose into glycogen energy and store it in liver cells. But that storage capacity tops out around 100g, and as the the liver fills, it transitions to converting fructose into fat (de novo lipogenesis “to create new fat”). For perspective there’s 100g of fructose in about ~200g of honey or 1900ml of cola (~12% HFCS, which is 55% fructose).

It’s not that fructose makes you fat, it’s that your body starts to turn it into fat sooner than it does other carbohydrates when one eats a caloric surplus. While there’s little research in long term surplus fructose consumption , short term research shows less than 1% of fructose eaten in a caloric surplus being converted into fat. But we don’t need long term studies to tell us eating a surplus of energy over the medium or long term means we get fat. I have read of fructose leaving more of a particular waste molecule that may contribute to our metabolism having more of a fat production bias. But again, this is solved by simply not eating too much of all foods too often.

“Do not subordinate fundamental principles to minor details.”

Fructose is special, but not special enough to pose any problem as long you just generally eat the right amount of mostly whole foods, and practice sustainable activity. This means flexible dieting (IF/IFYM) & the squat rack for me, but it could be vegan & swimming for you.