One of the most common beliefs on fasted cardio is that by exercising on an empty stomach, you’re more likely to burn fat. This is, theoretically, true. A meta-analysis of 273 people compared fasted cardio to ‘fed’ cardio and found that those who did fasted cardio burned more fat, but as mentioned in our fasted cardio guide, this is purely an acute effect. In other words, you may burn more fat during fasted cardio, but it won’t affect your body composition in the long run.
This has also been scientifically proven. One study compared 10 women who did fasted cardio every day for four weeks to 10 women who had a shake before exercising, and the results showed no difference in fat loss between the two groups.
Another study found that over a 24-hour period, the rate of fat burn could actually be slightly higher when training in a ‘fed state’ compared to after fasting.
Now, much of what we’ve touched upon so far relates to fat burning, so what if that’s not your goal? Well, training in a ‘fed state’ could also be better for your hormones and athletic performance, particularly as women. This is something Renee McGregor, a sports dietitian, and Dr Stacy Sims, a female physiology expert and author of ROAR, are keen to set straight, so we caught up with them for everything you need to know.
How could fasted cardio affect women?
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Before we go any further, we’re not saying you need to totally throw in the towel on fasted cardio. If done safely (which we’ll come onto further down), there are some benefits. It’s efficient, for one, and it may well be better for those of who you often feel nauseous if you eat prior to a workout (one study proved that this is more likely to occur on a full stomach).
Doing too much intense fasted cardio, however, could cause your hormones to go haywire. The source of this is an increased sensitivity to low energy availability (i.e. when you ‘deny your body of an important fuel source’) compared to that of males. ‘Kisspeptin is a neuropeptide responsible for sex hormones and endocrine and reproductive function,’ Dr Sims explains.
‘It plays a significant role in maintaining healthy glucose levels, appetite regulation and body composition. Women have two areas of this in their hypothalamus – their brain – whereas men have one, so women have a greater sensitivity to changes in their energy balance. This energy balance is disrupted through fasted cardio, as your brain perceives a deficiency in nutrients.’ Indeed, one study found that fasting decreases the production of kisspeptin, thereby disrupting the release of estrogen and progesterone – the two key female reproductive hormones.
Adding exercise into the equation exacerbates a hormonal imbalance further. ‘Firstly, stress hormones like cortisol rise even higher,’ Dr Sims adds. ‘As you keep increasing that stress with exercise, it reduces your ability to relax. Your thyroid activity is depressed, the key hormone for regulating menstrual cycles; the luteinising hormone (LH) starts to lose its daily pulse and this messes with your menstrual cycle.
‘Your body also starts storing more belly fat. Now you’re facing disrupted menstrual cycles, higher anxiety and stress, impaired performance and often weight gain. Additionally, fasting kicks up the sympathetic (a.k.a. fight or flight) drive in women, producing more anxiety, a greater risk of depression, and impaired sleep patterns (a significant reduction to REM, Rapid Eye Movement, and SWS, Slow Wave Sleep, the two patterns responsible for mental and physical growth and reparation) which can be detrimental to your hormonal health, performance, wellbeing, and body composition.’
McGregor adds that your cortisol levels are naturally ‘highest in the morning’, so by adding stress to your body and training at a high intensity without any fuel (note the use of high – low-intensity fasted cardio may well be beneficial for you, more on that to come), your cortisol levels could ‘become chronically high’. Not the one.
It’s not just your hormones that could do with safeguarding, either. While some studies on fasted cardio (albeit on men) have shown that fasted cardio could be beneficial for athletic performance in endurance training (since practicing fasted cardio in the long-term can lead to chronic molecular adaptations that mean your body learns to use fat for fuel when it runs out of glucose from food), fasted cardio could be the reason you’re not building strength or muscle mass.
‘If women train fasted, they’re more likely to tap into lean muscle as an energy source as the body is trying to conserve all fat and carbohydrate stores for critical reproductive functions,’ Dr Sims tells WH. Sure enough, a study on the livers of mice after intermittent fasting showed that the livers of male mice stopped producing energy storage molecules, whereas the livers of the females made use of all of their available resources, including muscle, in an effort to ‘stock’ the energy necessary to maintain their reproductive capacities.
Here’s the deal: ‘Biologically, women are “designed” to hold onto fat and carbohydrates because, as far as evolutionary fitness is concerned, her primary purpose is to feed, nurture, and cultivate an entire other human being inside her body for nine months,’ Dr Sims says.
‘Women have to create and support another life inside their bodies. They have to provide the food, the water, and the shelter at all times and under all circumstances. If something goes terribly wrong in the “outside world”, that nutrient flow to the foetus could be interrupted, thus putting her evolutionary purpose at risk. So, having adequate fat stores is key to the success of the baby. This is why a female is more likely to tap into her lean muscle mass to fuel her fasted training sessions than fat stores, further compromising strength and muscle function for performance.’
McGregor adds that besides strength and muscle function, the impact fasted cardio and fasted training could have on your reproductive health could also ‘reduce your cognitive function and balance’. In turn, those dreaded single-leg exercises may well seem a lot harder.
‘Fasted training may appear to work for the first few months, because nearly any new training technique will yield some effects in the short term,’ Dr Sims advises. ‘But the long-term effects for females can be endocrine (hormonal) dysfunction, thereby increasing abdominal fat, depression and potential fat gain. The bottom line is that you should focus on fuelling yourself adequately so that you’re able to cover all metabolic processes and replace the energy expended during exercise. To improve training adaptations and performance, fuelling before exercise, and recovering from exercise is critical.’
So, what about the ‘fed state’?
Both Dr Sims and McGregor are huge proponents of training in the ‘fed state’, for both physical and mental health, but what is it? Pretty much exactly what it says on the tin; training after you’ve had a good feed.
‘The fed state is defined as the metabolic state occurring during the first few hours after ingesting food in which the body is digesting food and absorbing the nutrients, and usually refers to 2-4 hours post eating,’ McGregor explains.
There is a particular ‘fed state’ formula you could follow to reap the biggest rewards, which we’ll come onto further down.
How can training in a fed state benefit women?
By maintaining a hormonal equilibrium (i.e. everything is balanced and working in harmony with one another), McGregor wagers that training in a fed state could yield the following advantages:
- Better athletic performance
- Improved glycemic (blood sugar levels) control throughout the day, preventing blood sugar dips and the potential to over consume energy later in the day
- Regulation to reproductive hormones
- Improved leptin and ghrelin control (the hormones responsible for regulating your appetite) which is essential in maintaining a healthy body weight
How much do I need to eat before exercise to reach a ‘fed state’?
This one’s all about striking a balance. Eat too much and you could end up feeling like you’re going to vom mid-squat. Eat too little and you’ll run the risk of falling victim to one or more of the above fasted cardio dangers.
‘In general, before a training session, it’s encouraged to consume between 0.5-1g of carbohydrate per Kg of body weight,’ McGregor advises. ‘But this will be individual and dependent on the type and duration of exercise being performed.’ For example, if you weigh 60kg, you’d want to consume between 30-60kg of carbs pre-workout, depending on how long you’re going to be training for and the intensity of training you’ll be doing.
When we asked Dr Sims for her advice, she advised ‘15g of protein before resistance training and 15g of protein + 30g of carbs before cardio’, adding that this ‘is enough to signal to your brain that nutrition is available’, but that these numbers only apply to workouts of 90 minutes or less. ‘If you’re going to be training for over 90 minutes, you’ll need to take fuel in to consume during your session.’
What food should I eat to achieve a ‘fed state’?
Heading to your local Maccas for a side of fries probably isn’t wise. Who knew? Both McGregor and Dr Sims advise that you go for something ‘easy to digest’.
McGregor adds: ‘A banana, large glass of juice, hot cross bun, slice of toast, or one Weetabix are all great options.’
How long before exercise should I eat to reap the rewards of a ‘fed state’?
That would depend on how much you’re eating. If you’re going in for a long sesh, McGregor advises ‘1-3 hours to digest your meal before exercising’. If your sesh will be less than 90 minutes and you’ll only be having a snack before starting exercise, ’10-30 minutes’ should do the trick.
But I like fasted cardio, how can I make it safe?
There are two factors to consider: how long each workout is, and how often you do fasted cardio. Providing you stick to the expert advice, fasted cardio could remain a fixture in your regular routine.
‘Fasted cardio sessions need to be performed at what is known as your first ventilatory threshold, in order for them to be safe,’ McGregor tells us. ‘This is where your breathing rate is higher than usual, but maintains an easy/recovery pace, each session lasts no more than 60 minutes, and you do them no more than twice a week.
‘The negative impact comes when your sessions are of a moderate to high intensity, or you do them too often.’
So, to recap, here’s what you should stick to:
- Duration: Max 60 minutes
- Intensity: Low, with your breathing rate at an easy/recovery pace
- Frequency: Max 2x per week
If your affinity with fasted cardio is less to do with goals and purely because you’re not a fan of the feeling of exercising with a full belly, Dr Sims recommends consuming ‘something as simple as a cold brew coffee with protein powder and 4oz of oat milk’.