Sugar: The Body’s Response And The Gut Microbiome Episode 2: Fructose



Glucose is used by every cell in our body. Fructose can ONLY be processed by the liver, where it is converted to glucose (glycogen) and fat. Fructose is a sneak and can lead to obesity and metabolic confusion. It sneaks fat around our vital organs where we can’t see it. Excess fructose also disrupts our gut microbes, causes inflammation and leaky gut syndrome. All of which are undesirable, to say the least. And food producers are hiding fructose (high fructose corn syrup) in so many processed foods and drinks, making it difficult for us to avoid. Read on for more detail.


With all the Christmas Shenanigans behind us, let’s crack on with episode 2 – fructose. For those of you that want to have a quick peek at episode 1 – Glucose.

Fructose is a monosaccharide carbohydrate which, along with glucose, forms sugar. It is found in fruit, vegetables and many, many processed foods, breads, pastries and sauces – often in the form of high fructose corn syrup (HFCS).  Unlike glucose, fructose is metabolised solely by the liver because only the liver has the transporter for fructose. This is worth focussing on – whilst almost every cell in my body can use glucose, ONLY my liver can deal with fructose. All fructose goes to my liver, where the mitochondria convert it to glucose (to be stored as glycogen) and FAT. My liver works its socks off converting fructose to fat.

Because fructose doesn’t directly trigger the hormones insulin and leptin (which regulate metabolism and feelings of fullness), diets high in fructose can lead to obesity and metabolic confusion. The cascade of hormones evolutionarily honed to occur in response to nourishment is confused by fructose and that causes interference with the normal appetite signals to the brain. Drinking too much fructose/glucose seems especially problematic, probably because our evolutionary response to nourishment is based on chewing triggering a chain of appropriate hormonal responses. Sweet liquid drinks bypass that whole process. When you give fructose to animals, they lose their ability to control their appetite, they eat more and they exercise less.

Gary Taubes, who has just published The Case Against Sugar, argues that it is not just about eating too much of this stuff, it is the fact that fructose (and glucose) has unique physiological, metabolic and hormonal effects on the human body that trigger diabetes and obesity. To him, it is a toxin and should be minimised in our diet.

Fibre in fruits and vegetables slows down the absorption of fructose into my bloodstream. Unprocessed fruit and vegetables also contain many other important nutrients that are needed for good health. The problem is the fructose hidden in highly processed foods, sweet foods, foods containing HFCS – the hidden fructose in our diet. Fructose metabolism drives more liver fat than we can export and eventually results in fatty liver disease. The fatty deposits around the liver stop the liver cells responding to insulin in the blood (ie the liver becomes insulin resistant) which drives the pancreas to produce more insulin. Insulin promotes fat formation and retention, which further prevents the liver responding to insulin in the blood – a vicious circle.

Non-alcoholic fatty liver disease (NAFLD) – the most common chronic liver disease in adults and children is basically the same disease as affects alcoholics but without the alcohol (the clue is in the name….). Certain dietary sugars, particularly fructose, are suspected to contribute to the development of NAFLD, obesity, insulin resistance and inflammation.


microbes and fructose - not a match made in heaven

Rodents on high doses of fructose develop fatty liver and increase their visceral fat (fat around organs) dramatically. There is concern that this “invisible” fat and the metabolic chaos that fructose causes is going unnoticed in many people, because we can be slim externally but still have a disordered metabolism and gut microbiome, along with high levels of fat deposited around our organs. Not a good combination for long term health. Interestingly, this has been shown to be reversed in rodents by using antibiotics, which suggests that our gut microbes are involved in some way. The antibiotics kill the gut bacteria that are facilitating the fructose/visceral fat cycle. It is likely that the gut microbiome is a crucial interface in this whole process.

From the perspective of my gut bacteria, when I consume excess fructose, it cannot all get absorbed into my bloodstream in the small intestine. This results in it reaching my large intestine where the pathogenic gut microbes think their ship has come in. Because it has! The unhelpful bacteria flourish on this food supply at the expense of the helpful bacteria and thereby disrupt the gut microbial balance. The pathogenic bacteria ferment the fructose and produce short chain fatty acids (SCFAs) and gases including methane, hydrogen, carbon dioxide and hydrogen sulfide. This can cause bloating, discomfort and pain. Also, methane gas is not inert – it is biologically active and can disrupt the work of the colon, causing pain and constipation.

Fructose has also been shown to significantly increase circulating lipopolysaccharides (LPS). LPS is a major component of the outer membrane of certain bacteria. Somehow, and most likely through its negative impact on the balance of the gut microbiome, fructose may be increasing gut permeability thereby allowing bacteria that belong in our gut to get into our blood stream. As far as our immune system is concerned, in that location, bacteria are trespassers. Their presence in our blood increases the production of inflammatory chemicals in our blood stream. Such inflammation, if prolonged, is associated with all sorts of illnesses including depression and dementia. Find out more about Inflammation - The Source Of Most Woes.

The amino acid tryptophan is a crucial precursor to the production of serotonin (the happiness neurotransmitter).  Tryptophan likes to latch on to fructose during digestion. When there is so much fructose in our gut that most of it can’t be absorbed into the blood, we lose that fructose to our colon and we lose the tryptophan attached to it too. Lack of serotonin is a factor in depression. Serotonin also has a role in making us feel full after a meal so fructose could be interfering not only with our mood but also with our feelings of fullness via depleted serotonin production.


so what?

By way of cheerful conclusion then, fructose is implicated in causing disruption to our gut microbiome, inflammation, leaky gut, fatty liver disease, fatty organs generally, obesity, insulin resistance, diabetes, metabolic and hormonal disruption affecting appetite control as well as affecting our mood and, possibly even our risk of suffering from depression and dementia.

That list is significantly reducing fructose’s appeal to me intellectually. The challenge is reducing its appeal to my tastebuds too!


As with glucose, we need to reduce our intake of highly refined processed foods in which food producers hide fructose. Look out for high fructose corn syrup on the label. 

We need to reduce the number of things we eat that even HAVE a label.

Ditch the sweet drinks. Including fruit juices.

Fructose in whole fruit is less problematic because it is accompanied by fibre and a range of beneficial nutrients.


Spreading straightforward information about the fundamental importance of a healthy gut microbiome.