One specific type of sugar, glucose, is the primary energy source of the body although not the only one. Pretty much anything you eat has to be broken down into glucose before it can be used.
Glucose is the most important of the large family of sugars, all of which have names ending in -ose. Glucose is commonly found in nature attached to another sugar, fructose, the two of which form a disaccharide (“two sugars”) named sucrose, also known as table sugar, the stuff that’s in your kitchen.
Fructose is fruit sugar, the stuff that makes honey and fruit taste sweet. It’s the sweetest naturally occurring carbohydrate. Fructose is also the added sugar in many food products. Consider this pair of cereal packages found in your neighbourhood grocery store.
The one on the right makes no bones about containing sugar. The one on the left has a prominent label advertising “No Added Sugar”. It also has a ** against “sugar” in the nutrition facts that the fine print clarifies as “sucrose”. That is, 1.4% of the product is sugar in the form of sucrose.
But go back to the ingredient list. Notice the unspecified quantity of “Apple Juice Concentrate”. The nutrition facts for apple juice say it is 10% sugar. A research study on the exact composition estimates (in grams per litre) 9.30–32.2 glucose, 66.10–96.00 fructose, and 8.5–55.10 sucrose. The range is because multiple varieties of apple were tested. Apple juice is mostly fructose. And fructose is a sugar! Fructose is the sweeter half of the sucrose molecule! In other words, the “no added sugar” claim is a lie because it redefines sugar as sucrose for the purposes of this label, while also refusing to identify exactly how much actual sugar is in this product.
Conspiracy aside, what exactly is the problem with fructose?
Your muscles and brain are powered by glucose. They cannot do anything with the fructose in your diet until your liver processes it. Fructose taxes your liver similar to how alcohol does.
When you consume sucrose, which is table sugar, it gets as far as the duodenum – the first part of the small intestine – where the enzyme sucrase splits it apart into the constituent glucose and fructose. This is why eating sugar or drinking any sweet juice gives you a rush of energy – the glucose goes into your blood and from there to your brain and muscles. Fructose is absorbed into the bloodstream along with glucose (and galactose), from where it is processed and stored by the liver.
This idea that different compounds are processed in different manners is called a metabolic pathway. There are multiple pathways around your body for the various components of your diet, and some of them are activated or deactivated based on certain conditions. Starvation, for example, triggers a metabolic state called ketosis wherein your body starts burning its own stored fat, using the produced ketone bodies instead of glucose for energy. This is the evolutionary purpose for which our bodies store fat: as a store for lean times. It is the same reason a camel has a hump, except their metabolism handles it differently.
There is something about the way fructose is metabolised in our bodies that has pretty nasty side effects. Dr Robert Lustig at the University of California has spent nearly two decades studying childhood obesity and is convinced fructose is the culprit.
He’s not alone.
Science author Gary Taubes has joined the fray with his new book The Case Against Sugar.
But do not just read press reports about sugar. Read Dr Robert Lustig’s paper or take out 90 minutes from your weekend to hear him as he takes you into a deep dive of exactly what fructose does to your body.
That was in 2009. In 2013, Lustig delivered another lecture addressing the criticisms of the first lecture.
Finally, we need to address the oft-stated refrain that it is only refined and added sugars that are bad for you while natural sugar is fine. This is false. Whether your fruit happened to be naturally sweet or you sprinkled some sugar on it, how does it matter to your digestive tract? The only distinctions that matter are the ratios of fructose and glucose, plus the role of fiber in slowing down digestion.
- Fruit: 40–55% fructose (but sometimes significantly higher or lower)
- Honey: 38–55% fructose
- Refined sugar (sucrose): 50% fructose
- HFCS (used in industrial food products): 42%, 65% or 90%
You are getting about the same amount of fructose no matter where your sugar comes from. Obsessing over added sugar is pointless because the problem is with all sugar. This leads to an inevitable conclusion: don’t drink fruit juice. You’re getting all that fructose as one gulp of a sugar bomb with none of the fiber required to temper it. Eat the whole fruit instead.
Hands-up if you ever felt good about going to a fruit juice stall instead of getting a soda. Fruit juice may be revered as a health food, but you got the same fructose dosage anyway. Do you see how culturally ingrained our sugar habit is, and how hard it will be to lose?
This article was first published on Kilter.