Fasted Training in Women
Is fasted exercise right for you and your body?
Have you ever wondered whether exercising on an empty stomach is suitable for your body and whether the evidence supports this type of training for women in particular? In this article, I’ll be taking you through what we know on this topic and what you might want to consider before making this a regular part of your fitness plan.
Fasted exercise is often hailed as a way of burning more fat for fuel with an implied ability to reduce body fat and even enhance performance. While some research supports these claims (1), the impact of fasted exercise on both fat loss and physical performance remains unclear.
What does the wider evidence say?
When it comes to performance, some studies show fasting has a negative effect on training outcomes, while others show no difference vs exercise in a fed state (1). More specifically, a systematic review found that eating before exercise supported cardiovascular (cardio) activity over 60 minutes but did not affect shorter-duration aerobic training (2).
When it comes to body fat, a study conducted in young women found no difference in fat loss following four weeks of fasted vs fed aerobic exercise (3). So, whilst you may burn more fat during exercise, fasted cardio is unlikely to affect your body composition in the long term. In fact, when it comes to the potential impact of fasted exercise on your hormones, it could even have the opposite effect.
In other words, the so-called “super” effects of fasted exercise are not as straightforward as you might think. This is especially true for women (or those with a female biology). The problem is that women are incredibly underrepresented in sport and exercise research (4). For this reason alone, it’s difficult to draw conclusions about women in exercise (4) and even more challenging to make appropriate fitness and nutrition recommendations for those with a female biology. However, our knowledge of female physiology can help us understand the potential implications of fasted training for women, especially when it comes to hormones.
How can fasted exercise affect women?
Firstly, we know that compared to men women are more sensitive to changes in energy balance and more susceptible to low energy availability (LEA) (5). LEA occurs when the amount of energy taken in from food is not enough to sustain the physiological functions needed to maintain optimal health. Those who do a lot of exercise have a higher risk of experiencing LEA. This is because the body will always prioritise the energy required by movement, altering (if needed) multiple systems in the body to support that activity when energy is scarce. In the long run, this can have detrimental effects on both health and exercise performance (6).
Some warning signs of LEA to look out for:
Fatigue, unrelieved by rest
Missing, very light or irregular periods
Low bone density
Gut issues or “IBS” symptoms
Mood changes and sleep disturbances
Frequent colds or infections
Poor exercise recovery or frequent injury
Performance stagnation
Low body mass, body fat % (although BMI may be in the normal range)
Exercise dependence
A poor relationship with food or your body
How can fasted exercise affect hormones?
Women’s increased susceptibility to LEA is the source of a cascade of changes to a multitude of hormones. One of the most important of these is kisspeptin (a protein in your brain that regulates reproductive hormones). After fasted training, women experience changes to this hormone, resulting in the downregulation of sex hormones. If this occurs for a prolonged time, ovulation can stop altogether, and you may find your periods go missing. This extended absence of ovulation, otherwise known as functional hypothalamic amenorrhoea (FHA), leads to a consistent drop in the ovarian hormones, oestrogen and progesterone. Low levels of these hormones, in turn, can cause several distressing side effects, including loss of bone density (7). For a closer look at FHA and some of the other health implications, you may find this article helpful.
Similarly, fasted training can also induce changes to thyroid function. This is because the thyroid axis adapts to reduced energy availability by altering two of the main thyroid hormones responsible for your body's metabolism, triiodothyronine (T3) and thyroxine (T4) (8). In fact, research has shown a significant drop in T3 levels when restricting energy intake in menstruating women (9). These changes can occur after only 4-5 days of consistent LEA, resulting in impairments to several body systems, such as heart, muscle, and digestive function, as well as brain development and bone maintenance.
Fasted exercise can also affect your main stress hormone, cortisol. In reality, cortisol rises in response to a range of independent factors such as exercise, low blood sugar or fasting, LEA, mental stress, time of day, and even different stages of the menstrual cycle. Exercising on an empty stomach first thing in the morning, may compound the effect further, especially if you exercise at a high intensity. This is because your cortisol levels are naturally on the rise and highest in the morning. Adding additional stress to your body, by exercising hard without any food in the system, can cause your cortisol levels to become chronically elevated.
Chronically high cortisol poses several threats to health and performance. For one thing, cortisol places the body in a “catabolic” state, meaning that tissues like muscle are broken down for energy. Research shows that women who experience menstrual dysfunction spend more time in a catabolic state (10) and have higher cortisol levels (11). Women are also more likely to use their lean muscle as an energy source during fasted exercise in an attempt to conserve fat stores vital for reproductive function. What’s more, cortisol exposure has been shown to influence where fat is stored on the body with high cortisol linked to a preference for fat deposition around the mid-section (12, 13, 14).
Consistently high cortisol can also affect your metabolism by making it more difficult for your body’s cells to absorb and use energy from food appropriately (insulin resistance). High cortisol can lead to increased blood pressure, a greater risk of anxiety and depression, sleep disruption, brain fog, immune and reproductive dysfunction, and reduced performance (8). Ultimately, the combination of under-fueling, exercising, and fasting over time could place unnecessary stress on your body. So, fueling appropriately before a workout is likely beneficial for your long-term health and performance goals (15), especially if you exercise frequently and/or at a higher intensity.
Reality check: Is fasted exercise ever appropriate?
Before going any further, I’m not saying that fasted exercise is never appropriate for women. If done safely and in moderation, at the right time and in the right person, there is evidence to suggest it could induce some beneficial metabolic adaptions (2). However, it’s clear that fasted training affects women very differently to men and doing too much or training fasted intensely can severely disrupt hormonal function.
However, there are instances where fasted training may be necessary. A busy schedule, for instance, can make it difficult to fuel before exercise. In these cases, research suggests that not eating before lower intensity exercise (less than 60 minutes) is generally OK (16). On the other hand, it’s important not to overdo this and be mindful of some of the wider impacts of fasted exercise on your body. Remember that we are all genetically and physiologically unique, and your individual capacity to withstand any potential hormonal disruption through fasted training, is just as unique.
It’s also worth considering the changes that take place across a natural female menstrual cycle. Over the course of about a month, your sex hormones fluctuate causing physiological changes in your body. At certain times of your cycle, your hormonal status may independently increase your need for adequate fueling. To give you an example, in the second half of your hormone cycle (the luteal phase), not only are your total energy requirements higher due to a general uptick in metabolism, but there is also evidence to suggest reduced access to your own carbohydrate stores for energy (17). This physiological limitation in carbohydrate availability suggests there may be a greater potential reliance on extrinsic sources of carbohydrate during exercise.
The late luteal phase also marks a time of heightened reactivity (cortisol response) to stress (18). For this reason, I recommend being mindful of the additional stress imposed by fasted exercise at this time, especially if there are any other factors such as poor sleep or a lot of work or other psychological stress in your life. Overall, current research on exercising around your menstrual cycle is somewhat conflicting and inconclusive. There is also a lot of between women and within woman variation, so, it is important to focus on your individual experience and listen to your body.
Why the continued focus on fasted training?
Unfortunately, society continues to subtly (or not so subtly) place emphasis on how bodies (especially female bodies) look with an ongoing rhetoric of 'thin is better.' The unsupported claims that fasted exercise (irrespective of an energy deficit) enhances weight loss only reiterates the thin ideal. The result is a conscious or subconscious focus on achieving a status of thinness rather than a focus on the wider health benefits and particularly the enjoyment experienced by physical activity (19). Fasting (in any capacity) and this mindset are a predictor of developing an eating disorder (20) or disordered eating habits (10), which bear extreme consequences for one's overall health and performance.
Is it time to switch the narrative? Taking the time to find and do what you love and moving your body in ways that make you feel strong, powerful, and confident will help you begin to redefine your relationship with exercise and ultimately improve your overall health. More widely, we need to continue to raise awareness of diet culture and fat phobia, whilst fostering communities of self-acceptance, in which we value ourselves for who we are and what we do, not how society dictates we should look. This work is hard, and the road is long, but I believe the more we talk about this (and related) subject(s) and the more compassion we show ourselves and others, the greater our chance of re-writing the story.
Basic fueling considerations moving forward
In summary, women have a unique physiology that places them at increased risk of energy imbalances through fasted training. Depending on training intensity, duration, frequency, time of day and, if applicable, your menstrual stage or personal hormonal circumstance, consuming a meal 2-3 hours before or a snack 1-2 hours (or less) before a workout is recommended. Additionally, I advise my clients to aim to refuel any exercise (especially fasted) within 20-30 minutes of completing that activity if possible. Where completion of your session falls away from a meal, a good basic option would be some chocolate milk (dairy or soya) and a banana.
For simplicity, an emphasis should be placed on all macronutrients and micronutrients around exercise for their distinctive role in supporting health. Put very simply (and rather crudely), carbohydrates provide an energy source to fuel your workout and reduce the potential adverse effects of our hormones on our metabolism (21), protein promotes muscle regeneration (repair) and good quality fats aid in reducing inflammation in response to exercise-induced stress. Additionally, dairy supports bone and immune health, whilst fruits and vegetables provide beneficial fibre, prebiotics, and a variety of essential electrolytes and micronutrients. Finally, staying hydrated with approximately 8 glasses (or 30ml per kg body weight) of water daily ensures appropriate cell functioning and electrolyte balance.
Many women experience low appetite or even nausea in the morning, which may affect their ability to adequately fuel before a morning session. If this is you, slowly introducing carbohydrate-rich foods or liquids, such as a banana, a small smoothie, a glass of juice or a few spoonful’s of yoghurt is a good place to start. Similarly, you can opt for a carbohydrate-rich meal for dinner the day before and top it up with fruit or juice in the morning.
Some basic examples of meals to consume 2-3 hours before a workout:
Overnight oats with milk and nut butter
Pasta with mozzarella and sauce of choice
A sandwich or wrap with chicken and raita/pesto
Tofu stir fry with noodles or rice
Chill con Carne with rice (vegetarian or with meat)
A bagel with salmon and cream cheese
Snacks 1-2 hours before might include:
Toast with honey or jam
A small bowl of porridge
A pot of natural yoghurt with fruit
A granola or other cereal bar
Oat cakes with nut butter
A smoothie made with milk or yoghurt
Snacks less than 60 minutes before working out (if a top up is needed) might include:
Fruit juice
Coconut water
A banana
A handful of dried fruit
Rice cakes with jam or honey
Sports drink
Take-home messages:
Fasted training may not be ideal for women and should not be considered for those with missing periods, resulting from functional hypothalamic amenorrhoea (FHA)
Women are more sensitive to changes in energy balance, which puts them at an increased risk of low energy availability (LEA)
LEA results in the downregulation of reproductive and thyroid hormones, and altered adrenal and metabolic responses, which compromises reproductive, bone and overall health
We are all genetically and physiologically unique, and your individual capacity to withstand any potential hormonal disruption through fasted training, is just as unique.
When it comes to exercise, we need to reframe our view of exercising for thinness to exercising for enjoyment to optimize the health benefits
Nourishing your body with carbohydrates and protein pre- and post- workout is recommended to maximize the health benefits of exercise
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This article was researched with the help of Noor Wadi, MSc GHP & HCPC Registered Diabetes Dietitian, and fabulous intern at Holly Dunn Nutrition.
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