2.6 Sugar, carbs and type-2 diabetes

Is it true that you can control diabetes 2 with a low carb diet? This question came up in a science discussion after someone had seen a TED talk to that effect.Advice from the NHS is clear that a healthy diet that balances a variety of types of food is the most important message. It should include “fruit, vegetables and some starchy foods like pasta, but sugar, fat and salt should be kept to a minimum”.

However as Diabetes UK says: “The problem with some starchy foods is that [they] can raise blood glucose levels quickly, which can make it harder for you to manage your diabetes.” It’s easy to find advice about diet, exercise and manging diabetes 2 via the internet, but each site uses words that most of us have only the slightest understanding of.

What is starch? How does it differ from carbohydrate? What counts as sugar? What are blood glucose and glycaemic index? In this blog we take the opportunity to explore some of the basic science behind carbs, sugar and type 2 diabetes.

Our bodies – and indeed all living systems – are the most marvellous examples of systems in balance. Too little oxygen and our cells will begin to die; too much and our lungs and eyes get damaged. Too high a temperature causes confusion, fatigue and ultimately death, but so too can too low a one. Bodily systems regulate this, restoring balance when levels stray either way. The thermostat in a heating system does much the same, causing a heater to click in or cut out depending on the temperature of the surroundings.

The same applies to levels of glucose in the blood: too much leads to tiredness and nausea, too little to shakiness and irritation. In extremes either can be life threatening. Regulation of the level of glucose levels is a vital task of the body. In diabetes the regulatory system isn’t working properly.


In science, the word ‘sugar’ refers not just to the single substance we might take with our tea, but to an entire class of chemical compounds with similar structures. Some, such as glucose, sucrose and lactose, are part of everyday talk. Others, such as ribose and galactose less so. The ‘-ose’ ending defines them as sugars. Many sugars are sweet to taste, largely as a consequence of evolution, which has developed out taste buds and brain circuits to desire them strongly. This is because enshrined in the molecular structure of sugars is a vital source of energy and evolution has led us to crave it. The very way the atoms in a sugar molecule hold together – the bonds between adjacent atoms – is the source of energy, much as the coiled up spring in a jack-in-a-box is a source of energy, just waiting to be released.

In our bodies the intricate process that releases energy from sugar occurs in every cell – in blood, muscle, nerves, bone and brain. Just one type of sugar, glucose, is used for this purpose. One of the key roles of our digestive system is to create glucose molecules by breaking down the larger molecules in the food we eat.

Glucose molecule

Once they have reached all the cells in the body, glucose molecules are broken down there and the energy in them captured for later use. This occurs everywhere – It is what enables us to move, digest and think.


The food we eat may be very varied in appearance but the important molecules in it, from the energy point of view, are of only four main kinds: starch, sugar, fibre and fat.

Fibre simply passes through the intestines undigested, though it has an key role nourishing our all-important gut bacteria. The other three kinds of molecule break down into simpler ones as we digest. Molecules of starch (found in potatoes, rice, pasta etc.) are just long chains as the diagram shows.

A starch molecule (amylopectin)

They are made up of the much smaller molecules of glucose and these are what is produced when starch is broken up during digestion:

starch = glucose + glucose + glucose …..

The main sugar we eat, sucrose, is not a long chain molecule but is made up of two simpler sugars: fructose and glucose (on the left and right of this diagram respectively . Digestion breaks sucrose down into these two: sucrose = glucose + fructose

Sucrose molecule

Fat molecules (known as triglycerides) are also chain-type molecules whose component parts are called fatty acids. These chains also break down in digestion into three separate fatty acids.

A ‘fat’ molecule made of three fatty acids

The energy for all our activities comes mainly from the breaking up of glucose and fatty acid molecules inside our cells, a process that releases energy associated with the bonds between their atoms.


As you digest your food, glucose and fats are released into the blood stream (though the fats, being insoluble oily molecules, get bundled up inside a soluble carrier called lipoprotein first). Both of these kinds of molecule convey the energy needed by all the cells of your body. The two have different qualities however, and work in complementary ways to ensure your cells get the supply of energy they need, even between meals. The fatty acids can be stored easily and are held back for use between meals. They get stored in fat cells in various parts of the body and in the liver. Your immediate need for energy is satisfied by the glucose molecules which go straight from the bloodstream to all the cells of your body.

To get into the cells, however glucose needs help. Cell walls are oily and glucose molecules won’t pass through them, just as water won’t mix with grease. The help they need is provided by insulin molecules. These, in effect, open up a ‘pore’ in the cell wall, allowing the glucose molecules to pass through where they release their energy.

To enable this to happen, insulin gets released from where it is stored (the pancreas) while you are digesting your food. This release is triggered by the rising levels of glucose in the bloodstream due to your eating. Insulin is doubly useful as it not only opens up all your cells to take in glucose, it also signals to fat cells and the liver to take in and store up molecules of fat from your food for later use. 

In between meals, as your glucose level begins to drop, your insulin level drops with it. This means the fat cells cease storing up fat but instead release it to keep up the supply of energy to all cells. Fat and glucose work together to keep you fuelled. That’s why you need both fats and carbs in your diet.

Diabetes 2

Problems arise, however, if your insulin system ceases to work properly. It can become less effective in helping glucose into your body’s cells and cause the liver to over-produce glucose that isn’t needed. This can lead to an  excess of glucose floating around in your bloodstream. This is what doctors are looking out for when they order a blood glucose test and ask you to fast beforehand. It is a sign of type-2 diabetes.

Like all aspects of our bodily systems, the digestive processes we have today evolved very slowly over countless millions of years. The system we have today suited early humans, but doesn’t work so well for us. Back at that time carbohydrates in food were encased in fibre, making them relatively hard and slow to digest. Sugars were more of a rarity then, too. Today with fibre stripped away and sucrose added to so much of our food, digestion is a much easier and faster process. As a result, as we eat, glucose floods into our bloodstream too fast for the balancing act that insulin normally maintains. Excess insulin enters the bloodstream in an attempt to cope. But our cells are overwhelmed; they just cannot process all the glucose coming their way. The fat cells can’t cope with the excess fat coming their way either. As a result half-processed glucose and fat and waste products abound.

At this point another of our bodily system gets called in: the immune system. In an attempt to clear up the mess, it releases chemical that reduce the effectiveness of insulin. It’s trying to protect our overwhelmed cells. So less glucose makes it into our cells and less fat gets stored in the fat cells. It has to go somewhere, so it stays floating around in our bloodstream and that is where the danger lies: too much glucose and fat in the bloodstream. The body can handle this imbalance from time to time (Christmas and Eid are OK!), but if it persists the heart and liver are threatened. That’s type-2 diabetes.


There’s a lot more to healthy eating than we can cover here – the role of fructose (one of the breakdown products of eating sugar), the role of ‘good’ bacteria in our gut and the effect of additives, for example. On top of this there is the part the hormones, leptin and grellin, play in telling our brains whether or not to feel hungry and our psychological tendencies which may drive us to compulsive or even addictive habits.

For practical purposes, however, there are some clear messages about giving our insulin a chance and avoiding type 2 diabetes. The Harvard School of Public Health summarises these and offers the encouraging conclusion that, in the vast majority of cases, prediabetes and type-2 diabetes can be avoided by making lifestyle changes. It advises:

  • Lose weight: being overweight increases the chances of developing type 2 diabetes seven-fold
  • Exercise: working your muscles more often and making them work harder improves their ability to use insulin and absorb glucose
  • Choose whole grains and whole grain products over refined grains and other highly processed carbohydrates
  • Skip sugary drinks, and choose water, coffee, or tea instead
  • Choose healthy fats. polyunsaturated fats found in liquid vegetable oils, nuts, and seeds can help ward off type 2 diabetes. Trans fats do just the opposite.
  • Limit red meat and avoid processed meat; choose nuts, beans, whole grains, poultry, or fish instead.

These are manageable, evidence-based tips. But if you are looking for a specific diet it’s worth knowing that simply following a dietary fad doesn’t generally work in the long run. You may lose a few of the easier kilos early on but it is very hard to sustain over the long term. The desire to eat sweet and fatty thing is very strong indeed, more or less hard-wired into our brains. Evidence-based commentators generally advise us to take it slowly, allow yourself some pleasures and remember how hard it is to overcome your built-in impulses, but always worth the struggle!

Further detail

 When glucose molecules find their way into the energy producing part of our cells (called the mitochondrion) they meet up with oxygen molecules that we have breathed in and passed into the bloodstream.

Mitochondria inside cells in the lung

Here the glucose and oxygen combine in a process that releases energy. It is analogous to the process of combustion in which oxygen combines with fuel molecules in a fire, releasing heat energy. Though  comparable to burning, this process in our cells (called cellular respiration) is much slower, and involves no rise in temperature.

The products of this ‘burning’ process (known as cellular respiration) are simply carbon dioxide and water. That’s why these two waste products feature so strongly in our daily lives: exhaling and urinating.

© Andrew Morris 4th February 2020