I thought I would write this to help people identify how, when and why potentially utilising both methods of FED and FASTED training protocols could be beneficial, depending on the situation and goal at hand. This can relate to anyone from an athlete or general health span aspirations.
So to identify what we're looking at by the term fasted, it is simply characterised by the absence of food / beverage intake containing 'energy' in the form of calories for a period of time, which may last from several hours to days (typically 4-6 hours between meals or 8-12 hours at night during sleep). Fed exercise is of course having consumed energy / calories prior to undergoing physical activity within 4 hours up exercise being performed.
FASTED TRAINING:
Fasting promotes lowering levels of insulin in the blood. So when aerobic exercise is performed in a fasted state, more oxidation of fat as a substrate is seen in comparison to when fed.
Fasted exercise has been shown to deplete stored glycogen sooner and therefore initiate fat oxidation sooner than fed training due to the lower blood insulin levels when starting exercise.
However sustaining long durations of performance may be compromised with lethargy and low energy, poor concentration becoming prominent.
When glucose is not present (fasted or depleted from current training) the body has to produce glucose for energy via energy pathways such as 'gluconeogenesis' whereby fat stores or eventually muscle tissue is used.
FED TRAINING:
When fed, higher insulin levels are present therefore increasing the amount of time until fat oxidation occurs due to higher glucose availability (from the calories consumed and available for energy).
The intake of carbohydrate-rich meals prior to training increases the availability of glucose during exercise and therefore favouring extended periods of exercise within increased intensity demands.
Timing and type of food intake is definitely important for FED training to preventing digestion issues or nausea during training.
Training intensity:
One important variable is the type of training in to be performed.
Studies have demonstrated that low to moderate exercise performed in the fasted state can increase the rate of fat oxidation at rest from 9 to 24 after exercise when compared with the same exercise performed after a meal.
Research has shown there to be little difference between fasted and fed training towards fat utilisation during higher intensity training...
" Fat oxidation, rather than the use of carbohydrate as a substrate, tends to be higher at low-to-moderate intensities of exercise, no >60–65 % VO2max, but is likely to decrease at an intensity >75 % VO2max".
Therefore, a simple form of low intensity exercise can be a great way to induce fat metabolism (used stored fat for energy) and improve insulin sensitivity levels at rest promoting fat reduction over time. Chronically, this can help to reduce the chance of type 2 diabetes, especially those who do not undergo consistent intense exercise protocols
Athletes
With physically active individuals, fat oxidation during sub-maximal exercise is improved during fed training, therefore favouring higher intensity and fed training as a regular approach in comparison to lower intensity fasted training, for longer term benefits to performance and body composition.
It is important, for sport specific athletes in-particular, to plan weight loss gradually over time to enhance performance with training whilst on a calorie deficit. Unplanned or too regular fasted training could lead to excessive muscle wastage, no energy during performance, inconsistent daily macronutrient intake, reduced cognitive focus and sub-optimal sleep / recovery plus many more.
Overall:
Generally, it would be strategically wise to use FED training for the majority of training, especially moderate to high intensity. Training with intensity needs to be sustainable over the duration of a program, for the important adaptations long term, such as elevated resting heart rate, blood lactate accumulation and carbon dioxide levels stimulating metabolic rate at rest.
This of course, makes weight management more realistic and consistent, promoting better appetite and being able to simply move more and enjoy more food. Fuelled workouts help to feel strong, cognitively undistracted, and reduce the chance of other poor nutritional choices post workout. Also preventing an unhealthy relationship with food and training (eating disorder territory) and making macronutrient targets easier to achieve throughout the day.
Low intensity steady state training (known as LISS) when fasted, can be used but consider it more-so as a tool for trained athletes depending on the circumstances and lifestyle variables. LISS should be performed more commonly by untrained individuals using using simple low intensity exercise such as hill walking, fast walking, easy cycling or other low intensity cardio methods to elicit fat storage depending on the goal and personal needs.
A good way to make sure you don't go excessively in relation to intensity is used a heart rate monitor or rate exertion levels on a scale of 1-10, aiming for around 3-5.
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References:
Vieira, A. F., Costa, R. R., Macedo, R. C. O., Coconcelli, L. and Kruel, L. F. M. (2016) “Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis,” British Journal of Nutrition, Cambridge University Press, 116(7), pp. 1153–1164.
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