Skip to main content

Time restricted fasting or intermittent energy restriction is potentially useful tool for weight loss  if done properly or even just to avoid weight gain but there are caveats.


So what is time restricted fasting or intermittent energy restriction?  (I will call this intermittent fasting (IF) going forward for ease).  There are a number of definitions but an accurate description is the voluntary avoidance of food and drink for a defined period of time.  It has been popularised by a number of people and some examples you may be aware of are:

  • The 5:2 diet adopted by Michael Mosley where one eats ‘normally’ (see below) for 5 days and restricted calories of 500-600 on the other two.
  • 16:8 diet popularised by Jason Fung where one fasts for 16 hours and eats within the remaining 8 hour window.
  • The Warrior Diet popularised fitness expert Ori Hofmekle where you eat only small amounts of fruit and vegetables during the day and one large meal around 4pm.

What these all have in common is that they are restricting excess energy consumption and allowing the body longer periods of rest in-between eating periods.

What are the known benefits of intermittent fasting?  IF has been shown to :

  • improve markers of glucose and lipid metabolism which can help protect against obesity, type-2-diabetes and metabolic syndrome (1) .
  • improve DNA repair – associated with improved structural integrity, reduced ageing and reduced risk of cancer (2).
  • improve markers of immune function (3) .
  • improve markers for circadian rhythm associated with improved sleep (4) .
  • upregulate proteins known to be protective against Alzheimers’ Disease (5) .

What are some potential negative impacts if not done properly:

  • changes in thyroid function which is closely tied with metabolism and can have many knock on impacts on the body (6) .
  • decreased testosterone and IgF which can influence fat free mass levels (7) . 
  • imbalanced blood sugar leading to hypoglycaemia and disrupted sleep patterns.
  • increased cortisol levels which has other long-term negative health impacts.
  • encouragement of obsessive-compulsive eating habits and eating disorders.

One of the key mechanisms that IF works by is to encourage the burning of fat stores as our body begins to realise that the sugar (carbohydrate) that is normally on-tap, is not there.   Not only can this help with weight loss but it can also help offset some of the deleterious effects on metabolism induced by our modern-day lifestyle of round-the-clock on-tap food (8).  For athletes who may not be concerned with weight loss it can help with metabolic flexibility which is a great benefit for endurance sport (see below).


Another mechanism that brings many benefits is that using fat as more of a primary fuel can switch on a pathway known as m-TOR and encouraging cellular repair and improving cellular ageing along with  increased production of mitochondria which are our cell’s energy powerhouses so thus one of the reasons many people may feel improved energy levels when following an IF eating strategy

The guidelines can really be very simple (caveat – for those who are not suffering from any other underlying chronic conditions):

  • Aim to eat within a reduced time window but what works for you is going to be very individual. You can shift this a little each day to adjust for training/work hours etc.  Remember some days you may just need to eat breakfast earlier due to other events that are gong in your life that can alter our hormonal balance on a day-by-day basis.
  • If you are used to eating within say a 14 hour window, build up to this gradually, slowly reducing the hours in which you eat.
  • Within this window if you listen to your body and avoid  processed foods and ensure all meals and snacks are balanced with fat and protein.   Dont use the restricted time window as an excuse to make poorer food choices.
  • In the early stages you are going to feel hungry at times you usually eat, gradually push out this time-window but only if you feel well.  Monitoring your blood sugat can be a useful tool and indeed an essential one if you have any condition where you do not metabolise sugar as you should such as in insulin resistance or T2D (see caveat below).
  • Think about changing some of your routines to make this easier for you.  For example, at night, if  it is usual to begin switching off in the kitchen or sofa with a glass of wine, how can you break this habit?  Go for a walk instead, making sure most of the food preparation was done earlier in the day can be really helpful.
  • Ensuring you start the day with a well-balanced meal at what-ever time you choose is really key to balance blood sugar.  So for example, think 25-30% of your plate as protein and a similar amount of healthy fats.  If you are aiming to lose weight, maintaining muscle is key so protein levels in the diet are important.  Athletes are likely to need to adjust these levels for more carbohydrates around certain training sessions.
  • Remember genetics plays a role. If something isn’t working for you, you may need to take the transition more slowly, or adjust the plan.

Additional considerations for Athletes

Athletes will need to incorporate more flexibility into any plan compared to those with very similar day-to-day structures, especially if you have double, or more, training sessions a day.  It is important to remember that fasted training is a stress response so understanding your body’s overall stress load is important.  In addition, when burning fat rather than glucose for energy, VO2 max reached is going to be lower, so harder more intense training sessions should not be done in a fasted state.  If done incorrectly can also impact immune and thyroid function so listening your body is important.

For endurance athletes specifically, metabolic flexibility is really useful for those longer races and reducing the reliance on gels and thus risk of gut upset, one of the most common reasons cited that races are not finished.  Metabolic flexibility means having the ability to switch between different energy stores as needed.

A useful way to incorporate it can be around a periodised nutrition and training plan and incorporating some IF techniques during lower-volume weeks such as recovery weeks or the off-season.  During the COVID-season, if training is volume and intensity is generally lower, this may be a good time to play around a little but understand the risks as well as benefit and, as always, listen to your body.

Juggling your timing of food around appropriate training sessions is needed.   Training in a glycogen depleted state for a specific period of time and possible periods of IF have been suggested to be a possible strategy to create a stress response for tissue adaptation (8) but should be done in controlled circumstances and ideally alongside some blood markers so the stress response,  thyroid function, immune markers, glucose and lipid metabolism can be monitored.

SUMMARY: To do IF it doesn’t matter which ‘popularised’ version you follow or just adapt your own routine that fits in with your lifestyle so long as you understand and follow the main principles.

CAUTION:  this should not be done if you either have an eating disorder (recognised or suspected) or if you cannot metabolise glucose well as in insulin resistance, type-2-diabetes or other chronic health conditions where fuel metabolism including fat metabolism may be impaired.  Under these circumstances, this should only be done under the supervision of a suitably qualified health-care professional.

If you want to find out more how you can run these tests and interpret the results  please contact Katherine here.