Fasted workouts have long been hailed as *the* way to boost metabolism - but new research suggests women have been misled. Here, top experts explain

Hands up if, like me, you've spent years of your life skipping breakfast after reading that fasted workouts are the holy grail when it comes to metabolism boost and fat burn? And, as a result, struggled through workouts, low on energy and feeling exhausted?

While everyone's body is different and what works for me likely won't work for you, it's definitely fair to say that professional opinions on fasted training are... mixed, and have changed a lot over the past few years. For most of the last decade, experts repeated time and time again that working out fasted was the thing to do if you wanted to supercharge your health, muscle and metabolism. But new research has seriously drawn that into question, especially for women.

You see, fasted training can actually have a serious impact on our hormones, with recent research indicating that regularly training fasted can put a huge amount of stress on your nervous system, interrupting your normal hormone function and leading to missed periods, poor mood and increased fatigue.

Below, we've spoken to top experts to debunk the wealth of research on fasted training, plus break down the pros and cons of training in the morning before eating. Keen to read more balanced nutrition advice? We've got features on the diet fads to avoid, team MC's go-to Em The Nutritionist recipes and tried and tested Glucose Goddess hacks for stable blood sugar levels, here, as well as explainers on intermittent fasting and how much protein you need.

Fasted workouts have long been hailed as the thing to do for metabolism and fat loss - why experts now warn they may be harmful for your health

What is fasted training?

First up, a definition for you. According to Michal Mor, chief growth officer and co-founder at Lumen, the term simply refers to working out after several hours without eating. "This often means working out in the morning after ten to twelve hours overnight without food," he goes on.

So, what's the point of fasted training? "The idea is that at this stage, your mitochondria – aka the energetic powerhouses of your metabolism - will have already burned through your body’s carb reserves, otherwise known as your glycogen stores, and will rely on fat as fuel for the workout."

Interestingly, though, recent data has also proven that men and women respond very differently to fasted training. "The vast majority of research is done on men - they’re simpler to study and don’t menstruate, which can complicate data," shares Liam Grimley, a qualified person trailer and co-founder of 432 Fitness platform. "Research implies that fasting could benefit men more than women, as fasting can have a serious impact on hormones and menstrual cycle. On reflection, it may not be worth it."

Is it good to train fasted?

This is where it gets interesting. According to fitness and lifestyle coach and founder of Sharp Fit For Life Martin Sharp, while there's been lots of research on the matter, many of the results are inconclusive, making things a little confusing. That said, "the patterns that emerge tend to centre on what type of training is being performed and also the specifics of the person being trained." In short - whether fasted training will work for you will totally depend on your gender, current fitness levels, type of training, and end goal.

His initial thoughts tie in with many of the research papers I've seen circulating - that ultimately, "fasting is a stressor on the body and will increase cortisol, among other things, which can impact sleep, repair and stimulate fat storage."

Mor agrees with this, adding that it's difficult to sum up all the research on fasted workouts. "That said, recent literature reviews reveal that there isn’t much evidence supporting increases in fat oxidation during fasted endurance workouts," he highlights.

That said, there may be benefits to fasting pre low intensity workouts like yoga, Pilates and walking. Mor goes on to explain that, generally speaking, during low-intensity workouts, mitochondria prefer fat as fuel to support sustained exercise and save glycogen. "So, if you’re planning a low-intensity workout, fasting may be effective for enhancing fat burn," he explains.

And Sarah Campus, personal trainer and founder of LDN MUMS FITNESS, agrees, pointing out that recent research has suggested that fasted training could possibly help with fat adaptation and insulin sensitivity.

What are the cons of fasted training?

However, there are many instances where underfuelling your workouts can result in less energy and therefore not as good a workout. "With resistance or weight training, you're likely to see a decrease in performance due to not having enough energy, creating a negative impact," Sharp explains. He also stresses that if you don't fuel properly post-workout - that is, eat a meal or snack containing a balance of complex carbs, protein and healthy fats within around a 60-minute window - then you risk affecting your body's post-exercise oxygen consumption (EPOC), key for muscle recovery and repair.

Mor agrees, adding that scientifically speaking, during high-intensity workouts, your mitochondria require carbs as fuel. "So, if carbs aren’t available, your body will break down muscle into carbs for fuel, meaning that performing high-intensity exercise while fasting can actually set you back in your muscle growth goals, rendering your workouts less effective." In short, fasted training could make it much harder to build muscle mass, improve athletic performance, and maintain a healthy weight.

Campus also adds that fasted training can sometimes lead to reduced performance due to lower glycogen levels, which can negatively impact high-intensity or long workouts, leading to fatigue and reduced power output. "You can also see more muscle breakdown as you have less fuel to support training, especially if long or intense, plus hinder your post-exercise recovery and muscle repair."

How does fasted training impact women, in particular?

As Campus details, women naturally oxidise more fat, so the added benefit of fasted training may be smaller. "Bottom line: it can be beneficial for low-intensity cardio but may disrupt hormones or recovery if overused."

And according to Grimley, fasted training has been shown in several studies to increase cortisol - aka your stress hormone - secretion, which can affect hormone balance and lead to negative changes in the menstrual cycle. "Fasted training has also been shown to be associated with low energy availability or Relative Energy Deficiency in sport (REDs) leading to low motivation, negative self-image and hormone changes," he goes on.

Not just that, but there's also a strong link between fasted training and the development of disordered eating behaviours, he shares, not to mention an increased risk of poor bone health, reduced metabolic rate (resting calorie burn) and ironically, loss of healthy muscle and retention of body fat.

My two cents? As a nine-time marathoner who's run three Boston Qualifying times, I feel a million times better when I fuel my body properly for my sessions. I trained fasted for years in my early twenties thinking it was *the* thing to do for fat burn, but often couldn't hit my sessions head-on because I felt so fatigued. Of course, every body is different, but I can't tell you how game-changing it's been to understand how important fuel is for my overall health.

Should you try training fasted?

Grimley's take? "In all honesty, I no longer recommend fasted training for active women. The are many drawbacks and I think they outweigh any suggested benefits."

He adds that women tend to exercise and perform at their physical best when fuelled, and he recommends aiming for around 10g protein and 20 to 30g carbohydrate prior to training (think some granola and Greek yoghurt, or sourdough and eggs). "This helps fuel the workout and support healthy muscle maintenance, and is thought to be particularly important during the high hormone phase of your menstrual cycle."

Bottom line: "It can be hard enough for women to build and maintain muscle so we should encourage women to provide the energy and building blocks for the workout," he concludes.