Your cholesterol result depends on when you go for the blood draw

What started this

Kate Tolo is co-founder of Blueprint, Bryan Johnson's longevity project. She is currently measuring her own menstrual cycle in a self-experiment: blood, brain, skin, saliva, grip strength. Her thread about it is going viral.

Her core claim: cholesterol shifts across the cycle — and nobody accounts for it.

We looked at the studies. She's right.

What the data shows

The key work here is the BioCycle Study (NICHD, 2010): 259 healthy women aged 18 to 44, up to 16 blood draws across two complete cycles.

Total cholesterol is highest in the first half of the cycle. After ovulation it falls. The low point sits just before menstruation. In the follicular phase, 14.3% of women were above the 200 mg/dL cholesterol threshold. In the luteal phase, only 7.9%. Just 5% of women were consistently above the threshold. But 19.7% crossed it at least once during their cycle.

Same women. Different day. Different result.

In 2023 this was confirmed in the largest group to date: 8,694 women in UK Biobank. Total cholesterol, LDL, HDL and the total-to-HDL ratio all shift with cycle phase — by 2 to 5% on average, and considerably more in some women.

That variation is larger than the analytical error of the lab assay. It's real.

Why this matters to us

Not because it produces waves of wrong diagnoses. Treatment decisions run on total cardiovascular risk, not on a single value.

It matters because it costs you comparability.

You change your diet. You start training. You take omega-3. Four months later you measure again.

If several percent of cycle-driven variation sits between those two measurements, you cannot tell a real improvement from chance. The cycle becomes the noise that hides the answer.

And cholesterol is only the best-known case. A metabolomics study in healthy women found 48 of 54 amino acids measured changing across the cycle. This isn't about one marker. It's about metabolism.

This isn't a women's issue

In 1977 the FDA recommended excluding women of childbearing potential from early-phase drug trials. That guidance stood until 1993. Sixteen years.

The reason was safety. The side effect was methodological: the cycle came to be treated as a confounding variable, best removed.

That's the pattern. Medicine has long treated variability not as information but as noise to average away — cycle, time of day, season, sleep. Ignoring those variables doesn't make a measurement more precise. It just makes it measure less.

The cycle is the most obvious blind spot. It affects half the population. And it follows a rhythm you can actually know.

What we do at speak23

It takes years for findings like these to reach guidelines and lab reports. We're not waiting.

At speak23 you can track your cycle — with us, or through whatever system you already use. We factor your cycle phase into how your blood results are interpreted. Across the full marker panel, not just cholesterol.

So you can tell what's real change — and what's cycle.

That's the difference between having a value and having an answer.