Glucose metabolism - what happens when you eat sugar and how can it all go wrong?

Love it or hate it, sugar is essential to life. So how does glucose metabolism actually work and how can it go wrong?

Written by

Dr Claire Merrifield MBBS MRCGP PhD

GP, PhD and our medical director


Key Takeaways

  • Glucose is essential for life and your body has complex systems to make sure you never run out.
  • Glucose in your bloodstream can either be used or stored as glycogen or stored as fat.
  • Excess energy intake can start your body on a journey towards insulin resistance and metabolic disease.
  • How well you sleep, how much you move, your stress levels and your overall diet all impact your glucose metabolism and your overall metabolic health.

Sugar is often spoken about in the same disgusted tones as smoking or taking illegal drugs. It’s not uncommon to hear someone proudly state that they’ve ‘quit sugar’. But is that even possible? Our experts take you on a whistle-stop tour of glucose metabolism so you can really understand what’s going on and how it can all go wrong.

Do we even need sugar at all?

Sugar, or glucose, is essential for life. If you have too little glucose in your blood, you’ll die. It’s the primary source of fuel for your central nervous system and provides the immediate energy for your cells to work properly. Your brain is the largest consumer of glucose in your body. It uses around 20% of all glucose-derived energy and needs around ~120g glucose a day1.

So the first thing to point out is that sugar is not “bad” for you per se, in the same way that petrol is not bad for a car. However, like anything, you can have too much of a good thing and in the case of sugar and food in general, we very often do.

Glucose is so essential to life that we’ve developed complex systems to ensure we always have enough glucose in our bloodstream. But when we consume more calories than we need, these systems can actually cause significant problems with our metabolic health, leading to obesity, insulin resistance, metabolic disease and ultimately conditions such as diabetes, heart disease, dementia and cancer.

So how do we metabolise glucose?

When glucose is absorbed from the gut, it enters your bloodstream causing blood glucose levels to rise. In response, your pancreas produces a hormone called insulin. Insulin has many effects and a major one is to help your cells absorb glucose from the blood, lowering your blood sugar.

When glucose enters a cell it has 3 fates:

What happens to glucose in your cells?

  • Some used for immediate energy by glycolysis or oxidative phosphorylation
  • Most is turned into glycogen (a storage form of glucose found in your liver and muscles - there’s limited space for this)
  • Excess is converted into fat (a longer-term storage solution, mainly occurs in the liver)

If you don’t top up your glucose by eating carbohydrate or you burn more glucose through exercise, your blood glucose levels will tend to fall. This causes your pancreas to stop releasing insulin and instead release a hormone called glucagon. This has a couple of effects.

Firstly, glucagon causes glycogen in your liver and muscles to be broken down into glucose. This process is called glycogenolysis.

Secondly, low insulin levels cause your body to break down fat stores in a process called lipolysis. Fat - or triglyceride - breaks down into glycerol and free fatty acids. Free fatty acids can be used as a fuel for your muscles or converted in the liver to ketones. Both your brain and muscles can use ketones as fuel. On the other hand, glycerol can be converted into glucose in the liver. You heard that correctly. Your body can and does create glucose, even if you’re not eating it in the form of sugars or carbohydrates. This is a process called gluconeogenesis (gluco- glucose, neo- new, genesis- create) where new glucose is created from the breakdown products of fats and proteins.

Lipolysis and gluconeogenesis are also the processes that allow you to survive without eating any carbohydrate at all - a so-called “ketogenic” diet because of the ketones produced. The main difference is that on a ketogenic diet, the fat being used for energy is coming principally from the diet rather than body stores.

So normally, in health, this system works very well to keep your blood glucose fairly constant. A healthy average blood sugar is usually less than 6.0 mmol/l.

Keeping your blood glucose levels above a certain threshold is essential for life.

So, just to recap, you absorb glucose from your gut into your bloodstream. Insulin then moves the glucose out of the bloodstream and into your cells. Once inside the cell, some glucose is used for any immediate energy needs but most is stored as glycogen in the liver or muscles. When glycogen stores are full, the excess is converted to fat. If there isn’t enough glucose in the bloodstream, you can convert glycogen stores back to glucose or break down fat. Broken down fat can be used as an energy source directly or via conversion into glucose. Keeping your blood glucose levels above a certain threshold is essential for life.

Where does glucose metabolism go wrong?

When you’re eating roughly the right amount of calories for your energetic requirements, your body’s metabolism is nicely balanced. In the modern world, an increasingly sedentary lifestyle and the ready availability of delicious food and drinks with added sugar, means it’s very easy to eat more than your body needs.

So what happens if you eat too much? An excess of energy causes a lot of problems. As we’ve already said, your body converts excess glucose to fat. Initially, fat is stored under the skin, as subcutaneous fat, which is a completely natural and healthy way to store some energy. But there’s only so much “space” in the subcutaneous fat stores. Over time, if we keep eating more energy than we’re using, the subcutaneous fat stores become full and your body has to store fat in the cells of your muscles and organs such as your liver and pancreas. Fat accumulation in your muscles and organs (called visceral fat) is the major cause of “insulin resistance” where your body is less sensitive to insulin.

Insulin resistance makes it harder to burn fat

If you’re insulin resistant, your body has to produce more insulin to keep your glucose levels in check. This high insulin level does a number of things to your body. If your insulin level is high, it actually tells your body to stop burning fat by lipolysis. This makes sense as your body is trying to encourage glucose use from the bloodstream. But paradoxically, it makes it harder for you to burn the fat which is causing the insulin resistance in the first place! We’ve entered a vicious circle!

Insulin resistance is one of the earliest warning signs we have that your metabolism is not functioning as well as it should.

Insulin resistance predates type 2 diabetes by around 10-15 years and is one of the earliest warning signs we have that your metabolism is not functioning as well as it should. As insulin resistance worsens, eventually you can’t keep your glucose levels in check even with really high levels of insulin. Your average blood sugar levels rise (hyperglycaemia) which over time leads to type 2 diabetes2.

How does your lifestyle affect the way you metabolise glucose?

So we’ve talked about how your body metabolises glucose and how it can go wrong, but how does what you’re doing day-to-day affect that?

Exercise helps to keep you metabolically healthy

Exercise has several beneficial effects on your muscles with regards to glucose metabolism3. It makes muscles more sensitive to insulin, allowing more absorption of glucose from the bloodstream. It also increases the amount of glucose that is taken up from the bloodstream without even needing insulin (so called “insulin-independent” uptake). Exercise increases the muscles’ immediate requirements for energy, so that more glucose can be used up straight away, rather than being stored for later. Low-intensity exercise stimulates your mitochondria to burn fat, so reducing fat accumulation in your muscles.

In the longer term, the more you increase the size of your muscles, the more energy they need and the more glycogen they can store. Your muscles effectively become bigger “sinks” for your blood glucose,keeping glucose levels under control and meaning less is converted to fat.

Stress can raise your blood sugar and make you store more fat

Cortisol is one of the main stress hormones. You’ve probably heard that when you’re feeling stressed out, in a high-pressure job or dealing with some other stress that’s hard to manage, your body will produce more cortisol. Cortisol is a type of steroid and unfortunately, far from creating a bodybuilder physique, an excess of cortisol is likely to create more visceral fat that’s hard to shift.

Part of the reason behind this is that cortisol triggers gluconeogenesis4. This raises blood glucose levels which in turn causes raised insulin levels and stops the body breaking down fat. Cortisol can also increase appetite and cause fat to be stored around the organs.

Poor sleep causes higher blood sugar levels

We’ve known for some time that not getting enough sleep is bad for us. It has been linked to an increased risk of cancer, cardiovascular disease and dementia. But did you know that insufficient sleep can increase your risk of obesity and type 2 diabetes?

Just a reminder that the magic number for sleep is 7-9 hours a night. Around a third of people don’t achieve this. People sleeping less than 6 hours a night are likely to have more visceral fat than people who get sufficient sleep. If you get less than 6 hours sleep you’re also more likely to develop metabolic syndrome which is one of the main reasons poor sleepers are more likely to die from heart attacks and strokes 5.

What’s startling is that changes in blood sugar as a result of poor sleep can happen over a very short space of time. Just 4 days of sleeping 4.5 hours is enough to cause insulin resistance, the precursor to type 2 diabetes6.

So what’s the bottom line on sugar?

Carbohydrates are not the enemy.

Hopefully you understand now how essential sugar is to life and how you can’t really be “sugar-free” no matter what you eat. The major problems come about when we have excess energy compared to our requirements and when we don’t exercise. However, it probably is true that some sources of energy may amplify this effect more than others. For example, fructose is metabolised differently to glucose and may be a stronger driver of metabolic dysfunction7.

Ultimately, sugar or carbohydrates are not the “enemy” and you can enjoy them as part of a healthy diet. But we should be avoiding processed foods and opting for complex and non-digestible carbohydrates (e.g. whole grains, fruits and vegetables) over refined sugars to allow our body the opportunity to metabolise sugar appropriately. Lifestyle plays a key role in this process. For optimal sugar metabolism, sleep well, move more and manage stress.

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