Exercise and blood tests: what 150 minutes a week changes

How exercise affects blood work: A1C, lipids, CRP and ferritin, results that spike after a workout (CK, AST, ALT) and how long to wait before your test.

Exercise and blood tests: what 150 minutes a week changes

9 min - BioSense Team - 2025-01-09

  • level:understanding
  • activité physique
  • bilan sanguin
  • HbA1c
  • ferritine
  • CK
  • transaminases
  • sport

How exercise changes blood tests: what 150 minutes a week does

Regular exercise, around 150 minutes of activity a week, shows up on a blood test. Some markers improve slowly with regular practice (hemoglobin A1C or HbA1c, triglycerides, HDL, CRP), while others spike for a few days after a hard session without signaling any disease (CK, transaminases, creatinine). This guide helps you tell real progress from a false signal caused by exercise, and choose the right time for your blood work.

Working out and blood tests: what regular activity changes, and what it does not

The Physical Activity Guidelines for Americans, like the World Health Organization (WHO, 2020), set a benchmark of 150 to 300 minutes of moderate-intensity activity per week (or 75 to 150 minutes of vigorous activity), plus muscle strengthening on at least 2 days. These benchmarks are explained in our article on official physical activity recommendations.

On a blood test, two kinds of effects need to be told apart:

  • chronic effects, which appear after several weeks or months of regular practice and reflect a real improvement in metabolism;
  • acute effects, which follow an intense or unusual session for 24 hours to a week, and can look like an abnormality.

| Marker | Effect of regular practice | Effect of a recent intense effort | |---|---|---| | HbA1c, glucose | Decrease, especially if it was high | Glucose sometimes temporarily altered | | Triglycerides | Decrease | May decrease the next day | | HDL cholesterol | Modest increase | Little effect | | LDL cholesterol | Small effect | Little effect | | CRP | Modest decrease | Possible rise for a few days after a very long effort | | Ferritin | May fall in endurance athletes | Possible temporary rise (inflammation marker) | | CK, AST, ALT | No lasting change | Sometimes very large rise, for up to a week | | Creatinine | Higher if muscle mass increases | Temporary rise |

Glucose and HbA1c: the marker that rewards consistency

Working muscle uses glucose and, with repetition, becomes more sensitive to insulin: this is one of the best demonstrated effects of physical activity.

For HbA1c, which reflects average blood glucose over the previous 2 to 3 months, the data are robust in people with type 2 diabetes. A meta-analysis of 47 trials (Umpierre, JAMA 2011) found that structured exercise training was associated with an HbA1c about 0.7 percentage points lower on average (about 7 mmol/mol), with a larger decrease above 150 minutes per week (0.89 versus 0.36 points).

In someone without diabetes whose HbA1c is already normal, the expected effect is much smaller: do not expect to see it on a single test. In the prediabetes range, however, the stakes are real. Prediabetes in the US is defined by the American Diabetes Association as an HbA1c of 5.7% to 6.4% (39 to 46 mmol/mol). In the US Diabetes Prevention Program trial, a program with goals of at least 150 minutes of activity per week and at least 7% weight loss reduced the onset of diabetes by 58% over an average of 2.8 years, in people with elevated glucose values. To place your own value, see our article on HbA1c, prediabetes and diabetes.

Triglycerides, HDL and LDL: real but uneven effects

Triglycerides are the lipid most sensitive to activity. A single endurance session can lower triglycerides measured the next day, and regular practice can keep them lower, especially when they were high to begin with. Activity works best alongside less alcohol and less added sugar.

HDL cholesterol rises, but modestly. A meta-analysis of 25 randomized trials (Kodama, Archives of Internal Medicine 2007) found an average increase of about 2.5 mg/dL (0.065 mmol/L). The threshold for an effect was around 120 minutes of exercise per week, and session length mattered most: each extra 10 minutes per session was associated with about 1.4 mg/dL (0.036 mmol/L) more HDL.

LDL cholesterol, the main risk factor for cardiovascular disease, moves little with activity alone. If your LDL is high, activity remains useful for your heart and arteries, but it replaces neither diet nor, when your doctor considers it necessary, treatment.

CRP: less background inflammation, but a spike after big efforts

CRP is a protein made by the liver in response to inflammation. Measured with a high-sensitivity assay, it is also used as an indicator of low-grade inflammation linked to excess weight, smoking or a sedentary lifestyle.

A meta-analysis of 83 randomized and non-randomized trials (Fedewa, British Journal of Sports Medicine 2017, more than 3,700 people) shows that regular training is associated with a modest fall in CRP. The effect is stronger when activity comes with a loss of body fat.

Conversely, a very long or very intense effort (marathon, trail race, competition) triggers a normal inflammatory response: CRP can rise for a few days, sometimes above the thresholds used to assess cardiovascular risk. A high CRP measured the day after a race therefore says nothing about your background inflammation.

Ferritin: the weak spot of endurance athletes

Ferritin reflects your iron stores. Physical activity, especially endurance, increases iron losses and needs: small digestive losses, destruction of red blood cells by foot strike in running, sweat. After each session, the body also produces more hepcidin, the hormone that slows iron absorption for a few hours.

As a result, according to a reference review (Sim, European Journal of Applied Physiology 2019), iron deficiency affects about 15 to 35% of female athletes and 5 to 11% of male athletes. Premenopausal women who run or cycle regularly are the most exposed, even more so with heavy periods or a diet low in meat.

There are two opposite pitfalls:

  • low ferritin can explain fatigue, unusual breathlessness or a drop in performance, even with normal hemoglobin. The American Society of Hematology's 2026 guideline treats a ferritin of 30 ng/mL (30 µg/L) or lower as diagnostic of iron deficiency in adults, while WHO has long used a lower cutoff in healthy adults, so lab flags may vary;
  • falsely reassuring ferritin: like CRP, ferritin rises with inflammation, including after a very intense effort. A normal ferritin drawn the day after a competition can hide low stores.

For the causes of high ferritin, see our article on high ferritin.

CK, transaminases and creatinine: the false signals of exercise

CK (creatine kinase) is a muscle enzyme. After an intense session, especially an unusual one or one with a lot of braking movements (downhill running, eccentric strength work), it can reach several times the lab's upper limit, sometimes far more, within a few days. A high CK without symptoms in someone who has recently trained is most often benign. However, severe muscle pain combined with dark urine (the color of tea or cola) after exercise requires urgent medical attention: it may be a sign of rhabdomyolysis.

Transaminases (AST and ALT) are often read as liver markers, but they are also found in muscle. In a study of 15 healthy men (Pettersson, British Journal of Clinical Pharmacology 2008), one hour of weightlifting was enough to raise AST and ALT markedly, and they stayed high for at least 7 days. Gamma-GT (GGT) and bilirubin remained normal. Raised transaminases with normal GGT and a high CK therefore point to muscle rather than liver. Our article on ALT and AST covers the other causes.

Creatinine depends on muscle mass. A recent intense effort, a very high-protein diet or a creatine supplement raise it, which can wrongly lower the estimated glomerular filtration rate (eGFR) and suggest reduced kidney function. Mention these factors if your creatinine is slightly high.

When to have a blood test if you exercise

There is no single official rule. The following delays are cautious benchmarks, to be adapted with your doctor or the lab:

| Situation | Suggested delay before the blood test | |---|---| | Walking, usual moderate activity | No particular delay | | Usual intense session (running, cycling, intervals) | 48 hours without intense effort | | Unusual strength training, restarting after a break | 3 to 7 days if CK or transaminases are measured | | Marathon, trail race, long competition | About a week if you want baseline CRP, ferritin or transaminases |

A few rules complete these delays:

  • come fasting if your test order requires it, well hydrated, without having run or cycled to the lab (plain water is fine): see our guide to preparing for a fasting blood test;
  • tell the lab and your doctor about any intense effort in the previous days and the supplements you take (creatine, iron, protein);
  • compare like with like: to follow the effect of your program on HbA1c or lipids, keep the same conditions from one test to the next and leave at least three months between two HbA1c measurements.

To learn how to build up to these 150 minutes, or beyond, without exhausting yourself, see our guide to zone 2 training and VO2 max.

Frequently asked questions

How long before activity shows up on a blood test?

Triglycerides respond within days. HDL and CRP take several weeks to a few months of regular practice. HbA1c reflects the previous 2 to 3 months: a test three months after starting a program is the minimum delay to judge an effect.

My transaminases are high and I do strength training: should I worry?

Not necessarily. An intense session can raise them for a week. Your doctor may order a repeat test after a week without intense effort, with GGT and CK. If the rise persists at rest, other liver causes need to be looked for.

Can exercise lower ferritin?

Yes, especially running and endurance sports in premenopausal women. Low ferritin with fatigue is worth discussing with your doctor, who will also look for other causes of iron loss before any iron supplement.

*This article is for information purposes only and does not replace medical advice. If you have chest pain, unusual breathlessness or dark urine after exercise, seek medical attention without delay.*

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Sources and methodology

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United States

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*BioSense methodology: our content is based on guidance from US authorities and medical societies (such as the USPSTF, CDC, NIH, FDA, ACC/AHA and ADA), UK bodies (NICE, NHS), European societies and the WHO. Sources are listed by country, and differences between countries are stated when they matter.*