Sugar has been hitting the headlines over the last decade or more, singled out as a cause of many ills that don’t just stop at rotting teach. There are also links with diabetes to contend with, obesity, cardiovascular disease and even cancer. For context, this focus on sugar has come from a combination of directions.
Firstly, observing populations, particularly in the US, it appears that the rise in obesity seen over the last 30 years didn’t really coincide with calories or fat consumption per head, but seemed to mirror an increase in the consumption of sugar, particularly sugar-sweetened beverages.
It also coincided with the introduction and wider use of high fructose corn syrup (HFCS) in foods in the US. At the same time, around the turn of the millennium, amid growing criticism over our focus on fat in the diet, a new hypothesis of obesity was proposed called The Carbohydrate: Insulin model (CIM).
The CIM placed carbohydrates, and especially sugar, at the centre of what is driving obesity and insulin resistance, and, by association, cardiovascular disease and other diet-related conditions. Championed by the high-profile journalist Gary Taubes and physicians such as David Ludwig and Robert Lustig, the movement culminated in a series of bestselling books and a new wave of diets focusing on low carbohydrate, and low sugar diets.
Nevertheless, this was not necessarily new thinking, as John Yudkin’s book Pure, White and Deadly back in the 1980s really set the scene, and Dr Atkins had already put low carb on the map in the 1990s.
Here we take a comprehensive, technical look at how we metabolise different sugars, the problem that can arise, and the situations where sugar can actually come in rather useful.
Metabolic Effect Of Sugar And HFCS
There is some clear metabolic effect to consuming excess amounts of sugar, explained mainly by knowing what “sugar” actually is. To all intents and purposes what we are looking at is added sugar, rather than naturally occurring sugars like fructose in foods such as intact fruit.
Added sugar typically refers to either sucrose, along with HFCS in the US. Sucrose is a disaccharide and is a pairing of glucose, the major free sugar circulating in our blood, and fructose, at a ratio of 50:50. Similarly, HFCS is glucose and fructose with a slightly different ratio of 55:45 fructose to glucose.
The potential issue here is when you get the rapid absorption of fructose and glucose at the same time in the liver. Although both glucose and fructose are absorbed in the same way, once in the liver fructose follows a different metabolic path to that of glucose.
Unlike glucose, the metabolism of fructose in the liver does not come under the same controls as that of glucose, whose metabolism in the liver and all tissues is controlled by the action of insulin. All fructose rapidly taken up by the liver is quickly converted into fructose-1-phoshate by the enzyme fructokinase; a far quicker entry and conversion compared to glucose.
The creation of fructose-1-phosphate depletes the liver of lots of phosphate, so to cope with this, the liver ends up producing the by-product uric acid to recycle phosphate. The resulting uric acid is released into the bloodstream and can lead to raised blood pressure and gout.
The rapid creation of lots of fructose-1-phosphate also has the knock-on effect of retaining more glucose in the liver as it keeps the enzyme that controls glucose entry active. Most of the fructose-1-phosphate itself ends up being converted into glycogen or glucose, further increasing levels of glucose inside the liver and increasing liver glycogen stores. The increase in glucose inside the liver due to fructose will lead to a prolonged increase in blood glucose and hence a prolonged insulin release.
The Creation Of Fat From Sugar
It doesn’t end there though, as a proportion of the fructose-1-phospate ends up converted into fatty acids in the liver by the process of de-novo lipogenesis, literally meaning “new fat creation”. In other words, you are creating fat from sugar. These fatty acids are not broken down for energy in the liver because there is plenty of glucose, so instead, they are packaged up and exported out into the circulation as new, very low density lipoproteins (VLDLs), contributing to a state of dyslipidaemia or disturbed blood lipids.
Some of these fatty acids also end up accumulating inside the liver itself which over time can contribute to fatty liver or non-alcoholic fatty liver disease (NAFLD). The presence of excess fatty acids and other by-products of fructose metabolism in the liver are likely to cause local inflammation which in turn interferes with insulin receptors on the liver, making your liver less insulin sensitive. The result of all of this is artificially high blood glucose, insulin resistance and dyslipidaemia, all the hallmarks of developing metabolic disease.
Sugar By Itself Is Less Of The Issue
Now all this hardcore metabolism sounds very convincing and is very much supportive of the notion that too much sugar is potentially bad for all manner of reasons. However, we need to put this into a little bit of perspective. Firstly, it is the dual presentation of glucose and fructose together in the liver that is causing the problem, not fructose by itself.
You can see the “toxic” effects of fructose by dosing liver cells with very large doses of fructose, but these are levels not realistic in most diets. In addition, the role of fructose does not actually support the carbohydrate insulin model (CIM) as fructose does not elicit an insulin response.
It has also been argued that actual intakes of sugar, in the UK at least, are still below the very high levels where you get these pronounced metabolic effects through sugar alone. Even in the US, it is only a proportion of people that have such very high intakes. We should recognise that much of the metabolic disturbance you get with sugar intake is not singlehandedly due to sugar but rather the combination of sugar and fat together.
Indeed, in the UK intakes of both sugar and fat, particularly saturated fat, both remain stubbornly higher than recommended. It should also be emphasised that sugar, alongside fat and salt, also contributes to the palatability of food, which can change eating behaviour. Sugary, fatty and salty foods are easier to overeat and therefore contribute to likely excess calories overall.
Sugary drinks can also contribute independently to feelings of hunger and overeating. Nevertheless, recognition for the metabolic “overload” and the negative effect on eating behaviour that sugar can contribute to, has led to the recommendations about free sugar intake being changed.
The Scientific Advisory Committee on Nutrition report on carbohydrates now recommends we consume a frugal five percent of our energy from free sugars, where currently intakes tend to be over 10 percent.
Other Considerations And Potential Benefits Of Sugar
The Benefits Of Sugar When Exercising
There are certain circumstances where a rapid delivery of carbohydrates into the system is useful, none more so than in exercising situations. Traditional dietary strategies in sports nutrition have traditionally focussed on supply of carbohydrate, or more specifically glucose, to the exercising or exercised muscle. This obviously involves strategies of carbohydrate feeding before and after, and possibly during exercise if required. The properties of sugar can be taken advantage of to deliver extra glucose into the circulation very quickly.
We have already seen that both sugar and HFCS lead to a significantly greater export of glucose out of the liver. However, the rates of absorption for sugar are also greater than other forms of carbohydrates, including maltodextrins, which along with glucose relies on a specific transporter for absorption called the sodium glucose transporter 1 (SGLT-1). This is a very fast route of absorption from the gut but there are limits to how much you can transport, due to the limited number of SGLT-1 molecules. Hence this route of absorption is said to be saturable.
In contrast, fructose is absorbed through a different transporter, namely GLUT-5. This is a passive and slow route relatively speaking, but separate to SGLT-1. The upshot of this is that sugar and HFCS use both SGLT-1 and GLUT5 routes of absorption, the combination of which can lead to greater absorption rates overall.
Multiple Transportable Carbohydrates
Modern sports nutrition products have taken advantage of this phenomenon by use of what are called multiple transportable carbohydrates. These sound very technical but simply describe carbohydrates containing fructose and glucose. HFCS and sugar as sucrose are two main examples of ingredients used in this way.
The use of sugars absolutely makes sense in terms of sports nutrition, when you couple this to the known increase in export out of the liver, as it means that the overall rate of delivery of glucose into the system from the gut is higher. Overall, this can be increased from around 1g every minute (60g per hour) for glucose derivatives like maltodextrins, to 1.5g every minute (90g/hour) or more.
Use of these ingredients in sports nutrition products such as drinks and gels is particularly useful in long endurance sports where you want to maximise delivery during exercise. It is also potentially useful in recovery, not just for the delivery of glucose for replenishment of carbohydrate (e.g. liver and muscle glycogen), but also aiding protein synthesis through the stimulus of insulin.
Pre-exercise, use of sugars can be useful as a way of “topping up” blood glucose and stimulating carbohydrate utilisation in preparation of a race or event, however, it can equally be detrimental as it can lead to reactive hypoglycaemia at the onset of exercise in some people.
Looking beyond sports nutrition, is there a role of sugar in the diet generally? Like anything, in moderation sugar itself is not necessarily damaging. Moreover, it depends on the wider diet, the lifestyle, and how and when this sugar is consumed. Whether sugar is eaten within intact fruit, or alongside protein, fibre (and fat to some extent), will alter the rate of absorption and how it manifests in the blood for example.
Put simply, there is a real difference between sugar within a cake or doughnut, versus part of a whole food such as fruit. It is also easier to consume high amounts of sugar through fruit juices, sugar-sweetened beverages, or foods with lots of added sugar. This is partly because it is easier to consume, but also because these foods also have other properties of palatability that make it easy to over-eat them.
Another way of thinking of sugar is really to appreciate what it does first and foremost, then use it accordingly. Think of it as a rapid fuel. If you are always rapidly fuelling, this is not a good way of maintaining a steady fuel supply. It will also mean an over-reliance on you keeping up this rapid fuelling.
Nevertheless, there may be times where this type of fuelling is useful, like around exercise, and sugar may also still be a feature of foods and meals at certain other times. Having a little sugar within a meal or as a dessert may well be less impactful for several reasons than say having a sugary snack or drink in-between meals. Removing sugar altogether from the diet has become a popular goal for some recent diets but thinking in absolutes is not necessarily helpful.