You Can Train Your Body To Burn Its Own Fat For Energy – But Is That As Good As It Sounds?

Becoming ‘fat adapted’ is the process of teaching your body to tap into its own, virtually limitless supply of fat reserves for energy. In doing so you can reduce your reliance on carbohydrate and potentially increase your stamina. All that seems great news for endurance athletes, but could everyone benefit from facing the fats? RSNG reveals six things you need to know before trying it for yourself…

1. You Don’t Have To Cut Carbs Becoming fat adapted doesn’t necessarily mean you need to follow a high-fat, low-carb diet (although that is one way of doing so). ‘To be fat adapted means that metabolic changes have occurred to allow your muscles to use fat stores as an energy source when carbohydrates aren’t readily available,’ explains qualified dietician and sports nutritionist Evelyn Toner (@activedietician). But those metabolic changes can be instigated by training, as well as diet.

‘The most popular method is to train in a fasted state over weeks, months, and longer,’ explains Don Maclaren, Emeritus Professor of Sports Nutrition at Liverpool John Moores University. ‘In these instances the body adapts to greater fat burning and very low carb burning.’

Morning fat burning is best achieved after an overnight 10-12 hour fast

But if fasted training sounds complex, it needn’t be – if you’ve ever been for an early-morning run or cycle the chances are you did so in a fasted state. ‘Morning fat burning is best achieved after an overnight (10-12 hour) fast,’ says Maclaren. In other words, if the last thing you ate was dinner at 8pm then you would train pre-breakfast, at 7am the following morning.

Train fasted regularly – once or twice a week – over a period of months, and your body will learn to tap into its existing fat reserves for fuel. Slow and steady is the name of the game and you should start with short sessions, building up gradually as your adaptation occurs, in order to avoid lightheadedness.

2. But A Low-Carb High-Fat Diet Will Also Work If the mere thought of training on an empty stomach makes your head spin, fat adaptation can also be achieved by altering your diet – although this is arguably a more radical change. ‘It would very much depend on the individuals weight, gender and activity levels as to what the actual quantities would be,’ says Toner. ‘What I would say would be that the diet would need to be high in fat-rich sources, like nuts, olive oil, rapeseed oil, avocado, fatty fish, eggs, and moderate amounts of full-fat dairy products.’

Although she hedges this with a warning: ‘I wouldn’t recommend such a diet, because it would be insufficient in fruit and vegetables, wholegrains and dietary fibre – all of which are essential for health and wellbeing.’

3. Fat Adaption Plus Carb Cycling Equals Peak Performance As Toner explains, most studies on the subject involve participants following, ‘a low-carbohydrate (less than 20% of total energy consumption), high-fat (over 65% total calories from fat) diet for a specified time period, alongside a structured endurance training program.’ As for the duration needed for such a diet to take effect, ‘studies vary, but some suggest it could be done for five to seven days before a competition, followed by one to three days of high-carbohydrate intake to replenish muscle glycogen stores before the event.’

That idea of putting carbs back into the system shortly before a race is one advocated by Maclaren, who says: ‘A number of studies have reported on athletes undertaking various shorter term training on a low-carb, high-fat diet (weeks rather than months) and then carbing up 24 hours prior to a laboratory time trial. The benefits of fat adaptation on fat burning were evident, but interestingly the results were favourable because the muscle glycogen stores elevated by the 24 hours of carbing up were used less – so, the body was fat adapted but could draw on the carbs when needed.’

Those doing exercise of the short and sharp variety should probably think twice

4. It’s For Marathons, Not Sprints Training your body to become fat adapted, so that it’s able to tap into its own fat reserves for energy, may sound too good to be true – and for some, it is. Because while endurance athletes can undoubtedly benefit, those doing exercise of the short and sharp variety should probably think twice.

‘Glycogen (produced by carbohydrate) is essential for high-intensity effort – even a sprint finish at the end of a marathon,’ says Maclaren. ‘So sprint and power event specialists should avoid fasted training; there is substantial evidence the quality of subsequent training in terms of power and speed will be compromised.’

But for the aforementioned endurance types, fat adaptation may well be the boost your stamina needs. ‘I think one fasted session a week can be a very valid training method,’ says running coach Karen Weir ( ‘I’ve used it myself and for me it’s actually quite important to get my clients – who generally are experienced runners – much more efficient at burning fat as an energy source.’

‘But it takes a while, so it needs to be part of a prolonged training programme: if you’re using it for a marathon campaign, you would start doing it from the outset. You wouldn’t try it for the first time during your longest run three weeks out from the race, because that would just be far too late.’

5. Beginners Should Think Again If you’re looking to improve endurance in a sport you’re new to, practice – rather than fasted training or low-carb dieting – will make perfect. Becoming fat adapted takes time and a considerable amount of hard work. At first, at least, those early-morning runs with no fuel in the tank, or the days without carbs, are going to be tough – and unnecessarily so if you’re in the early stages of training.

‘There are [experienced] runners, particularly within ultrarunning, who swear by racing in a fasted or fat-adapted state,’ says Weir. ‘But if you’re a beginner, don’t bother with it. You should focus, instead, on building up your routine and the simple enjoyment of running.’

6. The Studies Are Limited Reducing reliance on carbohydrate, and teaching your body to utilise its virtually limitless supply of fat reserves, has obvious benefits for endurance athletes, but there also lies the rub: most of the research has been conducted on well-trained, elite sportspeople. The benefits of fat adaptation for the rest of us is based largely on individual experience rather than scientific study.

According to Toner, actual athletes, ‘are better able to take up oxygen from the blood supply during exercise, and therefore better able to use fatty acids as a fuel source – as the presence of oxygen is essential for fatty acid oxidation. There’s not much research to show if this would even happen in untrained athletes.’

And even the pro studies have provided mixed results. ‘Some have observed impaired training responses after four weeks of low-carb dieting,’ says Toner, who goes on to warn that ‘metabolic changes do not automatically equal improved performance.’

There are runners, particularly within ultrarunning, who swear by racing in a fat-adapted state

WHAT NEXT? Find out more about how endurance athletes – and runners in particular – can benefit from becoming fat adapted...

Comments are for information only and should not replace medical care or recommendations. Please check with your Doctor before embarking on exercise or nutrition regimes for the first time.