Should Women Train Fasted? Understanding the Nuance
Challenging the Blanket Rule
There’s a rising interest in fasting, particularly its effects on exercise. However, much of the foundational research has been conducted on men, and emerging data over the last five years has revealed significant sex differences. This has led some to advocate for a blanket rule: women should not train fasted, or even fast at all.
However, the reality is more nuanced. It’s important to look beyond broad strokes and consider the specifics of a woman’s physiology, fasting duration, and exercise type.
Key Research on Women and Prolonged Fasting
Research into women's fasting indicates several negative effects, mostly tied to longer-duration fasts:
Stress Hormones: Prolonged fasts (e.g., 72 hours) in healthy women have been associated with marked increases in stress hormones such as cortisol.
Glucose and Fat Metabolism: Women tend to have lower plasma glucose concentrations and higher free fatty acid levels after fasting than men, making them more prone to hypoglycemia (low blood sugar) and causing fluctuations in energy levels due to the rapid mobilisation of fat.
Hormone Balance and Fertility: Alternate-day fasting has been linked to decreased androgen markers and increased sex hormone-binding globulin (SHBG) levels in premenopausal women, which can negatively affect fertility and hormone balance.
Bone Health: A four-day fast in healthy young women showed significant decreases in bone mineral density.
At face value, this data suggests caution, but the duration of the fast is a critical factor.
The Critical Difference: Shorter vs Longer Fasting
Negative research outcomes often stem from prolonged fasts (36 hours, 72 hours, or 4 days). When examining shorter, overnight fasts (12 to 16 hours), the results become more mixed and muddled:
Some studies in obese women found that short periods of intermittent fasting actually improved metabolic markers.
Other short-term fasting studies show mixed results, or even favourable ones, for women.
Practical Takeaway for Morning Workouts:
A common question is whether to eat before an early morning workout (e.g., after an overnight fast since dinner at 7 PM). While more research is needed, if you are concerned about the negative impacts of fasting, it is generally recommended to consume some form of carbohydrate beforehand.
Ideal Fuel: A rapidly absorbed carbohydrate source like white bread with jam, or a liquid carbohydrate (dextrin or maltodextrin mixed in water). These get into the bloodstream as glucose, which is the muscle’s primary fuel source.
Fruit Caution: While fruit is better than nothing, the high fructose content means it must be broken down in the liver, making it less ideal for immediate muscle energy compared to glucose-based carbs.
Considering Exercise and Overall Status
The decision to train fasted shouldn't be a one-size-fits-all rule, as the type of exercise and a woman’s overall nutritional status play major roles.
Exercise Type Matters
Short Duration/Lower Intensity: Fasting is less of a concern for short (30-45 minute) strength training or lower intensity workouts (e.g., Zone 2 cardio).
Long Duration/Hard Sessions: Longer, hard workouts (60+ minutes), such as marathon or triathlon training, differ significantly and are likely to require pre-workout fuel to support high energy demands and mitigate negative outcomes.
Overall Nutritional Status is Key
Undernourishment: Women who are highly active, very lean, or frequently bounce between restrictive diets are already in a state of under-nourishment. For this population, adding fasting is generally ill-advised, as it increases the risk of hormonal disruption.
Obesity: For those who are overweight or obese, fasting may be a convenient tool to reduce overall energy intake. In this context, the potential benefits of energy deficit may need to be weighed against the potential negative side effects of training fasted.
Additional Considerations: Stress and Sleep
A woman’s overall stress and sleep status further complicates the picture. One study on sleep-deprived women found that a 24-hour fast significantly elevated cortisol, disrupted the menstrual cycle, and impaired glucose metabolism. This highlights:
Stress Management: High chronic stress can already negatively affect sleep and cortisol levels. Layering a long fast on top of this can exacerbate these negative effects.
Ultimately, fasting is a complex topic and not a one-size-fits-all method. If you choose to incorporate fasted training, it's essential to consider the duration of the fast, the intensity and length of your workout, and your overall health, stress, and nutrition status.
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