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A different perspective on sugar

07 July 2021

Not all sugars are the same. Sugar expert Marthe Walvoort is researching how the structure of sugars corresponds to their function in the human body. ‘Even fruit contains unhealthy sugars.’

Text: Nienke Beintema; Photo: Reyer Boxem; Source: Broerstraat 5

There are healthy fats, such as those in fish and nuts, and unhealthy fats, such as those in fries and hamburgers. But that there are also healthy and unhealthy variants of sugar is less well known to people. ‘People think, for example, that the sugars in fruit are healthy,’ says Marthe Walvoort, Associate Professor of Chemical Glycobiology at the UG. ‘But that is a misconception. They are just as unhealthy as the sugars in cola.’

Marthe Walvoort
Marthe Walvoort

Good sugars

That’s quite a shock - especially if you’re aware that orange juice has just as many sugars as cola. Here is another misconception: that added sugars are always bad. ‘Of course not,’ says Walvoort. ‘All that matters is what kind of sugars these are.’ Bottle-feeding for babies contains added sugars that are actually very healthy.’ So how does that work? To answer that question, she dives into the chemistry of sugars – her expertise. ‘Yes, so I am not a nutritionist,’ she emphasises. ‘I am a chemist. I do research on sugars and their properties from a chemistry perspective.’

A ring of carbon atoms

Sugars, Walvoort explains, are molecules that are mostly made up of carbon, hydrogen and oxygen atoms. The carbon atoms form a ring with a single oxygen atom inside of it. The remaining atoms are attached to it in side chains These side chains determine the character of the sugar in question. Not only is their composition important, but also their placement within the ring: do they extend upwards or downwards? This is how you can turn the base molecule of glucose (with six carbon, twelve hydrogen and six oxygen atoms, usually written as C6H12O6) into eight different variants by changing how the side chains attach to the ring – and into even more different types of sugar by closing the ring at a different spot. That is why you can for example have fructose, which shares the formula of C6H12O6.


These rings can also be combined with each other. If you combine a glucose and a galactose molecule, you get lactose. And glucose with fructose yields sucrose, also known as saccharose: the sugar that you put in your coffee, derived from sugar beets and sugar cane. Our body converts all these different types of sugar into glucose. ‘That is the form your body can actually work with,’ says Walvoort. ‘Glucose is an important fuel for your muscles and your brain.’ But it is unhealthy when you consume too much of it, which is very easy to do. Your body turns any excess energy into fat. Moreover, too much sugar can result in distorted insulin levels, which can eventually lead to cardiovascular disorders and type 2 diabetes.

Too much fruit?

‘Nature has traditionally made sure that we don’t consume too much sugar,’ Walvoort explains. ‘Fruit for example was scarce and seasonal. And fruit contains a lot of fibre, which fills you up very quickly.’ Just try to eat three oranges in a row.’ But nowadays sugars are available at all times and everywhere. And if you squeeze the juice out of oranges, you lose the fibre and are mostly left with sugar. ‘You do get all the sugar and energy from those three oranges at once.’

Bacteria help out

You find lactose mostly in breastmilk. Isn’t that bad? ‘No, the baby needs those sugars because they’re growing so fast,’ Walvoort answers. But breastmilk contains more than 200 complex sugars besides lactose, that are all made out of two to five different sugar components. ‘The baby cannot digest those complex sugars in the usual way. That is the crucial difference between a sugar that can be harmful and one that cannot: can you digest the form easily or not? Complex sugars arrive through the mouth, stomach and small intestine wholly intact in the colon. Bacteria live there that can use those sugars as fuel. These are good bacteria, Walvoort emphasises: bacteria that help with the digestion of food, that produce vitamin K and play an important role in the defence against pathogens. ‘Good bacteria drive off the bacteria that make you sick and produce all kinds of useful substances, such as acids, which are harmful to pathogenic bacteria and substances that stimulate our own immune system.

Useful breastmilk

Adults can help their own good bacteria by eating a lot of fibre, such as those in grains and vegetables. Many of these fibres are themselves useful sugars. Babies get theirs through breastmilk. Especially during infancy, good bacteria could use a helping hand as a baby is still in the process of developing their gut flora. The immune system is not fully developed either. Walvoort: ‘Piece by piece, we are discovering what all the sugars in breastmilk are used for.’

Reproducing sugars

To better research these functions, Walvoort and her colleagues are working on methods to reproduce these sugars in a lab. That way, you can produce greater quantities in a pure form, but reproducing these sugars is difficult. ‘Researchers are much further along with proteins and DNA, but sugars are chemically speaking much more complex, because they are composed of many different components. In addition, you have all kinds of branched breastmilk sugars. We think that these side chains are especially important to the functions of these sugars.’ Researchers can now gradually research precisely which sugars or sugar chains cause this effect against pathogens and which stimulate the immune system. Walvoort: ‘Once we know this, we can maybe develop positive additions to bottle-feeding.’ She notes that those already exist to an extent. Higher quality bottle-feeding already has so-called galacto-oligosaccharides and fructo-oligosaccharides. These sugars are also known as prebiotics: they stimulate the helpful bacteria, which are known as probiotics. ‘But breastmilk contains many more varieties of sugar, with many more functions. We want to specifically reproduce those.’

First test

Walvoort and her colleagues, together with Wageningen UR and the UMCG, have already successfully reproduced and tested their first anti-pathogen sugar. This artificial sugar appears to inhibit the growth of harmful bacteria. ‘It is a proof of principle which we are very pleased with,’ according to Walvoort. ‘Now, of course, we want to produce more sugars. Maybe you don’t even need the whole sugar for this beneficial effect, but just a part of it. The possibilities are legion. This is truly an exciting time.’


Marthe Walvoort (1983) studied chemistry in Leiden, before she became a researcher at Oxford and returned to Leiden for her research project, for which she received her PhD with distinction. After a Postdoc at the Massachusetts Institute of Technology (Cambridge, US), she was attracted to the UG at the end of 2015 as a Rosalind Franklin Fellow. Since the start of 2021, she has been the Assistant Professor of Chemical Glycobiology at the Stratingh Institute of Chemistry of the Faculty of Science and Engineering. She is leading a group of six PhD students. In addition to this, she is a co-founder and member of the Young Academy Groningen (YAG).

Last modified:05 July 2021 2.03 p.m.
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