All You Need to Know About Fasting

Generally, when we talk about fasting, we are talking about intermittent fasting, and the function is often to lose weight.  What this mechanism does is reduce overall caloric intake, so by definition, if you reduce calories, there may be a resultant weight loss. We say ‘may’ because weight loss isn’t always that simple, there are many other factors at play like genes, microbiome composition, and more.

There are also many who tout fasting for metabolic health and that it offers a range of benefits to optimise overall health. There are also many who tout fasting for metabolic health and that it offers a range of benefits to optimise overall health.

Like most things in life, it perhaps isn’t that simple, so let’s take a look at fasting, the science and what the data says. 

What Is Fasting?

Fasting is not the same as starvation.  Starvation occurs when there are no reserves left in the body; with insufficient intake of nutrients resulting in the breakdown of vital tissues.  Fasting on the other hand is reducing calorie intake for a period of time.

We tend to consider human metabolism in two forms; we have the fed state and the fasted state.  

The fed state occurs after a meal and is also known as the absorptive state.  It is characterised by high insulin to glucagon ratio, largely in response to the blood glucose rise from 5mmol/L to 8-9mmol/L.  The reason we mention the hormones is that it is these guys that control much of what occurs in the body during this state.  

Anabolic metabolism dominates in the fed state largely to replenish fuel stores, and as we know, anabolism is the building-up aspect of metabolism. 

The fasting state occurs between meals and ensures maintenance of blood glucose at 5mmol/L.  This state is characterised by low insulin to glucagon ratio.  This low insulin to glucagon ratio overall promotes catabolism in comparison to the fed state; catabolism is the breaking-down aspect of metabolism.  It is this concept that has largely driven the touting of fasting as beneficial for health.  


It is thought that fasting is a possible trigger of autophagy. 

Autophagy occurs naturally in the body, and it is a process where cells remove unwanted molecules and dysfunctional parts – in short, the bad bits are catabolised.  

It must be remembered that the link between autophagy and fasting is in its early stages and some literature suggests that significant autophagy may take up to four days of fasting, which then has its own list of considerations, especially if you are active and need sufficient caloric intake to fuel your activities.  

Despite this, there are some supportive data around fasting and its health benefits. 

General health benefits

Fasting has been seen to decrease toxic load.  With little competition from substances absorbed after a meal, the liver is able to fully process waste products and excrete them efficiently.  

In addition, it has been demonstrated that during fasting, mitochondria downregulate.  The mitochondria are the power plants of the cell, where we get energy from.  But through the process, a high volume of free radicals are produced.  Therefore, with downregulation, fewer free radicals are produced, which reduces oxidative stress.      

Fasting has also been seen to decrease inflammatory molecules in many cell types (i). As we know inflammation underlies many chronic diseases and so it is thought that fasting reduces the incidence of:

  • Cancer
  • Arthritis
  • Cognitive decline
  • Diabetes (prevention)
  • Dermatitis
  • Liver, kidney, and heart disease



However, in cases of diabetes, caution must be exhibited. 

Diabetes is a disease that results from an absolute or relative lack of insulin. Insulin helps get glucose into cells to be used as energy; it, therefore, maintains the amount of glucose found in the bloodstream.  As we noted earlier, the fed and fasting state are defined by their blood glucose concentration.  Fasting with diabetes can result in dangerously low blood sugar levels and so if you do have any pre-existing conditions and are considering altering your caloric intake, seek advice from a qualified professional. 


If you are considering fasting there are a number of ways to do it:


🍳 The 16/8 method involves fasting every day for about 16 hours and restricting your daily eating window to approximately 8 hours.

🥘 The 5:2 diet involves eating what you typically eat 5 days of the week and restricting your calorie intake to 500–600 for 2 days of the week.

🌮 Eat Stop Eat involves a 24-hour fast once or twice per week.  Fasting from dinner one day to dinner the next day amounts to a full 24-hour fast.  Water, coffee and zero-calorie beverages are permitted during this time.  This approach can be difficult for some, however.  

Finally, if these approaches sound tough to start, you don’t need to follow a structured intermittent fasting plan to reap some of the benefits. Another option is to simply skip meals from time to time, such as when you don’t feel hungry or are too busy to cook and eat.  This is quite common when we skip breakfast for example.  

👉 In doing this we must consider our own metabolic health and mood.  

Maintaining regular blood-glucose levels is key to so many areas of health, including mood.  


One study of 329 adults showed that in women, those who did not consume breakfast had higher feelings of anxiety, men also had higher depression scores in the presence of breakfast skipping.  Furthermore, both men and women report higher fatigue and less vigour if they skipped breakfast.

It is also suggested that breakfast skipping is associated with Vitamin D deficiency. And we know the wide-reaching role Vitamin D has on both the body and mind! (i)

It is well established that in cases of poor mental well-being, meal-timing and content are particularly important. So if you have tried or want to try fasting, be mindful of how your feel and follow your body’s lead.  

Intermittent Fasting

There is no denying that there are some scientifically-backed benefits from intermittent fasting. But is this simply because of the reduced calorie intake, or even because we are making a conscious effort to improve our health?  For example, when we do eat outside of the fasting window, food choices are more beneficial to our health than usual.  

Intermittent fasting can be useful in weight loss programmes because it does reduce overall caloric intake. But then, like most diets, when the normal eating pattern is resumed, weight is often regained.  Is intermittent fasting sustainable long term for you?  The impact on mood in some also shouldn’t be ignored.  It may be that it’s more appropriate for some to avoid snacking between meals as a start. And ensuring that they consume well-balanced meals to support this; filling up on protein and fibre to support satiety for example. 

If you are considering any change to your diet, always seek the guidance of a qualified professional.