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

Yes, cholesterol changes across the menstrual cycle. Total and LDL cholesterol peak in the first half of the cycle and reach their lowest point just before your period: in the BioCycle Study, 14.3% of women were above the 200 mg/dL threshold in the follicular phase, but only 7.9% in the late luteal phase ².

That rarely changes a diagnosis. It does change whether two of your results can be fairly compared.

Why is everyone suddenly talking about cholesterol and the menstrual cycle?

Because Kate Tolo is measuring her own cycle more closely than almost anyone has before. Tolo is a co-founder of Blueprint, Bryan Johnson's longevity project. Across four cycle phases she is collecting data from blood, urine, stool, skin, imaging and fitness tests, with the stated aim of building a health protocol from female data ¹.

One of her key points has travelled fast since: cholesterol shifts across the cycle, and hardly any lab report accounts for it.

We went through the studies. On the substance, she is right. Where we would be more careful is in what follows from it – more on that below.

How much does cholesterol vary during the menstrual cycle?

A few percent on average, and noticeably more for some women. The key study is BioCycle, run by the US National Institute of Child Health and Human Development (NICHD). 259 healthy women aged 18 to 44 with natural cycles gave blood up to eight times per cycle over two cycles, with visits timed to cycle phases using fertility monitors ².

The pattern is clear. Total and LDL cholesterol are highest in the follicular phase, fall after ovulation and bottom out just before menstruation. HDL, often called "good" cholesterol, peaks around ovulation ². From the follicular to the luteal phase, median total cholesterol fell by 3.0% and LDL by 4.9% ².

That sounds small. It gets interesting at the cut-offs:

Share of women above the threshold	Mid-follicular phase	Late luteal phase
Total cholesterol ≥ 200 mg/dL	14.3%	7.9%
LDL cholesterol ≥ 130 mg/dL	17.8%	10.5%

Looking at individual women makes it even clearer. Only 13 of them – 5% – were above 200 mg/dL at every single visit. Yet 51 women, or 19.7%, crossed that line at least once during the study ².

You may also have seen the figure 19%. That is how much total cholesterol changed within the same woman across the cycle on average, roughly 28 mg/dL ². But this number captures the full range between a person's highest and lowest reading, which includes ordinary day-to-day variation as well as the cycle. The pure cycle effect is smaller – but it is systematic and predictable.

Have larger studies confirmed this?

Yes. In 2023, researchers looked at the largest group to date: 8,694 regularly menstruating women in the UK Biobank ³. Total cholesterol, LDL, HDL and the total-to-HDL cholesterol ratio all differed by cycle phase, by 2.2 to 5.3% on average ³. The biggest swing was in the cholesterol/HDL ratio, a number often used in risk assessment ³.

The authors put this in context themselves: the variation across the cycle is larger than the analytical imprecision of modern lab tests, which sits at around 1 to 2% ³. In other words, this is not a measurement artefact. In women with higher body fat and low physical activity, the variation was up to 25% greater ³.

One caveat matters here. In the UK Biobank, each woman was measured only once. The study compares different women at different cycle phases, not the same woman on different days. Taken together with BioCycle, which does exactly that, the picture is still consistent.

Does this lead to wrong cholesterol diagnoses?

Usually not, and we think that is worth saying clearly. Under European guidelines, the decision to treat is based not on a single lab value but on overall cardiovascular risk: age, blood pressure, smoking, diabetes, existing conditions and blood lipids together ⁴. A single reading just above 200 mg/dL does not turn a healthy 30-year-old into a patient.

So we would not sign up to the headline "women with high cholesterol are being misdiagnosed". The BioCycle authors put it more soberly: standardising when in the cycle blood is drawn could make results easier to interpret and reduce unnecessary repeat testing ².

Why does cycle-related variation still matter?

Because it costs you comparability.

Imagine you change your diet, start strength training and add omega-3. Four months later, you retest. If your first blood draw fell in the follicular phase and the second just before your period, part of your "improvement" may simply be your cycle. It can go the other way too: a real improvement disappears because the second test happens to land in the phase with the highest values.

The cycle becomes noise that hides the very answer you are looking for. If you want to follow trends rather than snapshots, you need to know how much variation to expect – and where it comes from.

Is cholesterol the only marker that changes with the cycle?

No, it is just the best-known example. In a metabolomics study, 34 healthy women gave blood at several points in their cycle. Of 54 amino acids and related metabolites measured, 48 changed significantly ⁵. Glucose, vitamin D, HDL and triglycerides also differed between phases; many values were lower in the luteal phase, while the inflammation marker CRP was higher ⁵.

This was a small study, and the 48 out of 54 are based on a fairly lenient statistical cut-off. After stricter correction for running many tests at once, 39 amino acids and derivatives still showed a significant drop in the luteal phase ⁵. Whether these changes matter for health is a separate question. The underlying pattern holds, though: the cycle does not shift one marker, it shifts metabolism.

Why has medicine overlooked the menstrual cycle for so long?

Partly because women were long kept out of the studies in the first place. In 1977, the US Food and Drug Administration (FDA) recommended excluding women of childbearing potential from early-stage drug trials ⁶. The main reason was protecting a potential pregnancy after the harm caused by thalidomide and diethylstilbestrol. Another argument was the variability of ovarian hormones and the menstrual cycle itself ⁶.

Only in 1993, sixteen years later, did the FDA lift that restriction. In the same guideline, it explicitly called for studying how menstrual cycle phase affects drugs, where feasible ⁷.

There is a pattern here. Laboratory medicine carefully controls many sources of variation: fasting, morning draws, a few minutes of seated rest. That is good practice. The cycle, however, was mostly not standardised – it simply was not recorded. A typical lab report still has no field for cycle phase.

Ignoring a variable does not make a measurement more precise. It makes it less informative.

And the cycle is the most obvious blind spot of all. It accompanies women for decades of their lives – and it follows a rhythm that can be known.

How can you make your blood test results comparable across your cycle?

With a few simple habits. None of them is an official guideline recommendation, but they follow directly from the study data ²,³ :

1. Track your cycle. Note the first day of your period, in an app or a calendar.
2. Retest in the same phase. If you want to compare results, try to have blood drawn in the same cycle phase each time, for example in the first week after your period starts.
3. Keep conditions the same. Same time of day, same lab, same fasting status.
4. Note hormonal contraception. The studies cited here looked at women with natural cycles. On the pill, a hormonal IUD or other hormonal methods, these patterns do not simply apply.
5. Keep single results in perspective. If a value sits just above a cut-off, a repeat test in the same phase is more useful than worry. Discuss unusual results with your doctor.

What does speak23 do with your cycle phase?

We are not waiting for this to reach guidelines and lab reports. That usually takes years.

With speak23, you can track your cycle, either with us or through whatever system you already use. We record your cycle phase at every blood draw and factor it into the interpretation of your Core Health blood test. For markers with well-established cycle effects, such as blood lipids, we interpret changes accordingly. For all others, we record the phase as context for your trend – so that patterns become visible once there is enough data.

Is this only a women's health issue?

No. The cycle is the most obvious case of a bigger question that concerns everyone: how much information do we lose when we treat biological rhythms as interference rather than as signal? The cycle is not the only rhythm in the body. But it is the one whose pattern we know best – and use least.

The good news is that this can change without any new technology. It only takes the willingness to ask one more question before interpreting a number. Medicine that understands rhythms does not need to measure more. It understands more.

That is the difference between a number and an answer.

Measure it yourself

Core Health Blood Test

Basic panel of membership - 75+ biomarkers

View the testPackages & pricing