Sugar and Diet

See also: What is Sugar?

Sugars are carbohydrates that have a short chain, generally cyclic structure and are sweet tasting. 

Sugars are found in almost everything we eat, in one form or another and to varying levels. 

Sugars, or simple carbohydrates as they are also known, consist of single sugar units, the monosaccharides glucose, fructose or galactose, or double sugar units, the disaccharides sucrose, lactose or maltose.

See our page: What is Sugar? for more information on each of these types of sugar, as well as common sugar alternatives.

The amount of sugar in our diets is rising and, according to the UN Food and Agricultural Organisation, the average annual consumption of 24kg per person in 1999 is expected to rise to 25.1kg per person by the end of 2015. 

One reason for this increase is that, in the 1970s, dietary fat was linked to heart disease, obesity and high blood pressure and so food manufacturers responded by replacing fat with sugar in many products. 

In addition, evolution has programmed humans to like sugar both to ensure that we have enough energy in our diet and because fructose, a component of sugar, helps us to store fat, which was an important advantage when food was scarce.

The Metabolism of Sugar

So, what happens when sugar is consumed?

The metabolic pathway depends on which sugar has been ingested, but in general the disaccharides initially divide into the two constituent monosaccharide units prior to being absorbed into the body. (See: What is Sugar? for more on monosaccharides and disaccharides)

Glucose is the primary energy source for most living organisms as it can be broken down to release energy more easily than either fats or amino acids. However it is more difficult for the body to store glucose because it is more water soluble. 

Glucose is therefore converted to a less soluble polysaccharide called glycogen for storage in muscles and the liver.

Glucose is absorbed directly through the intestinal wall into the bloodstream, which triggers a release of the hormone insulin from the pancreas. 

Insulin promotes the absorption of glucose into cells thereby reducing the blood glucose levels back to normal.  If energy is required by the cells immediately, glycolysis begins and the glucose is metabolized to release energy in the form of adenosine triphosphate (ATP). 

However in the absence of an energy demand, the glucose gets metabolized into glycogen and stored for later use.  When blood glucose levels drop too low, the glycogen is converted back to glucose either aerobically, which produces more energy per unit of glycogen and is important for endurance activities such as running, or anaerobically, which produces less energy per unit but is a faster process, for activities such as sprinting.

Fructose has a different metabolic pathway to glucose as it cannot be used directly for energy by cells. 

Instead, fructose is absorbed through the intestinal wall and transported to the liver where it is metabolized by one of two pathways: when either blood glucose or glycogen levels are depleted, fructose is metabolized first into glucose and then into glycogen but, if the glycogen stores are full, it is metabolized into triglycerides or fatty acids which are then stored in adipose tissue. 

High fructose consumption, either from excess fruit or fruit juice ingestion or more importantly from the sugar or high fructose corn syrup used to sweeten processed foods and drinks, is therefore a major concern in the fight against obesity.

Galactose is not generally found free in nature in large quantities, but forms part of lactose.  It is less sweet than glucose but can be metabolized into glucose and therefore glycogen when necessary.

Sugar and Health

Sugar has been associated with a number of health issues:

Dental Problems

Extrinsic, or added, sugar is known to contribute to the development of dental caries or tooth decay particularly in children, adolescents and the elderly.

In societies where sugar consumption is less than 60g/person/day, dental caries are rare, and this led the UK to set a proposed guideline that the average intake of extrinsic sugar should not exceed 60g or 10% of dietary energy per day.


Sugar contributes to weight gain in a number of ways.

Sugar provides four calories of energy per gram, but with no additional nutrients.  In addition, the very reason that we evolved to like sugar is that it encourages fat storage. Not only does excess sugar energy get stored as fat, anything else we eat at the same time tends to get stored as fat too.

This wouldn’t be so bad if we didn’t have that much sugar in our diets, but almost everything we eat has added sugar to make it taste nice.


Ingesting sugar has little effect on how hungry we feel.

Absorption of glucose causes the release of insulin which then forces the glucose into your cells. This rapidly reduces your blood sugar levels and leaves you feeling hungry again.

In addition, fructose inhibits the release of the hormone leptin, which tells the brain that we are full.  In the absence of the leptin signal, we never feel full  so we are not prompted to stop eating.

Insulin Resistance

Ingestion of glucose, or the glucose portion of sucrose, causes the pancreas to release insulin into the bloodstream, which facilitates the absorption of glucose into cells.

If there is a continued high glucose intake, cells become resistant to the effects of insulin and need more and more to trigger the glucose absorption, therefore the pancreas releases more insulin in order to maintain glucose regulation.  In an insulin resistant person, a high blood insulin level combines with a fairly normal blood glucose level.

Insulin has an additional role in fat metabolism, promoting the synthesis of fatty acids in the liver when the glycogen stores are full and inhibiting the breakdown of fat in adipose tissue when energy is required.  Therefore high insulin levels contribute to obesity.

Metabolic Syndrome

Metabolic syndrome is an umbrella term that covers a cluster of conditions including obesity or a large waist size, high blood triglyceride levels, low HDL cholesterol levels, insulin resistance and high blood pressure. 

In order to be classified as having metabolic syndrome, at least three of the risk factors must be present. 

Metabolic syndrome increases the risk of developing type 2 diabetes and heart disease.


Type 2 diabetes generally follows a period of insulin resistance.

As the cells become more and more resistant to the effect of insulin, there comes a point at which the pancreas is unable to produce sufficient insulin to trigger the uptake of glucose into muscle and liver cells.  At this point, blood sugar levels become too high and the result is diabetes.

High blood glucose levels can lead to hardening of the arteries and heart disease, kidney disease, blurred vision and nerve damage. Symptoms of type 2 diabetes include excessive thirst, frequent urination, increased hunger and weight loss, although many people do not experience symptoms immediately.


Sugar causes a release of the neurotransmitter, dopamine in the brain in a similar way to drugs such as cocaine, heroin and alcohol, and in a similar fashion dependency can result in needing more and more sugar to get the same good feeling.

For example, many people start off taking half a teaspoon of sugar in their coffee but this may increase to two or more spoons over time.

Studies at Princeton University have shown that laboratory rats fed sugar have exhibited all the classical components of addiction such as bingeing, craving, withdrawal and cross sensitization, and, although the same experiments have obviously not been carried out in humans, many individuals would report similar experiences.

Sugar withdrawal symptoms in humans can include headaches, nausea, mood swings, a lack of concentration and the shakes, however these will only last for a short while and can be lessened by slowly reducing sugar intake rather than cutting out all sugars at once.

Sugar: A Pure White Deadly Poison?

In 2009 Robert Lustig, a leading expert in childhood obesity at the University of California, San Francisco School of Medicine, gave a lecture entitled “Sugar: The Bitter Truth” in which he claims that sugar, irrespective of the calories, is a poison in its own right.

Although his view may seem extreme, there is much evidence that supports the argument and promotes the idea that moderation may well be the best approach to our sugar intake.

Unfortunately, our dependence on processed foods and drinks makes this difficult to achieve but it is still an effort worth making.

Giving Up Sugar

In 2002, the World Health Organisation (WHO) recommended that sugar should account for no more than 10% of the dietary calorie requirement, which works out to be around 50g per day for a person of average weight. 

However the most recent guidelines, issued by WHO in 2014, revised this to recommend that sugar should make up less than 10% of total calorie intake with 5% being the target.

However, reducing the amount of sugar in your diet, or even giving it up altogether, is easier said than done due to the amount of hidden sugar in many foodstuffs.

You will need to be determined to succeed in reducing or giving up sugar but here are a number of tips to help:

  • Read labels – anything ending in an –ose is definitely out, as are any syrups, honey, fruit juice concentrates, maltodextrin, etc.

  • Start preparing food yourself.  Processed food of any sort, even savoury foods, will contain sugar.

  • Watch out for the amount of fruit you consume. While fruit is an excellent source of vitamins and fibre, it contains large amounts of fructose.  Get in the habit of visualizing spoonfuls of sugar every time you look at an apple!

  • Fruit juice and smoothies are worse – fruit juice contains even more sugar than the fruit (it takes about 4 oranges to make 250ml of juice) and none of the fibre.

  • Allow more time to shop and cook.

  • Tell your friends and family you are going sugar-free – it will explain a lot and they will be able to offer support.

  • Throw out the forbidden foods or you’ll never be able to resist the late night cravings.

  • No alcohol. Alcoholic drinks contain high levels of sugar.

  • Be prepared for some withdrawal symptoms which may include headaches, irritability, cravings, muscle aches, etc.

  • Focus on the good –increased energy levels, better sleep, weight loss, and how much healthier you will be.

Good Luck!