Almost everything a woman is told about training came from research on men. Across six major sport and exercise science journals between 2014 and 2020, 31 percent of studies used male participants only and 6 percent used female participants only. Of more than 12 million participants, two thirds were male.^[1]

That is the origin of the advice most women are working from. Push harder. Add load. If you are tired, you are not trying. It is a reasonable playbook for a body whose main hormonal rhythm runs on a 24 hour cycle. It is a worse one for a body running a second rhythm across roughly a month, with a reproductive system that the body treats as optional whenever conditions look hostile.

28 was built on the position that intensity is not free for women. That is not a soft claim about listening to your body. It is an endocrine claim, and it is measurable.

Where intensity starts costing you

Cortisol response to exercise is not linear. It has a threshold. In a controlled study comparing 30 minutes of exercise at 40, 60 and 80 percent of maximum oxygen uptake against a resting control, cortisol changed by about 6 percent at rest, rose about 40 percent at 60 percent intensity and about 83 percent at 80 percent intensity. At 40 percent it did not rise at all. Once the researchers corrected for plasma volume and time of day, low intensity exercise actually lowered circulating cortisol.^[2]

Two things follow. The first is that gentle work is not a lesser version of hard work. Physiologically it is a different event, and it is the only one of the two that leaves you with less circulating stress hormone than you started with.

The second is that this study, the cleanest demonstration of the threshold we have, was run on 12 men. Which is the problem in one sentence.

Why women pay for it differently

Under sustained stress the body makes a triage decision, and reproduction is near the front of the queue to be cut. Dr. Jolene Brighten describes the logic plainly: when the body reads the environment as dangerous, it prioritizes survival hormones over reproductive ones, progesterone falls, and periods can arrive late, arrive early or stop. She is explicit that exercise is a stressor with real health benefits, and that the harm comes when it is overdone without adequate calories.^[7]

The threshold has been measured. In 29 regularly menstruating women, luteinizing hormone pulsatility, which is the signal that drives ovulation, was disrupted once energy availability fell below roughly 30 calories per kilogram of lean body mass per day.^[3] Energy availability means the food left over after exercise has taken its share. This is the crucial detail: the variable that broke the cycle was not how heavy the weights were. It was training load relative to fuel.

So the honest version of the claim is not that heavy training is bad for women. It is that heavy training writes a bill, and if she does not pay it in food and rest, the body pays it out of her cycle.

The part almost nobody is told

Most women assess their cycle by whether a period shows up. That test is far weaker than it sounds.

When researchers followed 67 exercising women across two to three cycles using daily urinary hormone measures rather than bleed dates, only about half had normal ovulatory cycles. Roughly 29 percent had luteal phase defects, meaning a luteal phase that was too short or produced too little progesterone, and roughly 21 percent were not ovulating at all. Their cycle lengths looked unremarkable.^[4]

A regular period is not proof that ovulation happened. Ovulation is what produces progesterone, and progesterone is what does much of the work women associate with feeling well in the second half of the cycle. A cycle can be quietly failing for months while the calendar says everything is fine.

Further along the same road sits functional hypothalamic amenorrhea, which has its own Endocrine Society clinical practice guideline and is typically driven by the combination of high exercise load, insufficient energy intake and psychological stress.^[5]The International Olympic Committee's 2023 consensus statement on Relative Energy Deficiency in Sport documents where chronic low energy availability leads, including effects on bone, cardiovascular and metabolic health.^[6] These are not fringe positions. They are the mainstream clinical understanding of what happens when training outruns fuel in a female body.

How 28 trains women, and why

Our method is built by trainers certified through the Gray Institute around low-impact, stability-based, functional movement. The principle behind it is that the body does not need constant cardio and heavy weights to change, and that varying intensity across the cycle means never working the body more than it needs to be worked.

In practice that comes down to four things.

• A low-impact base. Most of the week sits below the intensity at which cortisol climbs. That is not a compromise for beginners. It is where the adaptation is cheapest to recover from.
• Intensity placed, not sprinkled. Harder sessions belong where energy is genuinely higher rather than distributed evenly because the calendar says Tuesday.
• Deliberate easy weeks inside the month. Around menstruation and in the late luteal phase, lighter work is the plan rather than a failure of discipline.
• Fuel the work. Our nutrition guidance is built around nutrient-dense whole foods rather than restriction, because the evidence above is about energy availability. Training hard and eating less at the same time is the specific combination that breaks cycles.

None of this is an argument for training easy. Women can and should get strong. It is an argument against the default the fitness industry inherited, in which more is always better and fatigue is always weakness, applied to bodies that were never studied.

What we are not saying

• That high intensity training is harmful in itself. An acute cortisol rise is normal. Frequency, fuel and recovery are what decide whether it accumulates.
• That this replaces medical care. A missing period, a newly irregular cycle, or symptoms that are getting worse are reasons to see a clinician. Training changes are not a treatment for a diagnosis.
• That our specific protocol has been shown in a trial to protect ovulation. It has not. We are applying an established mechanism, and we would rather say that plainly than imply evidence we do not have.

The reason 28 looks gentler than what women are used to being sold is not that we think women are fragile. It is that the standard prescription was calibrated on bodies that do not lose ovulation when the math stops working, and women have been paying the difference without being told there was one.