Why blood glucose variability can worsen fatigue and brain fog

If you live with fatigue or brain fog, you may have noticed that how—and when—you eat can make a real difference to how you feel. One important factor influencing variability of symptoms is blood sugar. While dramatic highs and lows in blood sugar are well known in diabetes, they can affect us all, and fluctuations in blood glucose can contribute to fatigue, poor concentration, and energy crashes. This is why it is so important to manage blood glucose (sugar) levels carefully if you have ME/CFS, Long Covid or Fibromyalgia.

This blog explains why this happens—and what you can do about it.

What is blood glucose variability?

After you eat, your body breaks down carbohydrates into glucose (sugar), which enters your bloodstream and this causes:

• Blood sugars to rise

• The hormone insulin to be released

• Glucose to move into cells for energy

Ideally, this process is smooth and steady.

However, if a meal is high in rapidly absorbed carbohydrates (such as sugary foods or refined carbs), it can lead to:

1. A rapid spike in blood sugar

2. A large insulin response

3. A subsequent drop (dip) in blood sugar

Whilst it is normal for blood sugar to spike after a meal and then return to baseline, when those spikes happen often, rise high, or stay elevated for a long time this can have an impact on your health and how you feel. Then is not about a single moment, but more about your pattern of blood sugar over time.

Recent research shows that glycaemic variability is an important component of metabolic health. That means that in addition to your average glucose levels, the amount your blood glucose levels spike or go low is also an important indicator.

This means that:

• It’s not just “how high” blood sugar goes

• But how much it fluctuates that matters

An on-going “spike and crash” pattern of blood glucose levels can reduce energy levels and increase brain fog.

Hypoglycaemic episodes

When blood glucose has peaked to a high point, this can be followed by a signficant and rapid drop in blood glucose, so that glucose levels are abnormally low. Such a dip in blood glucose, which is also known as a hypoglycaemic episode, occurs when blood glucose falls below 4mmol/L. Hypoglycaemia can lead to symptoms such as sweating, shaking, dizziness, hunger, and irritability. These episodes are resolved by taking 15-20g of fast-acting carbohydrates such as juice or glucose tablets.

This Photo by Unknown Author is licensed under CC BY

Impact in ME/CFS, Long Covid and Fibromyalgia

In people with ME/CFS, Long Covid and Fibromyalgia, the body is often already under stress as a result of impaired energy metabolism, nervous system dysregulation and increased sensitivity to physiological changes. This means blood sugar swings can have a greater impact and can lead to a worsening of symptoms such as fatigue and brain fog.

1. Blood glucose variability and energy levels

At a basic level, your body relies on glucose to produce energy in the form of ATP (adenosine triphosphate) via the mitochondria. However, this system works best when glucose supply is stable and predictable.

When blood glucose rapidly rises and falls, several biological processes are disrupted that can impair energy production.

Mitochondrial dysfunction and reduced ATP production

Mitochondria convert glucose into ATP through oxidative phosphorylation. However:

• Fluctuating glucose levels disrupt mitochondrial balance

• This affects processes such as mitochondrial fusion, fission, and turnover

• Leads to less efficient energy production

Research shows that oscillating glucose levels impair mitochondrial function, including changes in mitochondrial membrane potential and energy metabolism.

In simple terms:

• Your “energy factories” become less efficient

• Cells produce energy less reliably

  
  

Increased oxidative stress damages energy systems

One of the most consistent findings is that glucose variability increases reactive oxygen species (ROS)—harmful molecules that damage cells.

• Fluctuating glucose produces more oxidative stress than constant high glucose 

• ROS directly damage mitochondria and enzymes involved in energy production

This leads to:

• Reduced ATP generation

• Increased cellular “wear and tear”

• Greater fatigue at a whole-body level

Conditions such as ME/CFS, Long Covid and Fibromyalgia are characterised by impaired energy production, which makes it even more important to try to maintain stable blood glucose levels.

Fatigue and energy crashes

When blood glucose levels drop rapidly:

• The body activates stress hormones, like adrenaline and cortisol which temporarily increase glucose availability

• This process can lead to feelings of fatigue or “wired but tired”

• Sudden need to rest

In people with underlying fatigue conditions, this can feel like:

• A mini “crash”

• Worsening of baseline fatigue

2. Blood glucose variability and brain function

The brain is the most metabolically active organ in the human body, and has a high and constant energy requirement, using around 25% of available glucose when resting.

The brain relies heavily on glucose and when glucose levels drop quickly:

• Energy supply to brain cells becomes unstable

• Neurons (nerve cells) cannot maintain optimal function

Experimental studies show that glucose fluctuations can:

• Disrupt brain cell metabolism

• Increase inflammation and oxidative stress in neural tissue

Glucose fluctuations can therefore lead to symptoms such as:

• Brain fog

• Poor concentration

• Mental fatigue

This Photo by Unknown Author is licensed under CC BY

Common triggers of blood glucose variability

You may be more likely to experience swings if you:

• Eat meals high in refined carbohydrates (e.g. white bread, sugary snacks and drinks)

• Skip meals or go long periods without eating

• Eat carbohydrates on their own (without protein or fat)

• Rely on quick “energy fixes” like sweets or caffeine

Signs your blood sugar may be fluctuating

Whilst it may be difficult to differentiate symptoms of fluctuating blood glucose from the ups and downs of a condition such as ME/CFS, Long Covid or Fibromyalgia, the following signs may indicate that your blood sugar levels are less stable:

• Mid-morning or mid-afternoon crashes

• Feeling shaky, lightheaded, or irritable

• Strong cravings for sugar or carbohydrate-rich foods,

• Experiencing energy dips after meals

• Have brain fog that comes and goes

How to stabilise blood sugar

The goal is not perfection—but to gradually achieve more stability in blood glucose levels which should help support more stable energy production.

Here are some suggestions on how to stabilise blood glucose levels:

1. Eat balanced meals

Try to make sure that meals are balanced and include the following:

• Protein rich foods, such as eggs, fish, meat, tofu, cheese, yoghurt

• Include plenty of green, leafy and colourful vegetables

• Include healthy fats, e.g olive oil, fish, nuts, seeds

• Use whole grain and minimally processed carbohydrates

• Keep sugary foods to a minimum

Eating this way will slow the release of glucose after a meal, and this will help reduce spikes.

2. Avoid highly refined sugars and carbs, and don't eat carbs on their own

Avoid eating sugary foods, refined carbohydrates and sugary drinks. Whilst these foods might provide an initial boost to energy levels, they usually lead to a dip in blood glucose afterwards. Chose snacks which contain protein, such as nuts, cheese, yoghurt.

Try not to eat carbohydrates on their own.

For example:

• Instead of eating fruit alone → add nuts or yoghurt

• Instead of toast alone → add eggs or peanut butter

This Photo by Unknown Author is licensed under CC BY

3. Eat regularly, small meals might suit

Long gaps between meals can increase the risk of glucose dips in some people. For most people having a regular pattern of eating with three meals each day is better than leaving longer gaps between meals.

Some people may prefer to eat more often and find eating smaller meals more frequently suits them better than eating three larger meals, as this can be easier on the digestive system.

4. Choose slower-releasing carbohydrates

Eating fewer refined carbohydrates, and instead choosing wholemeal carbohydrates will help stabilise blood glucose levels. The absorption of carbohydrates from wholemeal foods is slower, and this slower release of glucose will reduce the glucose spike after a meal . Some examples of slow releasing (low GI) carbohydrates include:

• Whole grains, such as brown rice, quinoa, rye bread and buckwheat

• Lentils and beans (if tolerated)

• Root vegetables

These foods provide a more gradual rise in blood sugar and help to avoid blood glucose spikes, which are so often followed by blood glucose dips.

This Photo by Unknown Author is licensed under CC BY-NC-ND

5. Order of food eaten

Research suggests the order in which you eat the food on your plate may affect how high your glucose spikes after a meal. Researchers have found that eating vegetables and protein-rich foods in advance of carbohydrate foods reduces the glucose spike experienced after a meal.

6.Start with your first meal of the day

A breakfast high in refined carbs can set up a day of blood glucose swings. Avoid eating foods high in carbohydrate for breakfasts, e.g pastries and breakfast cereals. Instead make sure your breakfast has a good portion of protein: For examples:

• Eggs: scrambled, boiled or poached or in an omelette with vegetables

• Continental-style breakfast with cold meats and cheese

• Yoghurt with nuts and seeds

• A protein-rich smoothie

6. Be careful with caffeine and sugar

Caffeine can temporarily mask fatigue and can contribute to later crashes. Foods high in sugar, especially when eaten without protein, can cause an immediate glucose spike, which will often be followed by a glucose dip.

In summary

When you have ME/CFS, Long Covid or Fibromyalgia, energy production systems are already compromised. Maintaining stable blood glucose levels, will help minimise the disruption caused by the peaks and troughs of fluctuating sugar levels. This helps prevent the added burden of the symptoms caused by blood glucose variability, including its impact on energy production and cognitive function.

References

Klimontov VV, Saik OV, Korbut AI. Glucose Variability: How Does It Work? Int J Mol Sci. 2021 Jul 21;22(15):7783. doi: 10.3390/ijms22157783. PMID: 34360550; PMCID: PMC8346105.

Magistretti PJ, Pellerin L. Cellular mechanisms of brain energy metabolism and their relevance to functional brain imaging. Philos Trans R Soc Lond B Biol Sci. 1999 Jul 29;354(1387):1155-63. doi: 10.1098/rstb.1999.0471. PMID: 10466143; PMCID: PMC1692634.

Peters, R., White, D., Cleeland, C. and Scholey, A., 2020. Fuel for thought? A systematic review of neuroimaging studies into glucose enhancement of cognitive performance. Neuropsychology review, 30(2), pp.234-250.

Zhou, Z., Sun, B., Huang, S. et al. Glycemic variability: adverse clinical outcomes and how to improve it?. Cardiovasc Diabetol 19, 102 (2020). https://doi.org/10.1186/s12933-020-01085-6