The eternal struggle, killing the fat

Thursday, 18 July 2019

Eternal Struggle Cover

Metabolic flexibility, fat burning and fasting

Exercise and nutrition go hand in hand and should always be considered together to get the most out of training and enhance desired muscular and cellular adaptations. My blog post today will introduce the concept of metabolic flexibility, fat burning and the all popular, intermittent fasting…. Hope you enjoy!

How the body fuels itself

Our bodies are constantly using a mixture of fat and glucose (from carbohydrates) to fuel us throughout the day. However, the relative contribution of each fuel source is determined by two factors; our diet (particularly the fat and carbohydrate content), and the type of activity we are doing, i.e. the intensity and duration of the exercise.

Typically, the rate at which we burn fat increases as we go from low to moderate intensity exercise, i.e. walking to running. However, as the intensity increases to a fast run or sprint, carbohydrates become the primary source of fuel and the relative contribution of fat decreases. The reason for this is that glucose provides an instant energy source and is the most efficient to fuel the demands of high intensity and/or anaerobic activity (i.e. sprinting). Whereas fat, although supplying the body with a much greater store of energy, is much slower to break down and use, thus fat is preferentially utilised for lower intensity exercise.

What is metabolic flexibility and why is it important?

The ability to effectively use a range of fuel sources and to efficiently respond to the types of fuel we provide our bodies is a term known as metabolic flexibility and is desirable since it enables us to be adaptable to the varying demands of exercise and available fuel sources and will also improve our metabolic health, helping protect us against illnesses such as diabetes.

Most of us will be fairly good at using carbohydrates as fuel, since it is the easiest fuel source to burn and also generally makes up a large proportion of a typical daily diet. But how do we train our bodies to be better at burning fat?

Before I explain this further and reveal some ways in which you can enhance metabolic flexibility (especially fat burning), it’s important to be clear of your goal, training plan and personal preferences because as with any nutritional advice, there is never a one size fits all rule and often the best plans are those that are periodised. This means that different types of nutritional stimuli are matched with different types of training sessions to enhance specific training outcomes. For example, a low intensity cardio session could be conducted fasted to enhance fat burning, whereas a high intensity weight training session, where muscle growth is the goal, might be conducted when efficiently fuelled so optimal performance can be achieved.

Too many words in health
Some of us get confused with all the different information out there

Training your metabolism and fat adaptation

1. Time restricted eating and fasting

Our bodies are designed to respond to states of fasting and states of being fed, and these mechanisms are rooted from our ancestors where food availability could rapidly switch from being abundant to scarce. When we eat, insulin is produced, which works on the cells of our body to increase the absorption of glucose (carbohydrates) from our food to either be used as energy or stored (as glucose repositories or fat). The concurrent response to the rise in insulin, is a decrease in the use of fat for fuel. On the flip side, if we are fasted, insulin release decreases and instead the hormone glucagon is released which increases our ability to break down and utilise our fat stores for energy. Based on this rationale, fasting (or the easier version of time restricted eating) has become a new favourite for many people trying to lose weight, increase lean muscle and increase their efficiency at burning fat.

However, in addition to increased fat burning, fasting is also associated with a plethora of other health benefits….

 Helps protect cells from free radical damage

Free radicals are produced everyday by the body in response to the environment, exercise, and some normal biochemical reactions. However, free radicals can cause damage to our cells and DNA, enhancing cellular aging and the risk of disease. The beauty of fasting is that it shocks the cells in our body in to lock down mode, stepping up their natural defences and reducing the permeability of their membranes to foreign invaders and free radical damage. This means fasting can slow cellular ageing and even help protect against disease.

Increases lean body mass

This is an interesting outcome of fasting, since restricting calorie intake seems counterintuitive to increasing muscle mass. However, the mechanisms behind this are based on the hormones that are produced in response to a fasted state, in particular, Human Growth Hormone (HGH). This hormone is converted into IGF-1 in the liver which consequently increases the breakdown of fat and also triggers a signalling pathway that promotes muscle protein synthesis.

However, increasing calorie intake is still important for muscle gain so if that is your goal, then try to only use this technique once or twice a week on a light training or recovery day. Or simply use time restricted eating in preference to fasting, and make sure that you hit your daily calorie count in the time window you allow yourself to eat.

So…. are you convinced about fasting yet?

Although a 24 hour fast is still my ultimate goal, I also love food and get a serious case of ‘hangry’ without it. So, for me, the best solution is to implement time restricted eating only on certain training days

Although a 24 hour fast is still my ultimate goal, I also love food and get a serious case of ‘hangry’ without it. So, for me, the best solution is to implement time restricted eating only on certain training days; this is where I restrict the hours in which I eat to a certain time frame, for example, from 12-7pm. Consequently, I achieve a 17 hour fast (which is enough for me) and I have already noticed improvements in muscle mass, reduction in fat mass and I also feel better in myself!

2. Fasted training

On the days I fast, I also train in the morning which allows me to undergo a fasted training session. The rationale for fasted training is based on the fact that if you consume food prior to exercise, elevations in insulin cause our muscles to favour the use of glucose as a fuel and downregulate fat use. However, fasted training limits the amount of glucose available and forces our muscles to start breaking down fat.A quick tip…. ON AVERAGE fat oxidation peaks around 65% of maximal heart rate, so try and train within this heart rate zone during a fasted session (but remember that this is only average figure, everyone’s peak fat oxidation will vary depending on a range of factors).

3. Low carb, high fat diets

Research has shown that 1-3 days of a low carb, high fat diet can increase fat burning during submaximal exercise and similar to the methods above, long term use can induce cellular adaptations that increase the body's capacity to use fat as a fuel source.However, you should be careful if you are using this diet to increase fat loss as research has also shown there is no improvement in endurance performance at submaximal intensities compared to those on a high carbohydrate diet. Furthermore, the ability to perform high intensity sprints tends to be impaired in those following a low carb high fat diet, as the muscles lose the contribution of glucose, which primarily fuels high intensity, anaerobic exercise! Therefore, if you want to try this diet, it should only be followed periodically so that you can still appropriately fuel for the work required!

So when would a LCHF diet be appropriate?

  1. Muscle glucose stores require many days to fully replenish, whereas fat stores are much larger and last longer, therefore the ability to primarily use fat could be beneficial to endurance athletes performing events over continuous days where time to replenish carbohydrate stores is limited.
  2. Events involving prolonged low-moderate exercise with no requirement for higher intensity efforts (although this is rare, since everyone wants to sprint to the finish line at the end of an endurance event!)
  3. If you find it hard to consume enough carbohydrates to meet your target due to gastric distress or the inability to access carbohydrate supplies during the event, then being able to more effectively utilise fat could be beneficial!

Hopefully you now have a good idea of some methods you could try to increase your ability to use fat but remember that your nutritional needs will vary depending on the type and intensity of training you perform, and your ultimate goal….so bare this in mind when designing your own meal plan!


This piece was written by Beth Cragg, Brookes University Masters Graduate in Sports Nutrition. You can find Beth on Instagram

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