Fructose is the sweetest of all sugars and is naturally found in fruit and honey. Table sugar or sucrose, is made up of equal portions of glucose and fructose. Over recent years we have seen a huge rise in the amount of sugar in our diets and sugars, including fructose, are heavily used by the food industry. But fructose is now considered a major contributor to the rise in obesity and diabetes and a primary cause of non-alcoholic fatty liver disease (NAFLD). So should we cut down on eating fruit and drinking fruit juice? Here we take an in-depth look at what makes fructose unique amongst sugars, why it puts us at risk of metabolic health problems, and suggest ways we can cut down.
What is fructose?
Fructose is a type of sugar known as a monosaccharide – a single sugar. it occurs naturally in fruits, but is also found in foods such as honey, sugar beets, sugar cane and, in very small quantities, in vegetables. It is also a component of table sugar or sucrose, which is made up in equal parts of glucose and fructose. Fructose has four calories per gram, which is the same as other sugars. However, fructose is the sweetest naturally occurring carbohydrate, being 1.2 to 1.8 times sweeter than table sugar and it is used extensively by the food industry as a food sweetener.
What’s the problem with fructose?
Whilst we’ve always eaten fructose, the amount of fructose we eat today is far greater than it would have been even forty years ago. Fructose is commonly known as fruit sugar, and found naturally in fruit and vegetables, but it also a component of sucrose, or table sugar, and is also commonly used by the food industry in processed foods. With the rise in ultra-processed foods, we have seen a huge rise in sweetened foods in our diets. Traditionally it is thought that humans would have eaten between 16-20 grams of fructose per day. However the average consumption in the US is now estimated to be between 85-100grams per day. We may eat slightly less fructose in the UK as the food industry use a different formulation of corn syrup in foods, however, consumption remains very high. This is a problem, as it seems that the unique way in which fructose is metabolised makes it worse for our health than glucose.
What is unique about fructose?
Fructose is metabolised in a different way to glucose and other sugars. Rather than getting absorbed into the blood stream, fructose travels to the liver and undergoes a different form of metabolism, which involves an enzyme called fructokinase.
Fructose has no effect on blood glucose and does not trigger the release of insulin, which all sounds very positive, however the process of fructose metabolism has some very negative consequences.
a) Depletion of energy
Normally the process of metabolising our food uses up some energy, but then creates more energy. ATP (adenosine triphosphate) molecules, which are energy molecules, are used up during the process of metabolism. However, more ATP is then generated, and eating food usually generates more energy than it uses. However, this is not the case with fructose and instead fructose metabolism uses up ATP very rapidly, causing ATP levels to fall steeply and energy to be reduced. Instead of producing energy, this particular metabolic pathway leads to the production of uric acid which causes oxidative stress in the mitochondria. So, fructose metabolism depletes energy and produces uric acid as shown below.
Diagram showing how fructose is metabolised in the liver
Adapted from the book Nature wants us to be fat by Richard. J. Johnson.
b) Fructose metabolism leads to uric acid which leads to fat storage
The uric acid produced by fructose metabolism can cause gout, but it also has a number of other detrimental effects:
It causes oxidative stress in the mitochondria, and this further reduces energy production
It stimulates the production of fat for storage
It blocks the burning of stored fat
Diagram showing impact of uric acid on mitochondria
Adapted from the book Nature wants us to be fat by Richard. J. Johnson.
c) Fructose, non-alcoholic fatty liver disease and metabolic health
Fructose is uniquely placed to help us store fat, which is why it is thought to be a major contributor to the rise in obesity. At a population-level, the increase of fructose in our diets over recent years tracks closely with the rise in obesity, and also to the rise in type 2 diabetes and other metabolic diseases.
The risk of non-alcoholic fatty liver disease (NAFLD) rises in proportion to the consumption of fructose - the more fructose you eat, the greater your risk of NAFLD. Fructose is now considered the primary cause of NAFLD.
The build-up of fat in the liver initiated by high fructose levels can ultimately lead to insulin resistance, type 2 diabetes and other metabolic health problems.
Fructose also slows metabolism, causes appetite dysregulation and leptin resistance – leptin is the hormone which usually tells the brain when we are full after eating.
Diagram to show link between high fructose and metabolic health issues
The Metabolic Survival Switch
Given that fructose metabolism reduces energy and causes fat storage it appears to be pose a distinct disadvantage to humans. So why have humans evolved with these adaptions? Researchers now believe that genetic mutations in the past made humans more sensitive to the effects for fructose to help them to survive during colder times. Through these genetic changes, the fruit and honey our ancestors ate could be converted into fat and stored, which would have helped them to survive harsh winters when food was scarce. However, today, when we consume large quantities of fructose, this leads to obesity and other metabolic health problems.
Fructose is also used by other animals as a way to store fat and to help them survive difficult times in a similar way to how our ancient ancestors may have needed to store fat. For example, the fat tailed dwarf lemur gorges on fruit to help it survive periods of drought.
Ways to cut down on fructose
There are four ways to reduce fructose in your diet:
- Minimise ultra-processed foods
- Cut right down on added sugars
- Eat fruit but don’t drink it
- Minimise dried fruit
1. Minimise ultra-processed foods
Many ultra-processed foods contain a great deal of sugar and fructose, so it makes sense to keep these foods to a minimum. Many ultra-processed foods contain syrups made of a combination of glucose and fructose:
High Fructose Corn Syrup (HFCS) is a sweetener made from corn starch – used in many highly processed foods in the United States and two main types are used:
HFCS 42 = 42% fructose, 58% glucose – used in foods, e.g cereals
HCFS 55 = 55% fructose, 45% glucose – used in soft drinks
Glucose-fructose syrup (GFS) or iso-glucose is a sweetener made from corn and wheat starch and used by the food industry in Europe and the UK. It is made out of a combination of 20-30% fructose and 70-80% glucose. It is commonly found in biscuits, cakes and ice-cream.
In the UK and Europe soft drinks are mainly sweetened with sucrose, which contains 50% fructose, rather than Glucose-fructose syrup.
Minimising intake of sweetened foods, especially biscuits, cakes and ice-cream along with soft drinks will help you to reduce fructose in your diet.
2. Cut right down on added sugars
Table sugar, or sucrose is 50% glucose, 50% fructose, so to reduce your intake of fructose you need to avoid added sugars. Try and reduce your intake of sugar in drinks and in cooking and minimise sweet treats such as cakes and biscuits.
Don’t forget, honey is very similar to sugar, with roughly equal proportions of glucose and fructose. Agave syrup is worse, as it is approximately 80% fructose.
Be aware of hidden sugars in foods and check labels – it’s surprising how often sugar turns up in the most unlikely places, like tomato sauce or mayonnaise!
3. Eat fruit, but don’t drink it
All fruit contains some fructose, however when you eat whole fruit it comes with fibre which slows the release of fructose and lessens it’s impact. Whole fruit are also beneficial as they contain vitamins and phytonutrients.
Avoid eating too many highly sweet fruits, i.e tropical fruits, as they have a high glucose content which can raise blood glucose.
Try and limit fructose to no more than 8 grams of fruit in a single portion. For example, choose a small apple or pear, or just half a piece of fruit in order to limit the impact of fructose.
The fructose content of common fruit is shown below, along with their carbohydrate content. Apricots and plums have the lowest fructose content, followed by berries, with grapes and pears containing the highest levels.
Fruit juice
Fruit juices are high in fructose, with apple juice and grape juice having the highest amounts – grape juice has nearly 32g in 450 mls. Even a very small glass (150g) of grape juice contains nearly 11g of fructose. Fructose consumed as juice is also rapidly absorbed within the body, and can overwhelm the liver. It is better to eat fruit than to drink it, but if you do want to drink a little juice, orange is the best option.
4. Avoid dried fruit
All dried fruit have a very high fructose content, as the process of drying the fruit concentrates the sugars. Dried fruits are also very high in carbohydrates, so are likely to raise blood glucose.
In conclusion
Our ancestors benefited from genetic changes that enabled them to convert fructose to stored fat and to slow down metabolism to conserve energy to survive harsh times. Now we live in a time of plenty and these genetic adaptions are no longer serving us well! So whilst whole fruit can form a part of a healthy diet, avoid eating added sugars, drinking soft drinks or fruit juice and save the dried fruit for Christmas!
Further reading/resources
'Nature wants us to be fat' - Richard J Johnson, MD, BenBella Books available from Amazon at https://bit.ly/46edPjI
Johnson, R et al Fructose metabolism as a common evolutionary pathway of survival associated with climate change, food shortage and droughts https://onlinelibrary.wiley.com/doi/pdf/10.1111/joim.12993
Peter Attia interview with Rick Johnson https://peterattiamd.com/rickjohnson2/
DiNicolantonio JJ, O'Keefe JH, Lucan SC. Added fructose: a principal driver of type 2 diabetes mellitus and its consequences. Mayo Clin Proc. 2015 Mar;90(3):372-81. doi: 10.1016/j.mayocp.2014.12.019. Epub 2015 Jan 29. PMID: 25639270.
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