Sugar Basics & How It Causes Weight Gain

Sugar has been demonised in the media for sometime now and quite rightly so. Refined sugar in any form provides no nutritional value. This is such a big topic so for this article we’re just going to focus on the basics of sugar and how it effects weight.

The Different Types of Sugars

As most people are aware, sugar is a carbohydrate, which is a rich energy source. Carbohydrates are classified as simple or complex depending on the structure of sugar molecules in their chemical structure. Simple carbohydrates consist of one or two sugar molecules and are found in the following forms:

  • Glucose – sugar in its simplest form and is present in all carbohydrates. It is the brain’s only fuel unless there has been prolonged starvation (1).
  • Fructose – the form naturally present in fruits and vegetables, is available as an alternative sweetener and is often present in numerous processed foods in its refined form
  • Sucrose – this is table sugar as we know it. Its consists of 50% glucose and 50% fructose
  • Galactose – found in milk, fruits and vegetables
  • Lactose – found in milk products, 50% glucose, 50% galactose
  • Maltose – two glucose molecules joined together

Sugars & Weight Management

Blood sugar levels must be tightly controlled to maintain health. All of the above sugars with the exception of fructose, will directly raise your blood glucose levels, therefore triggering a spike in the hormone insulin, which lowers blood glucose levels by acting like a key to allow glucose to enter your body’s cells. Simple refined sugars that are not naturally present in whole foods are digested very quickly. This includes the sugar present in cakes, chocolate, biscuits, pastries, white bread, white pasta and white rice. Consuming high quantities requires large amounts of insulin to bring blood glucose back to a safe level. Following this, excess glucose is stored in the form of glycogen in the liver and muscles. If glycogen stores are full, insulin will facilitate the storage of excess glucose as fat, which promotes weight gain if you are frequently overconsuming sugar.

Chronically high insulin levels can lead to insulin resistance (2), where cells no longer listen to the insulin signal and glucose accumulates in the blood. In response, the pancreas produces even more insulin leading to even higher levels of insulin, creating a viscous cycle. As one of insulin’s main functions is to store excess glucose as fat to keep blood sugar levels in check, persistently high insulin levels will lead to weight gain. Essentially, insulin is a fat storage hormone. Not only will sugar make you fat, but the sugar high will shortly be followed by a sugar crash. A sugar crash stimulates the release of stress hormones, which triggers the release of stored sugar from the liver. When this occurs we rarely use up this released energy so again insulin causes it to be deposited as fat (see figure 1).

Figure 1

Fructose does not affect blood glucose levels, instead it has to be processed by the liver. The naturally occurring small quantities present in whole fruits and vegetables are of no concern. Fructose causes a problem when it is consumed in its processed form in high quantities, especially high fructose corn syrup (HFCS), which can contain up to 90% fructose. Fruit juice is also one to avoid, as consuming the juice of a fruit without the fibre is problematic. One issue with fructose and weight control is that it fails to stimulate the satiety hormone leptin, meaning you are less likely to recognise that you are full (3). Over-consumption of fructose has been linked to fatty liver, high blood triglycerides and insulin resistance, leading to problems with weight control (4).

So, how do you avoid sugar and what’s the alternative? Keep an eye out for our next article about the names commonly used to try and disguise sugar in food products and guidance on natural and artificial sweeteners!


1. Berg J, Tymoczko J, Stryer L. Biochemistry. 5th ed. New York: W.H. Freeman and Co.; 2002.

2. Samuel V, Shulman G. The pathogenesis of insulin resistance: integrating signaling pathways and substrate flux. Journal of Clinical Investigation [Internet]. 2016 [cited 9 April 2018];126(1):12-22. Available from:

3. Sáinz N, Barrenetxe J, Moreno-Aliaga M, Martínez J. Leptin resistance and diet-induced obesity: central and peripheral actions of leptin. Metabolism [Internet]. 2015 [cited 9 April 2018];64(1):35-46. Available from:

4. Dornas W, de Lima W, Pedrosa M, Silva M. Health Implications of High-Fructose Intake and Current Research. Advances in Nutrition [Internet]. 2015 [cited 9 April 2018];6(6):729-737. Available from:

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Author: Sarah Turner is a BANT registered nutritional therapist and blogger, recipe developer and researcher for Louise Digby Nutrition. She is also currently studying physiotherapy and plans to provide a fusion of nutrition and physiotherapy upon graduation!

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