Why Your Blood Test Gets Worse the More You Train

You train five or six days a week, you eat like you mean it, and the annual physical comes back with more red arrows than last year. It happens to a lot of endurance athletes, and it almost always triggers the same thought: is all this training actually hurting me?

Usually the answer is no. A standard blood panel was built around a largely sedentary reference population. Training changes your muscle mass, your plasma volume, your hormone axis and the way your kidneys handle waste, so a handful of markers that read abnormal in a general population are simply what a trained body looks like at rest. The job is to tell those apart from the small set of results that genuinely needs a clinician.

This article is general education, not medical advice, and it does not diagnose anything. Reference intervals differ between laboratories. Bring the actual report to a clinician who knows your training history.

Key takeaways

• Several abnormal results in trained athletes are adaptation, not injury. A resting heart rate in the 40s and a high HDL are the two most common.

• What you did in the 48 hours before the draw moves creatinine, urine protein, CK, ALT and AST more than your long-term health does.

• Muscle mass raises creatinine. If creatinine is high but urine protein is normal, ask for cystatin C before you worry.

• A high total cholesterol driven by high HDL is a different picture from one driven by high LDL. Read the four numbers, not the headline.

• Hard efforts raise uric acid partly because lactate and uric acid compete for the same clearance pathway in the kidney.

• The fix for most athletic abnormalities is unglamorous: 48-72 hours of easy training, normal hydration, and a normal-protein day before the test.

Match the stress to the marker

Not every athlete gets flagged for the same thing. The table below maps the training stress to the markers it tends to move, so you can jump straight to the section that applies to you.

Three things that distort your results before the needle goes in

Hard training in the 48-72 hours before the draw

Creatine kinase (CK) is the enzyme that leaks out of muscle cells when their membranes are stressed. After a hard or unfamiliar session it typically starts rising within about 12 hours, peaks somewhere between 24 and 72 hours, and takes around five days to come back down, longer after downhill running or heavy eccentric work. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) travel a similar curve, because AST lives inside muscle as well as liver.

In other words, a Monday morning blood draw after a Sunday long descent is reporting on Sunday. If you can, schedule the test after two or three easy days.

Under-drinking while you fast

Fasting before a blood draw means no food. It does not mean no water. Plenty of people stop drinking at 8 p.m. the night before and arrive at the clinic having had nothing for twelve hours. Concentrated urine pushes urine protein up, concentrates uric acid and nudges creatinine higher. Drink normally through the day before; just skip the food, and skip the morning coffee if your lab asks you to.

A high-protein day right before the test

If you eat 1.6 g/kg of protein most days and eat the same way the day before the test, then add a hard lifting session and a shake on top, you have maximised the chance of a misleading urine protein result. Two easy days and a moderate protein intake beforehand cost you nothing and make the report readable.

Urine protein: what the plus signs mean

A dipstick grades protein in urine from trace to 4+. It is a semi-quantitative test, so the brackets below are approximate and laboratories differ, but the shape of the scale is genuinely useful.

One of our hosts sat at 2+ for two consecutive years. Her clinician asked about training and diet, then asked her to follow a very low-protein diet for three days and repeat the test. Everything normalised. That is a common pattern, but it is one person's story rather than a protocol, and you should confirm it with your own clinician instead of copying it.

If you land on 1+ or 2+

• Cut protein roughly in half for 7-14 days. If you normally eat 1.3-1.5 g/kg, drop toward about 0.8 g/kg temporarily, not permanently.

• Halve training intensity over the same window. Walking, easy spinning, mobility and yoga are all fine.

• Drink water steadily across the day rather than in one large bolus before the test.

• Skip the all-nighter. Sleep debt and travel fatigue move these markers on their own.

• Retest after one to two weeks, ideally with a first-morning sample.

Creatinine, and the test that settles the question

Creatinine is a waste product of muscle energy metabolism, cleared by the kidney. Serum creatinine is a good filtration marker in a general population, but it scales with muscle mass, body size, sex and age. A 90 kg man with a decade of training behind him, especially one supplementing creatine, can sit slightly above the population reference range and have entirely normal kidney function. For reference, Mayo Clinic Laboratories lists adult male creatinine at roughly 0.74-1.35 mg/dL and adult female at roughly 0.59-1.04 mg/dL, with the usual caveat that intervals are lab-specific.

If creatinine is high but urine protein is normal, ask for cystatin C. It is produced at a fairly constant rate by all nucleated cells, it is much less influenced by muscle mass, and it tends to rise before creatinine does when filtration genuinely falls. If cystatin C is normal, the elevated creatinine is far more likely to reflect your muscle than your kidney.

The next rung up is a measured creatinine clearance, which needs a full 24-hour urine collection: every void, kept cold, for a full day. It is awkward, and it is also the most useful number available for a heavily muscled person, because it compares what you clear against what you produce.

Cholesterol: read all four numbers

A lipid panel gives you total cholesterol, HDL, LDL and triglycerides. Only one of those four is a summary, and it happens to be the one people stare at.

• HDL - higher is generally the direction you want, and regular aerobic training raises it.

• LDL - lower is generally preferred; intake of saturated and trans fat is the main dietary lever.

• Triglycerides - track closely with visceral fat, added sugar and alcohol, and drop quickly when you add aerobic volume.

• Total cholesterol - HDL plus LDL plus the rest. If your HDL is high because you train, your total can read high on a panel that is otherwise reassuring.

There is a second pattern worth knowing. If total cholesterol has been flagged at every test for years while HDL, LDL and triglycerides are all normal and your lifestyle is clean, ask about familial hypercholesterolaemia, a genetic pattern that raises cholesterol independently of diet and training.

It is also worth remembering that cholesterol is raw material. Your body synthesises a substantial amount each day and adjusts production against intake, and cholesterol is the precursor for steroid hormones. That does not make a high LDL harmless. It does mean one number out of context tells you very little.

Uric acid: the training connection

Uric acid is the end product of purine breakdown. Purines come from food - shellfish, organ meat, beer, some mushrooms - and from your own cells. Hard training increases purine turnover on its own, which is why athletes who drink nothing and eat no seafood can still run high.

Three mechanisms stack up after a hard session:

• More purine turnover from accelerated muscle energy metabolism means more substrate for uric acid.

• Heavy sweating without replacement drops urine volume, so the same load is cleared in less water. Picture a salt solution losing water: crystals form sooner.

• Lactate and uric acid share transport pathways in the kidney and compete with each other. A session that floods your system with lactate slows uric acid clearance at exactly the moment production is highest.

Reference intervals vary. Mayo Clinic Laboratories lists roughly 3.7-8.0 mg/dL for adult men and 2.7-6.1 mg/dL for adult women, and defines hyperuricaemia most commonly as above 8.0 mg/dL in men or 6.1 mg/dL in women. Many panels simply flag anything above the local upper limit, so a result marginally over the line is not automatically a diagnosis.

What actually helps: cap the intensity spikes, finish sessions with real fluid replacement instead of waiting until you are thirsty, and treat hydration as part of training rather than an afterthought. This is also where an electrolyte drink earns its place, because replacing sodium and fluid together restores urine volume far better than plain water alone after a long, salty session. If a joint becomes hot, swollen and exquisitely painful, or if you pass dark or cola-coloured urine, stop reading articles and go to a clinician.

Resting heart rate: the one most athletes misread

Sinus bradycardia, a resting heart rate between roughly 40 and 60 beats per minute, is one of the most common findings in endurance athletes and one of the most consistently misread. It is usually adaptation, not pathology.

The heart is a muscle. Sustained aerobic training increases stroke volume, the amount of blood ejected per beat. A heart that moves more blood per beat does not need to beat as often to supply the body at rest. The analogy that tends to land: a strong worker carrying ten bricks per trip makes fewer trips than one carrying five.

It stops being good news when it comes with symptoms. Dizziness, near-fainting or fainting, unusual fatigue, breathlessness, chest pain or palpitations, or an ECG that also reports conduction block or ST-segment changes, all warrant a cardiology opinion regardless of how fit you are. A resting heart rate persistently below about 40 with no symptoms is common in long-time endurance athletes; the same number with symptoms is not.

CK, AST and ALT: muscle damage or something worse

CK normally lives inside the muscle cell, and blood CK is usually under about 200 U/L, though exact ranges vary by laboratory. Heavy or unfamiliar work - a new lifting block, a long descent, a sudden jump from 30 minutes to two hours of aerobic work - stresses muscle membranes and lets CK leak into the blood, along with soreness and heaviness.

The threshold commonly used to diagnose exertional rhabdomyolysis is CK at least five times the upper limit of normal, in the right clinical context. Two details matter more than the number itself:

• The classic triad of severe muscle pain, weakness and dark tea- or cola-coloured urine appears in under 10% of cases, and more than half of those affected do not report obvious muscle pain. Check your urine colour after brutal sessions; it is the cheapest early warning you have.

• Risk of acute kidney injury climbs sharply once CK passes roughly 5,000 U/L, because myoglobin released from broken-down muscle obstructs the kidney's filtering tubules.

Dark urine, rapidly worsening weakness, or CK that keeps climbing instead of falling after 48-72 hours of rest is an emergency, not a recovery problem. Go to urgent care or an emergency department.

AST and ALT rise for two overlapping reasons: they leak from damaged muscle alongside CK, and the liver has to process a larger load of metabolic by-products during and after hard efforts. Both are usually transient. Rest, hydrate, and recheck after a week of easy training before you assume anything is wrong with your liver, and be honest with your clinician about alcohol, training load and supplements, because that context changes the interpretation completely.

White blood cells: the immune open window

A complete blood count with slightly raised total white cells, neutrophils or lymphocytes is another common finding in people who train hard, particularly in the 24 hours after a long race or a heavy block. Exercise causes white cells that were marginated along vessel walls to detach and enter circulation, driven largely by adrenaline and cortisol. The count goes up because the cells moved, not because you are fighting something. It is transient.

The more useful read is the pattern over time. Endurance athletes who race or do long sessions frequently sit in what gets called the immune open window, a short period after hard efforts. The practical response is not a supplement; it is sleep, enough food, and not stacking another maximal effort on top of the last one.

How to prepare for your next blood test

• Take 48-72 hours off hard training. Easy aerobic work and mobility are fine.

• Eat normally, not heroically: moderate protein, normal carbohydrate, no blowout meal the night before.

• Drink water normally right up to the draw, even while fasting from food.

• Sleep. Travel, shift work and a four-hour night move markers on their own.

• Tell the clinician your weekly training volume and what you did in the 72 hours before the draw. It changes how the report gets read.

• If you are re-testing a specific concern, test at the same time of day under similar conditions so the two numbers are comparable.

When to stop self-interpreting

• Urine protein at 3+ or 4+, or any protein result alongside blood in urine.

• Creatinine rising across successive tests, or cystatin C above range.

• Triglycerides and fasting glucose both elevated, especially alongside a waist that keeps growing while your limbs stay lean.

• Resting heart rate under 40 with dizziness, fainting or chest symptoms.

• CK above five times the upper limit of normal, or any dark urine after exercise.

• The same number flagged at every test for several years running.

What this comes down to

Training is a stressor, and blood tests read stress. Most of the worsening athletes notice is the measurable signature of a body that has adapted: a bigger stroke volume, higher HDL, more muscle turnover, more waste to clear. The minority that matters usually announces itself, either by climbing across successive tests or by arriving with symptoms.

The unglamorous levers do most of the work. Progress training load gradually, because the familiar ten percent rule exists for the simple reason that muscle and connective tissue adapt on different timelines. Replace fluid and sodium during and after long or hot sessions. Eat enough protein, but not a heroic amount. And treat sleep as training, because it is where the repair you are measuring actually happens.