The Diet Therapist
How are different forms of sugar metabolised?
Are natural sugars "healthier?"
Posted by The Diet Therapist, 18th March 2017
The Simple & Complex...
When people hear the word “sugar” – many instantly think of a bowl of white sugar, which is sucrose. Sugar comes in lots of different forms however, and these forms are classified according to chemical structure. “Simple sugars” are known as monosaccharides because they are made up of single sugar molecules (e.g. glucose, fructose and galactose). If two simple sugars are joined together with a chemical bond they are then called disaccharides – the most common we know as table sugar (sucrose) or lactose in milk (which is made up of galactose + glucose), or maltose in cereals (which is two molecules of glucose). Fibre and starches have lots of simple sugars joined together, and any carbohydrate that is made of more than 2 simple sugars is called a polysaccharide. So longer polysaccharides would make up much of the sugars present in plant-based sources like root vegetables & pulses.
* All sugars have to be broken down to monosaccharides to be absorbed – glucose, galactose or fructose.
There are a ton of different forms of natural and added sugar which have different chemical structures. When you are looking at ingredient lists, all of the following are types of sugar:
fruit juice concentrate
high fructose corn syrup
So…Do we metabolise different sugars differently?
All complex carbohydrates have to be broken down into simple sugars like glucose and fructose before they can be absorbed and used by the body. Often you will see articles looking at the chemical composition of different sugars and their ratio of glucose-fructose.
This is because glucose and fructose are not metabolised in the same way. Though they actually have the same chemical formula, their atoms are arranged slightly differently which influences how we taste their sweetness and how they are processed.
Every cell in your body can use glucose which is why it is the main energy source for your body. Glucose is taken up by tissues once absorbed into the bloodstream and your cells burn it as fuel. In contrast to glucose, fructose is mostly taken up and metabolised by the liver.
Excess glucose and fructose will be converted to fat and stored by your body, however, research has shown that fat made from glucose is more likely to be stored in fat tissue, whereas fat accumulation from too much fructose is more likely to accumulate In the liver and is associated with other hormonal and metabolic disturbances and non-alcoholic fatty liver disease. However, the research which has uncovered the biochemical mechanisms behind this tend to use high doses of pure fructose to look at metabolism; it’s important to remember that in nature, fructose exists within lots of food matrices including fruit. This is significant because the fibre in foods changes the way that the fructose is absorbed**
So: too much of either has health consequences.
Why have I read that certain sugars are “healthier” than others then?
Different sugars will contain different ratios of glucose to fructose which affects how they are absorbed and metabolised. For example, honey and maple syrup have quite different chemical profiles.
Honey is approximately 50% fructose, whereas
Maple syrup is predominantly sucrose (fructose + glucose).
Agave is approximately 80% fructose
Molasses are thought to be about 55% sucrose and 25% fructose,
Granulated white “refined” sugar (table sugar) = glucose 50% + fructose 50%
Honey = about 41 % fructose,
The theory that excessive fructose is damaging to health and leads to metabolic disorders - high fructose sweeteners like honey and agave shouldn’t be consumed in excess. Although originally many of the theories around fructose metabolism have looked at the administration of pure fructose, sucrose and High Fructose Corn Syrup (HFCS).
HFCS is a liquid sweetener commonly used in the US but rarely by British manufacturers. It is produced using corn, is about 55% fructose and is cheaper than sugar which is why it progressively replaced sugar in lots of products in America over the past 30 years. If you listen to Dr. Robert Lustig’s talk here > he talks more about fructose metabolism and HFCS in the US.
Glucose-Fructose Syrup (GFS) & Fructose-Glucose Syrup (FGS)
I’ve seen this in the UK on processed foods and it’s basically derived from glucose-syrup (in a similar process to HFCS) made from a form of starch like corn or wheat.
The fructose content of “glucose-fructose syrup” can range from 5 to 50% - but if the fructose makes up more than 50% of the syrup it has to be listed as “fructose-glucose syrup.” GFS is found in processed products like Kipling’s cakes, McVitie’s Jaffa Cakes and Biscuits and jams.
What about Fructose in Fruit? Does this mean I shouldn’t eat it?
Many of the research projects looking at fructose metabolism used “hypercaloric” conditions (effectively over-feeding experiments), and large amounts of pure fructose which has a different metabolic effect to that found naturally occurring in fruit.
in our everyday diet where we encounter fructose bound in fruits or honey, this isn’t thought to have the same adverse metabolic effects – either because of the impact of dietary fibres or antioxidants in honey.
What about the Glycaemic Index values of different sweeteners? Isn’t low Glycaemic “better?”