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Metabolic Flexibility & Weight Loss

Metabolic flexibility is the ability to switch between energy coming from carbohydrate in the diet and from burning fat. Most people have thousands upon thousands of calories that they are carrying around in the form of fat but are exhausted and unable to access those calories to produce any energy. Instead they crave carbohydrates all day long and cannot go for more than a few hours without needing to eat. Metabolic flexibility gave our ancestors the ability to eat plenty of foods when there was plenty available, then be able to burn fat to survive when times were much leaner. If we eat often and eat a diet high in carbohydrates, the body never really has to enter its fat stores, and like many other skills, that skill appears to be lost. The old adage of use it or lose it is relevant here.


Metabolic inflexibility is one of the causes of weight gain and the related metabolic diseases. Physical exercise triggers metabolic flexibility. In todays modern world we have sedentary jobs, & much of our leisure time is spent in front of screens. We have a fridge full of foods that require no hunting, gathering or farming. They are high calorie yet require no energy from us. This is not how it was for previous generations and the truth is we have simply not evolved to have such an abundance of food, for so little effort. So what can we do? Well we need to expend more energy. Now obviously we are not going to be out hunting, so we need to walk, run, cycle, lift weights, go to classes, do yoga- whatever floats your boat, but we need to expend more calories every day, as our diets are simply too high in calories and in particular carbohydrate.


Another method to induce metabolic flexibility is to fast. There are many different forms of fasting and a few are listed below.

  1. Some people prefer to eat healthily for 5 days per week then reduce calories drastically for 2 days a week.

  2. Some people prefer one day a week of fasting with only water or black tea / coffee / green tea / herbal teas.

  3. Others prefer longer fasts for several days.

  4. 16:8 - fast for 16 hours & eat over an 8 hour period. Most people choose a late breakfast at 10 or 11am and then finish eating for the day at 6 or 7pm. This is also known as time restricted eating.

Most commonly in clinical practice, I use time restricted eating, (TRE) often I start at 12 hours (dinner at 7pm then breakfast at 7am) then work up to 16 hours of not eating by pushing out breakfast later and later over the following weeks or months. I find this way to be achievable for men and women regardless of their employment. Shift workers for example have a high incidence of diabetes & obesity can use TRE to combat these risks.

The following image shows the effect of fasting on body systems.

Intermittent fasting (IF) results in reduced levels of insulin and leptin & increases in insulin and leptin sensitivity; reduced body fat; elevated ketone levels; reduced resting heart rate and blood pressure, and increased heart rate variability, reduced inflammation; increased resistance of the brain and heart to stress and resistance to diabetes. IF can delay onset and slow the progression of neuronal dysfunction and degeneration in animal models of Alzheimer’s, Parkinson’s and Huntington’s diseases.

If you want to lose weight, have more energy, reduce cholesterol & reduce the risk of diabetes, support the brain and the cardiovascular system then taking small steps towards metabolic flexibility is key. Slowly incorporating movement / exercise & periods of fasting are 2 ways that you are able to achieve this safely for majority of people, but there are some people for whom fasting is not suitable so it is always important to chat to your health care provider whenever planning any major change. Start slowly, building periods of exercise and fasting gradually into your life, as this will have far reaching, beneficial consequences. for all aspects of health & wellbeing not just weight.....


Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects-A Narrative Review of Human and Animal Evidence. Behav Sci (Basel). 2017 Jan 19;7(1):4. doi: 10.3390/bs7010004. PMID: 28106818; PMCID: PMC5371748.

Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017 Oct;39:46-58. doi: 10.1016/j.arr.2016.10.005. Epub 2016 Oct 31. PMID: 27810402; PMCID: PMC5411330.

Rynders CA, Blanc S, DeJong N, Bessesen DH, Bergouignan A. Sedentary behaviour is a key determinant of metabolic inflexibility. J Physiol. 2018 Apr 15;596(8):1319-1330. doi: 10.1113/JP273282. Epub 2017 Jul 4. PMID: 28543022; PMCID: calories for energy.

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