There is no doubt that fasting is an effective form of weight loss. But with any extreme technique there are a huge number of factors that need to be taken into consideration before anyone chooses to take this route. I look mainly at the effect fasting will have on an MMA Athlete who regularly trains and competes; however many of these effects are transferable to non-competitive athletes.
When we talk about fasting most people will refer to a form of ‘Intermittent Fasting’ which is defined as ‘an umbrella term for various diets that cycle between a period of fasting and non-fasting’  a popular example is the 5:2 Diet. However, Intermittent Fasting is a style of eating, and not a diet as such. The 5:2 Diet suggests that a person should eat ‘normally’ for a period of 5 days, followed by 2 days of consuming under 500 calories. The method behind this relies on the very simple equation that all humans can swear by: if calories consumed are less then calories expended this will result in weight loss.
However, long periods of fasting can be counterproductive. The human body is incredibly clever and will generally adapt to whatever extreme you put it under. During long term fasting, your body will quickly use up what it can find for fuel, the easiest and most accessible is Glycogen. Firstly from your muscles, and then from your liver and other organs. Secondly, fat stores which is why the 5:2 diet can be effective. And lastly protein; but not only the protein you hold in your large muscles such as your quads. This protein will be drawn from all muscles including your heart. An athlete will be unable to function fully after even one day of fasting due to the glycogen depletion; drawing strength and stamina away from their whole performance. People assume this is ok to do during ‘training’. Training is directly linked to competitive performance, but an athlete who is unable to drill and train for a real life situation due to extreme fatigue and loss of power through shocks, strikes etc will never gain the most from a training session.
The common practise for Intermittent Fasting in athletes is to fast on their rest days. I could easily get carried away at how detrimental this can be but simply put: Just because your mind is resting that doesn’t mean your body is to. Working on the theory that extreme weight loss is generally only used on a fight camp to make weight you can assume that any athlete will be training extremely hard; the body NEEDS those days to repair and rebuild the damage done from the previous training sessions; and this cannot be done on an extreme calorie deficit. It is the job of the Nutritionist to ensure minimal loss to a fighters muscle and therefore loss of power throughout the whole camp to create an athlete who peaks perfectly for their fight night with maximum strength, physically and mentally.
It is incredibly important to take into account a fighters state of mind during a fight camp. For some fighters the harder they grind, and the harder they find the camp, the stronger they become. For others it is the complete opposite. The relationship with food varies from person to person, and the satisfaction and reward from eating will vary too. You need to understand whether a technique of extreme measures such as Intermittent Fasting will break down your athlete and make them weaker and resent the fight which will have detrimental effects on their performance.
My job would be incredibly easy if what people say they do, and what people actually do, were the same. Binge eating is unfortunately common place in any weight loss programme, and it usually comes hand-in-hand with feelings of guilt and embarrassment following the binge due to the added pressure on a fighter to make weight. Therefore a fighter is less likely to tell you if they have had a binge. I work on the theory that extreme measures should be avoided during any camp, and truly believe that the best preparation comes outside of camps. A slow and steady ‘bulk’ will generally lead to an easier weight loss and therefore a positive camp. If an athlete feels they need to binge eat I often ask the question why? Have I made the cut too quickly? Are the calories too low? Have I messed up their insulin levels? Have I created a ‘sugar dump’? Are they struggling mentally?
With periods of Intermittent Fasting you are unable to control the cravings and this may lead to huge binges. Which in the long run will mean that a slower and steadier cut would have been more productive. Tortoise and Hare style.
It is wrong to believe that Intermittent Fasting has been proven to speed up the metabolism, in fact there are studies that provide conflicting evidence to both sides and so far the result is inconclusive. But for the purpose of using IF for a fighter it is clear that in the long term, fasting will slow down the metabolism. An athlete will generally burn more calories sitting still then a non-athlete due to the higher calorie consumption of their larger muscle mass. If you continually eat into that muscle mass as mentioned above you will decrease it over time, therefore slowing the rate of energy expenditure. It is important to look at the longevity of an athlete. A young fighter just starting out will need to make many cuts in their competitive life, and slowing down ones metabolism will mean harder and harder cuts in future, and could potentially make or break a career.
But, fasting is still effective. Personally, I have many cards up my sleeve to help a fighter cut. I chose to start off with a very simple calorie restriction, and get more creative or extreme depending on how the body responds to the cut and the training etc. It is helpful to work with a coach to create a plan tailored to the camp itself and meeting the demands put on the athletes body. A person who has continually made cuts that have been too extreme, too long and too often may find that fasting is a great way to kick start their cut; but there are many options to explore first that aren’t quite as extreme or demanding.
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Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials.