Every January, the same health trends resurface.
Cold plunges. Intermittent fasting. Keto. HIIT.
They’re often described as game-changers – quick, efficient, powerful ways to optimise your health. And for some people, they genuinely are. However, here’s the piece that’s often missing from the conversation:
Most biohacks were researched in men.
That doesn’t make them wrong, but it does mean they don’t always translate well to female physiology. Women’s bodies are sensitive to stress, fuel availability and recovery. When a health trend ignores that, it can leave you feeling worse, not better.
1. Cold plunges: when “toughening up” creates more stress
Cold exposure is everywhere right now, from ice baths in gyms to viral cold-plunge challenges.
In men, short, sharp cold exposure has been shown to:
- Increase adrenaline
- Support testosterone
- Improve muscle recovery
The difference for women is how we react to stress.
Cold plunges activate the sympathetic nervous system, triggering a rise in cortisol. This is a normal physiological response, but research suggests women often experience a higher and longer-lasting cortisol spike than men. When cortisol remains elevated, particularly in women already juggling busy lives, poor sleep or hormonal transitions, it can:
- Disrupt sleep
- Interfere with progesterone balance
- Leave you feeling wired but exhausted
- Encourage fat storage (particularly around the middle)
A more supportive alternative
Cold exposure doesn’t have to be all or nothing. Many women respond far better to gentler cold stress, such as cold-water swimming. The water is often slightly warmer, movement keeps circulation going, and the social aspect can help buffer the stress response.
2. Intermittent fasting: why longer fasts aren’t always better
Intermittent fasting is often promoted as a powerful tool for metabolic health, and much of the research supporting it has been carried out in men. In male physiology, skipping meals can improve insulin sensitivity, support fat metabolism and is often well tolerated from a hormonal perspective.
In practice, intermittent fasting frequently leads to very long overnight fasts, with breakfast skipped and the first meal of the day delayed until lunchtime. For many men, this pattern generally works well. Male hormones are less sensitive to short-term energy restriction, and testosterone helps maintain muscle mass, metabolic rate and blood sugar control even when meals are delayed.
For women, however, this same approach can send a very different signal to the body. Skipping breakfast and pushing the first meal of the day back until midday can be interpreted as energy scarcity, particularly when combined with busy mornings, caffeine, exercise and everyday life stress. This triggers a cortisol response, and when cortisol remains elevated over time it can suppress progesterone, disrupt oestrogen balance, affect sleep and appetite regulation, and increase cravings later in the day.
Research shows that fasting beyond around 12 hours may raise cortisol in women and alter reproductive hormone signalling. Rather than improving metabolic health, longer daily fasts can leave women feeling tired, wired, and more prone to energy crashes or overeating later on.
For many women, this means breaking the fast earlier, ideally before 10am, with a nourishing, protein-rich first meal. Eating earlier in the day helps stabilise blood sugar, reduce cortisol output and support hormonal balance. Leaving 3–4 hours between meals further supports steady energy levels and appetite regulation throughout the day. This gentler, more consistent approach works with female physiology rather than against it, and is often far more sustainable long term.
3. Keto and very low-carb diets: when fast results slow your metabolism
Very low-carb and ketogenic diets are often praised for rapid results, particularly when it comes to weight loss. Much of the research behind these approaches has, again, been conducted in men and tends to focus on short-term outcomes such as weight loss and blood sugar markers.
In men, severe carbohydrate restriction can be relatively well tolerated. Testosterone helps protect muscle mass and metabolic rate, and male bodies are generally better equipped to cope with short-term fuel restriction without significant hormonal disruption.
For women, however, long-term very low-carb eating can send a different message. Carbohydrates play an important role in thyroid hormone conversion, particularly the conversion of inactive T4 into active T3. T3 is a key driver of metabolic rate, energy production and body temperature regulation.
Research shows that prolonged carbohydrate restriction can reduce active T3 levels, effectively slowing the metabolism. When this lower metabolic output is combined with elevated cortisol, which often rises when the body perceives ongoing stress or fuel scarcity, many women begin to notice changes such as fatigue, feeling cold, low mood and stubborn weight gain, particularly around the abdomen.
This is why keto can feel brilliant initially for some women, appetite is reduced and weight may drop quickly, but over time energy dips, progress stalls and belly fat becomes harder to shift. This isn’t a lack of willpower; it’s the body adapting protectively to what it perceives as a long-term stressor.
Rather than extreme carbohydrate restriction, most women do better with balanced meals that provide clear signals of safety and fuel availability. Including carbohydrates from whole, fibre-rich sources such as vegetables, pulses and root vegetables supports thyroid function, gut health and hormone metabolism.
Combined with adequate protein and healthy fats, this approach helps stabilise blood sugar, reduce cortisol output and support a more resilient metabolism. Over time, this steadier approach is far more likely to support energy levels, body composition and long-term health without the cycle of restriction and burnout.
4. HIIT: when pushing harder starts working against you
High-intensity interval training (HIIT) is often promoted as the most efficient way to exercise. Short workouts, maximum effort, big results. Much of the research supporting HIIT focuses on male physiology and performance outcomes, where higher levels of testosterone support muscle recovery and adaptation to repeated high-intensity stress.
For women, however, the picture can look very different. HIIT places a significant demand on the body by activating the stress response and raising cortisol. This isn’t a problem in itself, cortisol plays an important role in adaptation, but when high-intensity training is frequent and recovery is limited, cortisol can remain elevated for longer periods.
Over time, chronically raised cortisol can interfere with recovery, disrupt sleep and encourage fat storage, particularly around the abdomen. Many women notice that despite exercising harder and more often, they feel more fatigued, inflamed and frustrated by stubborn belly fat that refuses to shift.
This response is not about fitness levels or motivation. It’s about total stress load. When intense exercise is layered on top of busy schedules, poor sleep, under-fuelling or restrictive diets, the body prioritises protection rather than progress.
For most women, a more supportive approach is to treat HIIT as one tool, not the foundation of their movement routine. Limiting HIIT to one or two sessions per week allows you to gain cardiovascular benefits without overwhelming the nervous system.
Balancing higher-intensity workouts with strength training, walking and gentler movement such as stretching or yoga helps support muscle mass, bone health and metabolic resilience while also allowing cortisol levels to settle. Movement should leave you feeling energised and capable, not depleted.
Why one-size-fits-all rarely works
The common thread across these popular biohacks is not that they are “bad” or ineffective, but that they are often applied without considering female physiology. Much of the research behind cold exposure, fasting, low-carb diets and high-intensity exercise is based on male bodies, shorter timeframes and narrow outcomes.
Women’s bodies are highly responsive to signals of stress, fuel availability and recovery. When those signals suggest scarcity or overload, the body adapts in protective ways, often showing up as fatigue, disrupted sleep, stubborn belly fat or a slowing metabolism.
A more effective approach is rarely about doing more. It’s about doing what your body can respond to well, consistently and sustainably. Gentler cold exposure, shorter fasting windows, balanced meals and varied movement patterns often lead to better energy, steadier moods and more resilient health over time.
If you’d like to explore these topics in more depth, I discuss each of these biohacks in detail in my January podcast series, episodes 275–278, available on all podcast providers. Each episode looks at the science in more detail and offers practical, hormone-aware ways to adapt these trends so they work with your body, not against it.