If you've heard the term fasted cardio bandied about, you're one of a large majority. The training technique of exercising before eating anything (usually before breaking your overnight fast) is one that bodybuilders, endurance athletes and your average fitness pal all have in common. And it's growing in popularity thanks to its innate free-ness.
However - and this is crucial - fasted cardio should be combined with an already healthy routine. It should never be the be-all and end-all. Concentrating on a balanced workout routine with adequate rest, proper nutrition and good stress management is always the foundation of a safe and sustainable fitness journey. Also, put simply, it's not for everyone.
Who should avoid fasted cardio?
As with anything that works by tweaking your food or exercise schedule, there are going to be some caveats about who it's appropriate for and who should probably avoid it.
If exercising without eating beforehand triggers any thoughts of restriction or eating disorder tendencies, pull back and ask for help. Your health is always the most important thing.
If you, or anyone you know, is struggling with an eating disorder, contact Beat, the UK-based charity who hope to end the pain and suffering caused by eating disorders.
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"It's important to remember that fasted cardio is a personal choice," says trainer Sarah Craske, owner of functional fitness studio F45 Cambridge Station. "Some find it difficult to eat prior to a workout whereas others need the energy from food – it's completely dependent on the individual."
If you identify with any of the following medical concerns, it's best to speak to your GP thoroughly first.
You are pregnant or breastfeeding
You are someone with diabetes
You have low blood pressure
You have an adrenal-related medical condition, like Addison's disease
You are a teenager or a child
What is 'fasted cardio'?
Put simply, it's the act of doing cardiovascular exercise without having eaten before. Ever gone for a jog before breakfast? That's fasted cardio.
For a deeper dive into what fasted cardio actually means, we asked registered dietitian and Fuelbetter's lead sports dietician, Anna Hardman, to fill us in.
"If you’re exercising in a fasted state, it means you’re trying to exercise when your liver and muscle tissue are glycogen-depleted. Glycogen is your body’s way of storing carbohydrates, and your body needs carbohydrates to fuel the brain and to fuel high-intensity exercise," she explains.
The two different types of fasted cardio
"Exercising in a fasted state has two levels," explains Hardman. "Level one is when your liver is low on glycogen, but your muscle glycogen stores are still decent. You can achieve this by having an early dinner, and then exercising the next morning before breakfast."
(For non-elite athletes or very techy amateurs, level one is the type of fasted cardio you're more likely to fall into.)
"Level two is when you’ve gone to the trouble of depleting your muscle glycogen stores and deliberately not replenishing them – for example, by exercising intensely for a week and minimising carbohydrate intake during that time."
Reducing your carbohydrates like this is something that's normally reserved for elite and higher-level athletes. Cutting any food group in this way should always be done with professional supervision and guidance: it's not just about chucking all the pasta in the bin and going cold turkey. Remember, smart fitness is sustainable.
What are the benefits of fasted cardio?
Like all benefits, they're going to slightly differ between individuals. This is because body types, genetics, age and activity level will play some part in influencing how your body reacts to exercise in a fasted state.
Equally, if you don't feel able to put in the work when you're sans fuel, you won't reap the benefits of a good workout. So, remember these benefits are felt by some not all.
1. Fat loss
Fasted cardio has long been a tool used by people trying to lose weight well and get the most out of their fat loss efforts. However, it's not as clear cut as you might think. While exercising without eating can slightly accelerate fat loss, it's not the only way nor the most enjoyable.
"There is a lot of attention currently focused on the idea that exercising in a fasted state can help accelerate weight loss by encouraging the body to rely more on fat and less on carbohydrates as fuel," describes Hardman.
"However, there is a catch: not only can this make training feel like extra hard work, but in a fasted state your body may also breakdown muscle mass to provide the fuel it needs to exercise. In addition, the reward, for those able to stick fasted exercise, is only a very modest increase in the amount of fat burned as fuel."
So, don't take fasted cardio as a one-way ticket to your body composition goals.
Looking after your nutrition, sleep, stress and incorporating other forms of exercise like resistance and strength training exercises is what will get you there. This tweak to your training is something to consider when you've got all other pieces of the puzzle in place first.
Half of the battle of morning workouts can be waking up with enough time to eat something and let it digest before you start. Fasted cardio means you can get up, get dressed and get on with it.
For some people, the feeling of working out before can make them feel faster, lighter or more agile. Craske says that this can be highly beneficial before plyometric workouts or short runs.
4. Improved performance
Before we jump into how fasted cardio can improve your athletic performance, there's a crucial note to make: the benefits on performance will mostly be felt by serious endurance athletes – something the majority of us are not.
"Studies have shown that training in a Level Two fasted state may help to improve race performance," Hardman says. "This is because training while muscles are glycogen-depleted encourages the active muscle tissue to rely a little more on fat and a little less on carbohydrates as their fuel source.
"For endurance athletes, this helps them preserve glycogen stores until a little later in the race – giving them an edge on their competitors by allowing them to run faster for longer."
Are there any side effects of exercising in a fasted state?
If not done safely, there are some warning signs to look out for.
"Symptoms that indicate unsafe fasted cardio exercise mainly relate to low blood sugar and dehydration," says GP, founder and medical director of The Lifestyle Code Clinic Dr Mishkat Shehata. "These include lightheadedness, confusion, shakiness, weakness and pallor."
"The important thing is to listen to your body and know when it is telling you to stop," says Craske. "You must also always ensure you are drinking water throughout any workout, as this will have a huge impact on the way you feel."
How long should a fasted cardio session be?
Anything moderate, under an hour, is best, advises Craske. Go over the 60-minute mark and you could start to reap the grizzly side effects of your body being low on fuel.
So, a medium-paced run or spin on the exercise bike, try to keep your effort at a low to moderate intensity. LISS (low-intensity steady state) training falls into this category too, like walking and gentle swim workouts.
How to eat after a fasted cardio workout
You've finished your workout and now it's time to eat. However, because you've put your body under a little more strain, there is a smart way to refuel.
"After exercise, you really want to focus on a nutrient-rich meal with a balance of carbohydrates and protein," says Hardman. Think eggs on toast, protein pancakes, a balanced smoothie with vegan protein powder or Greek yoghurt with overnight oats, porridge or cereal.
Need some inspiration? Check out these healthy fast-breaking options
15 Joe Wicks' overnight oats recipes
17 high protein breakfasts that aren't eggs
"I did fasted cardio for two weeks: here's how I found it"
After reading research published by the American Physiological Society, Women's Health's Camila Wood found that working out pre- breakfast uses stored fat – rather than that bowl of granola – to fuel metabolism, her mind started to turn.
Another study from Northumbria University found that fasted cardio can help you burn up to 20 per cent more body fat than post-breakfast training (with no appetite increase), was the final nail. She decided it was time to take the empty-bellied plunge.
How Camilla changed her workouts
"To go a little easier on myself, I swapped my six usual workouts for four spinning ones and, with two extra rest days a week, I was pretty excited about the prospect of losing more body fat with less effort," she says. "I didn’t find myself lacking energy when I trained first thing at 7 am – but my 10 am Saturday sessions were a different story. I was starving and found myself regularly (and surreptitiously) turning down the resistance."
Was it worth it?
"By the end of two weeks, I had lost an impressive 2kg and 2 per cent body fat. For me, it feels like a totally sustainable change to the way I usually work out – and now that I’ve ditched the caffeine, I have much more energy throughout the day."
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