It depends.
When deciding whether or not to do fasted exercise, some elements need to be taken into consideration:
- Length of the fast
- Type/intensity of the exercise
- Health goal/issue (eg: weight loss, optimizing fertility, obesity, type 2 diabetes, etc.)
#1 The Length of the Fast
Fasting and exercise are hormetic stressors. They have many benefits, but at the same time, they can also be harsh on a woman’s hormones (eg: estrogen, thyroid, cortisol)[3].
Until more studies are available on women exercising while fasting, it’s a good idea to stay on the safe side and follow these simple rules:
- Exercise is generally safe on 16-hour fasts or shorter.
- The exercise routine should be no longer than 1 hour (unless it is very low impact).
- The exercise routine should be done at the end of the fasting window with a refeed soon after (ideally within 1 hour, for muscle recovery).
#2 The Type of the Exercise
There are 3 main types of exercise: endurance, strength, and mobility/flexibility. Low impact exercises are the best option for women in a fasted state.
For best exercise results, opt for mobility and flexibility exercises like yoga, stretching, pilates, or walking.
The second best type of exercise is some form of endurance exercise, like jogging, cycling, or swimming.
Last comes strength training. To be effective, strength training requires a form of quick energy (glucose or glycogen). In a fasted state there is no exogenous intake of glucose, so the liver will break down glycogen. The amount of glycogen is limited, and it is quickly consumed during strength training, leaving you depleted.
Nevertheless, it is okay to do strength training in a fasted state if you feel good and have the energy to do it, but remember to eat soon after the session (ideally within an hour) to help with muscle recovery.
#3 The Health Goal/Issue
Not all women are advised to exercise while in a fasted state. If you are trying to get pregnant, underweight, or suffering from an eating disorder, it is not recommended.
Exercising during the fasting window speeds up ketone production. Ketosis is considered a low fertility state. In this state, the body receives signals to shut down reproduction as there is insufficient food available to support a healthy pregnancy. [2]
For underweight women, fasting could lead to an even lower body fat percentage and the resulting hormone balance disruption (eg. amenorrhea).
Women with a history of eating disorders are also not advised to exercise in a fasted state. It increases hunger and could lead to binging/restricting pattern cycles.
On the other hand, women who already suffer from a chronic condition like PCOS, type 2 diabetes, or obesity [1] are encouraged to do some form of exercise while fasting. For them, exercise can increase the benefits of fasting, including better insulin sensitivity, higher fat oxidation, energy increase from ketone production, and less brain fog.
References:
[1] Intermittent Fasting and the Possible Benefits in Obesity, Diabetes, and Multiple Sclerosis: A Systematic Review of Randomized Clinical Trials, PubMed, PMID: 34579056. Link
[2] Private podcast: Qualys, Peter’s thoughts on fasting and ketosis for females, Peter Attia MD (fasting & longevity expert). Link
[3] Effects of Intermittent Fasting on the Circulating Levels and Circadian Rhythms of Hormones, PubMed, PMID: 34474513. Link