How sugar impacts your health

Published May 18, 2022

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What is sugar?

Natural sugars are abundant and found in many forms and are needed by the majority of life on Earth. Our hunter-gatherer ancestors would have eaten sugar in fruit sporadically and infrequently as they competed with birds and other animals for the quick energy burst the sugars in fruit would have given.

But now, chemically refined and highly purified sugar in all of its many forms (sucrose, high-fructose corn syrup and approximately 59 other names) is abundant in our modern diet of processed foods. While refined sugar is in 75% of packaged foods, these highly refined sugars do not occur in the natural world except in rare cases, such as honey production by bees.

Are there different types of sugar?

Carbohydrates, or sugars, are classified as either complex carbohydrates or simple carbohydrates. Complex carbohydrates are made up of multiple simple carbohydrates, joined together by chemical bonds — the more chains and branches of simple carbohydrates, the more complex the carbohydrate. Complex carbohydrates take longer for your body to break down, delaying the release the sugar into your bloodstream. Complex carbohydrates include wholegrains such as rolled oats, brown rice, spelt, rye and barley.

Simple carbohydrates, or sugars, are either monosaccharides (one sugar molecule) or disaccharides (two sugar molecules). They are digested quickly, and the sugars are released rapidly into the bloodstream. The two main monosaccharides are glucose and fructose. The two main disaccharides are sucrose (composed of glucose and fructose) and lactose (a combination of galactose and glucose). High fructose corn syrup is a common manufactured sugar made up of fructose and glucose.

What is glucose?

Glucose is the primary source of energy every cell in your body relies on it to function.  When you eat carbohydrates, your body breaks them down into separate units of glucose. When blood glucose levels rise, the pancreas releases insulin, signalling cells to take up glucose from the blood. As the cells absorb the glucose from the blood, levels start to drop. When the term’ blood sugar’ is used, it refers to glucose in the blood.

The nutritional profile of glucose

The glycemic index (GI) is a ranking of how quickly foods make your blood sugar levels rise after eating them. Glucose is the defining standard and has a GI value of 100. High GI foods are very easily broken down into glucose and can cause a quick rise in blood sugar. Glucose on its own does not taste sweet, compared to fructose and sucrose, so food manufacturers will often combine glucose with fructose to use as a sweetener.

How does glucose affect your body?

Research suggests that, as glucose stimulates insulin release from the pancreas, it also influences other hormones that help regulate appetite — leptin and ghrelin. Leptin suppresses appetite and ghrelin the increases appetite. It has been shown that lower GI foods (such as wholegrains, proteins and those lower in glucose) suppress ghrelin, therefore helping you feel full.

What is fructose?

Fructose or fruit sugar is a simple sugar naturally occurring in fruit, honey, sucrose (see below) and high fructose corn syrup. Fructose is one-and-a-half times sweeter than sucrose (white sugar).

The nutritional profile of fructose

Fructose has a GI of around 19, as it is absorbed directly into the bloodstream during digestion and has no impact on insulin production or blood glucose levels.

How does fructose affect your body?

The body handles fructose in a different way to glucose. Fructose is metabolised in the liver. As a result, blood sugar levels do not rise as rapidly after eating fructose compared to other simple sugars. When you eat too much fructose, the liver can’t process it fast enough and starts to make fats that are carried in the blood and stored as triglycerides.

Studies have shown that eating large amounts of fructose may increase your appetite by impairing the body’s ability to suppress circulating ghrelin (the appetite-stimulating hormone).

What is sucrose?

Sucrose is crystallised white sugar made from the refined juice of sugar cane and is in households and foods around the globe. Sucrose is a disaccharide made up of 50% glucose and 50% fructose and is broken down quickly into its monosaccharide parts.

The nutritional profile of sucrose

Sucrose has a GI value of 65. The fructose component is metabolised in the liver (see notes on fructose above). Due to its glucose content, sucrose does lead to an increase in blood glucose.

What is lactose?

Lactose is a sugar found in milk. It consists of glucose and galactose. It is broken down by an enzyme called lactase. Once broken down, the simple sugars are absorbed into the blood.

The nutritional profile of lactose

Whole milk has a GI value of 46. Being broken down slowly helps the absorption of minerals such as calcium and magnesium. Some people cannot tolerate lactose due to a lack of the enzyme lactase. Lactose intolerance can lead to diarrhoea, bloating and other digestive symptoms.

What is high fructose corn syrup?

High fructose corn syrup is a syrup made from corn that contains fructose and glucose

Because of the worldwide increase in the consumption of processed foods and beverages sweetened with high fructose corn syrup (HFCS), fructose intake has quadrupled over the last 100 years.

How is HFCS made?

High fructose corn syrup is made from corn. The corn is milled to produce corn starch, which is then processed further to create corn syrup.

Corn syrup is mostly glucose. To make the taste more like table sugar, some of that glucose is converted to fructose using enzymes.

The most commonly used HFCS is HFCS 55, which contains 55% fructose and 42% glucose.

HFCS 55 is similar to table sugar which is 50% fructose, and 50% glucose.

How does high fructose corn syrup affect your body?

Natural fructose from fresh fruit and vegetables is beneficial for your health. Fruits and vegetables contain a small amount of fructose, plus fibre and nutrients. Processed forms of fructose, such as HFCS contain no fibre or nutrients and may have adverse health effects.

A literature review conducted in 2017 showed that eating excessive amounts of high-fructose processed foods can lead to:

  • Inflammation
  • Increased tummy fat as it may alter how the body breaks down fats and carbohydrates
  • Weight gain Increased appetite, as it can stop you feeling full

How much sugar should you be eating?

The World Health Organisation in 2015 recommended, for optimal health, that free sugars (sugars added to foods) should be less than 5-10 per cent of our total calorie intake. For the average adult eating 2,000 calories per day, five per cent would amount to 100 calories or approximately six teaspoons (25 grams) of sugar per day.

What are the short-term symptoms of too much sugar?

Some people can experience the following symptoms after overindulging on sugar:

  • Low energy levels — A 2019 study found that thirty minutes after sugar consumption participants reported increased fatigue and after sixty minutes felt less alert than a control group.
  • Low mood —A 2017 prospective study found that higher sugar intake increased rates of low mood  in males.
  • Bloating — Several types of sugars can cause bloating and gas in people who have digestive conditions.

How can too much sugar affect your long-term health?

Overeating sugar over time can impact your health in a variety of ways:

Tooth decay

Foods and drinks high in sugar are one of the leading causes of tooth decay. When the bacteria in your mouth break down sugar, they produce acid. This acid dissolves the tooth’s surface, which is the beginning of tooth decay. Tooth decay may not cause any pain. But if you have dental caries, you might experience:

  • Toothache — Either continuous pain keeping you awake or occasional sharp pain without an apparent cause.
  • Tooth sensitivity — Feeling tenderness or pain when eating or drinking something hot, cold or sweet.
  • Spots — Grey, brown or black spots appearing on your teeth.
  • Halitosis — Bad breath may also be a sign of tooth decay


  • Horrible taste — An unpleasant or metallic taste in your mouth may also be present with tooth decay.

Ageing skin

Excess sugar in the diet leads to the formation of advanced glycation end products (AGEs), which can affect collagen formation in the skin. There is some evidence to suggest that a high number of AGEs may lead to faster visible ageing, due to the impact of elevated sugar on collagen. Weight gain Research exploring the type of sugar people eat has shown that fructose can reduce the hormone leptin — the hormone that tells your brain that you are full — potentially causing weight gain.

How to reduce your sugar consumption

You can reduce the amount of added sugar you eat and drink by:

  • Checking labels — Check food labels for sweeteners. Some of the ingredients to look for include fructose, glucose, sucrose, maltose, honey, corn sweetener, corn syrup, high fructose corn syrup and malt syrup. As a general rule, look for the sugar content to be less than 5 grams of sugar per 100 grams.
  • Reduce foods and drinks with added sugar — Reduce foods/drinks with added sugar, such as yoghurts, sauces, salad dressings, fruit juices and soft drinks
  • Eating wholefoods — Avoiding processed foods, in general, will reduce your exposure to hidden sugars.

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