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Should you fast before training?

A boy is running
(Image credit: Unknown)

Yes...says Richard Scrivener, consultant personal trainer for Universal Pictures UK

To burn more fat than normal you need to create a calorie deficit by getting your body to use its own energy reserves. It takes six to eight hours to fully empty your digestive tract and use up most of the carbohydrates stored in your muscles as glycogen to fuel your movement – the point at which you begin using fat as fuel. Also, insulin levels increase after eating, and according to research, raising insulin can reduce fat-burning by up to 22%. 

Muscles need stored glycogen to lift heavy weights so it’s not a good idea to do all types of exercise in a fasted state. Cardio workouts such as rowing, running or swimming are the best exercises to do. Mixed-weight, total-body circuits are also effective. Just steer clear of heavy low-rep strength sessions unless you want to see your PBs disappear – or stars, depending on how fasted you are. 

There are no strict rules about timing, so experiment. You could eat your evening meal and then fast until the following evening; train then eat immediately afterwards; or start and finish a fast at breakfast time, again training just before you break the fast if that fits your schedule better. Either way, start with gentle workouts and only ramp up the intensity and amount of workouts when you’re used to exercising on an empty stomach.

As a bonus, you’ll also realise you can happily cope without eating every hour or two, resulting in a better relationship with food in general. And believe me, nothing tastes better than the meal  you eat after a fasted workout.

No...says Scott Baptie (www.foodforfitness.co.uk (opens in new tab)), a nutritionist specialising in fat loss and sports performance

Afasted cardio workout in the morning is a staple of various bodybuilding training routines. The logic is that low glycogen (stored carbohydrate) levels will cause your body to move away from using carbohydrate and start to use body fat instead. However, according to a recent study published in the Journal Of The International Society Of Sports Nutrition, training when fasted has no discernible difference on your body composition compared with doing it in a fed state. 

Other studies show that consuming carbohydrates before training doesn’t blunt fat loss, and in some cases can even increase fat loss after training because your work capacity is greater, giving you more of a long-lasting metabolism boost. If you find that exercising first thing on an empty stomach – whether through choice or because you find eating first thing makes you nauseated – is the best option, then go for it. Just don’t worry that you’re going to scupper your fat loss efforts if you ever do succumb to hunger and eat something before training – you won’t.

Fundamentally there is no substantial evidence in favour of fasted exercise, so I suggest having a meal rich in protein and carbohydrates before training to provide fuel for your session and minimise any potential muscle loss. It’ll allow you to go harder during training which will keep you burning fat for longer after you’ve finished and optimise your muscle growth, helping you to achieve the muscular, lean body you’re after.

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