This piece was written by one of contributors; medical student w/ BSc in human nutrition and MSc in clinical and public health nutrition – Rebecca Fox.
A spoonful of sugar: it helps the medicine go down, right?
Well, the NHS recommends consuming no more than 30g of free sugars per day (about 7 sugar cubes or 7 teaspoons worth of sugar) . However, according to the most recent UK National Diet and Nutrition Survey, UK adults take in an average of 64.3g of free sugar per day for men, and 50g per day for women  – well over what the NHS (and Mary Poppins) recommend.
But what are “free sugars”?
“Free sugars” are defined by the Scientific Advisory Committee on Nutrition as sugars that are “added to foods by the manufacturer, cook, or consumer, plus sugars naturally present in honey, syrups and unsweetened fruit juices” . Oftentimes, they are baked into biscuits or added inconspicuously into otherwise healthy foods like yoghurt. They take many forms and are referred to by many different names on food packaging labels, such as: dextrose, fructose, lactose, sucrose, honey, invert sugar- and many more!
However, free sugars are not to be confused with the sugars naturally found in fruits, vegetables, and milk. Sugars that are found in these foods do not need to be limited in your usual diet (except in certain individual medical cases) because these foods are very nutritious and these sugars don’t hit our bloodstream as quickly as free sugars.
How might they impact our gut?
Most of what we eat (including most sugars) gets digested and absorbed in the small intestine, which is made up of three parts: the duodenum, jejunum, and ileum- each with slightly different absorptive functions. Then, whatever hasn’t been absorbed in this region makes its way into the large intestine, where it is fermented by our gut bacteria (or is excreted).
There are thousands of different types of bacteria in our gut, comprising our gut “microbiome”. Each person also has a different gut microbiome composition. This means that we all have a variety of bacteria that depends on individual factors such as our environment and what we eat. The individuality of each person’s gut microbiome may make it difficult to study scientifically, although much research is currently being done to understand how our gut microbiome is linked to our overall health.
Some types of bacteria have been identified as likely having a “protective” role in our gut. These protective functions include (but are certainly not limited to!) creating chemical messengers called cytokines that influence other organs and body systems, such as the way our body responds to insulin . On the other hand, there are some bacteria that, if present in large numbers, may be harmful for the gut and surrounding tissues .
So where does sugar fit into all of this?
Many links have been made between what we eat and types of bacteria that make up our microbiome . Bacteria require a source of energy in order to proliferate. However, different bacteria prefer different types of energy derived from broken down carbohydrates, protein and/or fat- most of which comes from the food we eat.
Dietary fibre, for example, is a type of carbohydrate. There are many different types of fibre-some of which are fermentable and others which are not. The structure of dietary fibre makes it difficult for the enzymes in our small intestine (where carbohydrates are mostly absorbed) to break down. Therefore, dietary fibre makes its way into the large intestine where fermentable fibre is fermented by bacteria (and the non-fermentable fibre is excreted). Short chain fatty-acids (SCFA’s) are an important product of this fermentation process. SCFA’s such as butyrate may improve colon health as they provide both a source of energy for other good bacteria, as well as energy for the cells lining the intestinal wall [6, 7]. This is one of the many reasons why the NHS recommends consuming around 30g of fibre per day .
On the other hand, free sugars are hypothesised to have a different effect on the bacteria in our gut. These free sugars are normally largely absorbed in the small intestine rather than making their way into the large intestine. However, more recently, it has been hypothesised that consuming excess amounts of free sugars may lead to an overgrowth of pro-inflammatory (or “bad”) bacteria in the colon .
One study published in Scientific Reports showed that a high sugar diet may increase susceptibility of a type of inflammatory bowel disease called colitis, in mice . The researchers showed that mice fed a high sugar diet for only two days had increased gut permeability and inflammation. One of the reasons for this, posited by the authors, was that having a diverse range of bacteria present is likely an important factor in maintaining a healthy gut. However, in this case, only one type of sugar was given to the mice, and may therefore have limited the different types of bacteria that could have normally grown on a more diverse diet. Different bacteria interact differently with each other and may produce distinct signals and products when they are in each other’s presence . For example, having a greater diversity of bacteria may increase the types of SCFA’s that are produced; thus aiding gut health. This is an interesting study, but of course we are not mice so research investigating this in humans is needed.
Similarly, another hypothesis has been put forward regarding the body’s response to excess intake of refined sugar. Since most sugars are absorbed in the small intestine there isn’t much time or available substrate for bacteria to grow there. However, if excessive amounts are consumed, this may lead to elevated growth of pro-inflammatory bacteria in the small intestine . This overgrowth of pro-inflammatory bacteria may be associated with increased inflammation and potentially causing an impaired immune response. However, this theory has not been experimentally validated in humans and remains at the stage of hypothesis.
What about artificial sweeteners?
Just like other components of our diet, artificial sweeteners may also play a role in altering our microbiome, though there have been many mixed findings. On one hand it has been shown that continued consumption of artificial sweeteners may change the composition of gut bacteria [13,16,17,18]. One study in particular found that giving Splenda® to mice for six weeks appeared to elevate numbers of pro-inflammatory bacteria, in addition to increased inflammation in the gut . Similarly, it has been found that consuming aspartame may change how glucose is metabolised and tolerated studies have largely been conducted in animals .
Now, it is important to note that artificial sweeteners can be a useful sugar substitute for health reasons such as dental health or diabetes. So, while there may be some evidence to suggest that artificial sweeteners may impact the gut microbiome, most of this research has been done in animals and remains inconclusive.
Conclusions and Take-Home
- Dietary fibre is a type of carbohydrate that has been shown to be beneficial for our gut microbiome and overall health.
- The jury is still out on how refined sugars directly impact the gut microbiome, although current research has shown that consuming large amounts may be harmful for the diversity of the gut microbiome. This is in line with the current guidelines that call for limiting our intake of refined sugar to 30g per day for adults .
- Artificial sweeteners may also have a negative impact on the health of our gut microbiome, although further research is needed to quantify these effects.
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