If you are considering fasted cardio but want to know more about how it may impact you, read on for a summary of the pros and cons so that you can decide whether it is for you!
Pros of fasted cardio
Potential calorie restriction tool: Fasting can be a potential tool for restricting total calorie intake. If you are seeking weight loss, then creating a calorie deficit is necessary. If practising fasted cardio aids overall calorie restriction, then this will be a benefit for you.
Improved digestion: Eating close to training can cause gut distress, known as runner’s gut syndrome. There are various factors that can influence this, but eating close to training, and/or consuming high carbohydrate sports drinks can be one of them.
Fasting before training could alleviate this. However, experimenting with options such as a simple shake, juice or coconut water that still provide calories and a mixture of carbs and protein, is an alternative to fasting and may aid performance.
Beneficial training adaptations: For athletes, training in a fasted state may be associated with enhancing training adaptations, such as increased muscular oxidative capacity, improved blood sugar regulation and capacity to resynthesis glycogen.
In conjunction with sufficient periods of training in a fed state, it may be considered part of a specific periodised nutrition programme to optimise performance.
Cons of fasted cardio
Being in a catabolic state: Fasted training may place you in a catabolic, otherwise known as break down state, where muscle protein breakdown is enhanced. The degree of this will depend upon the individual: their body fat levels, their energy availability and their length of fasting, as well as total protein intake and distribution. Further research is required to determine whether this translates into a significant difference in lean body mass.
Elevated levels of cortisol: Both fasting and exercise increase levels of the stress hormone cortisol. Depending upon the individual, health status, lifestyle, volume of training and fasting, this may contribute to chronically elevated cortisol levels.
Chronic elevation of cortisol may be associated with increased risk of chronic diseases, increased fat storage around the middle and more muscle breakdown (1). It may also be associated with reduced levels of sex hormones in male and female athletes.
Reduced training capacity: Training in a fasted state may be associated with fatigue, reduced training intensity and decreased performance levels. This may be detrimental for any athlete or individual who enjoys training.
Additionally, if the goal is weight loss, these factors will can contribute to a reduction in overall energy expenditure.
Impaired recovery from training: Reduced nutrient availability and increased muscle protein breakdown may impair the process of muscle protein repair.
Negative health impacts: Especially for individuals who are especially lean, train a lot and/ or are athletes. Studies indicate that within-day energy deficiency, which may occur with fasted training, may reduce levels of sex hormones – testosterone for males (2) and oestradiol for females (3).
Additionally, if fasted training is not accompanied by sufficient nutrient availability throughout the day, it may impair an athletes recovery.
May be more challenging for females: Whilst research indicates that fasting may beneficial for especially overweight individuals the same may not apply for females who are of a normal/healthy weight. Fasting may be too much of a stressor and be associated with dysregulation of blood sugar management (4).
Females are more sensitive to stressors and changes in energy availability. Within-day changes in energy availability may be associated with increased levels of cortisol, reduced levels of oestrogen and dysregulated menstrual cycles. Thus the combination of fasting and exercise may exacerbate both levels of cortisol and impact levels of sex hormones.
Consider the purpose of fasted cardio: if you are trying to lose weight, does it help or hinder your progress? If you are trying to optimise your performance, is it helpful?
Consider the impact of fasted training on your individual status: are you male or female? How lean are you? What’s your overall health status and energy intake?
Consider the timing of your nutrition: if you do your cardio fasted, then your overall nutrient timing will become more pertinent. The level of importance will depend upon the intensity and duration of your training, fasting period and nutrient intake and distribution. Protein intake and distribution is relevant for optimising recovery for endurance athletes
What do you think about doing your cardio fasted? Is it something that you do? What are your experiences?