How Diet & Exercise Affect Your Blood Tests
What you ate, when you trained, and which supplements you took can all skew your results. Here is how to get the truest snapshot of your health.
The Biotin Warning - This Saves Lives
Biotin (vitamin B7) is in nearly every hair, skin and nail supplement on the market. Doses above 5 mg cause major interference with troponin and thyroid hormone assays - they can mask a heart attack or fake hyperthyroidism in some lab platforms.
Stop biotin supplements at least 72 hours before any blood test - and tell your doctor what supplements you take.
How Exercise Affects Your Blood
Acute exercise (the last day or two) pushes muscle enzymes, immune cells and stress hormones up. Regular exercise lowers fasting glucose, triglycerides and inflammation in the long term. The key is to know which effects last hours and which last days.
| Activity | Marker affected | Effect | Recovery |
|---|---|---|---|
| Heavy resistance training (last 48h) | CK (Creatine Kinase) | Can rise 5-50x normal | 48-72 hours |
| Marathon or endurance race | Troponin (transient) | Mild elevation, mimicking cardiac event | 24-72 hours |
| Heavy exercise | AST (and to a lesser extent ALT) | Elevated from muscle, mimics liver injury | 48-72 hours |
| Acute exercise | White cells (neutrophils) | Transient rise of 2-3 x10^9/L | 4-24 hours |
| Endurance training | Haemoglobin (slight dilution) | Slight drop from plasma volume expansion | Chronic adaptation, not a problem |
| Regular aerobic exercise | HDL cholesterol | Beneficial rise | Long-term benefit |
| Regular exercise | Fasting glucose, HbA1c, triglycerides | Beneficial decrease | Long-term benefit |
| Intense session same morning | Cortisol | Significantly elevated | 4-6 hours |
What Food Does to Your Results
Recent meals push glucose, triglycerides, urea and uric acid around. Even a healthy meal of grilled chicken and a banana will shift several markers temporarily. That is why fasting matters for certain tests.
| Food / situation | Marker affected | Effect | Recovery window |
|---|---|---|---|
| Recent meal (any food) | Glucose, triglycerides | Both rise. Triglycerides up to 30%. | Fast 8-12 hours |
| High-protein meal | Urea, uric acid | Both increase | 12-24 hours |
| High-purine food (organ meats, sardines) | Uric acid | Spike that can trigger gout | 24-48 hours |
| Bananas, oranges, tomatoes | Potassium | Mild rise | 4-12 hours |
| Asparagus / beetroot | Urine colour | Pink/purple urine (harmless) | 24 hours |
| Liver, kidney, beef | Iron, B12 | Acute rise | 12-24 hours |
| Vitamin C high meal | Glucose (some meters) | Falsely elevated finger-prick glucose | 4-8 hours |
| Caffeine | Cortisol, glucose, blood pressure | Mild increase | 4-8 hours |
Alcohol and Blood Tests
Alcohol affects multiple markers for days after a heavy night. The most affected are:
GGT (Liver enzyme)
Sensitive marker of alcohol use. Rises within days of heavy drinking, takes 4-6 weeks of abstinence to fully normalise.
MCV (Red cell size)
Heavy drinking enlarges red cells. MCV stays elevated for weeks after stopping. Often the first hint to a GP of unrecognised alcohol use.
Triglycerides
Spike sharply after binge drinking. Can read 2-3x baseline if you drank heavily the night before.
Glucose
Can drop dangerously low during a fast after heavy drinking, or rise if alcohol was consumed with sugary mixers.
AST and ALT
Long-term heavy use raises AST more than ALT - the “AST/ALT ratio above 2” is a classic alcohol pattern.
CDT (specialist test)
Carbohydrate-deficient transferrin - the most specific alcohol marker, used in addiction medicine and court contexts.
Supplements That Interfere
| Supplement | Tests affected | Severity |
|---|---|---|
| Biotin (B7) - in hair/skin/nail supplements | Troponin, TSH, FT4, FT3, vitamin D, hCG and many hormone tests | CRITICAL - can mask heart attacks or fake hyperthyroidism |
| Iron supplements | Serum iron, transferrin saturation | High - skews iron studies if taken in 24h |
| Vitamin C (megadose) | Glucose, occult blood test in stool, copper | Moderate |
| Vitamin B12 injection or high oral dose | Serum B12 (artificially elevated) | Moderate - look at active B12 instead |
| Calcium supplements taken with sample | Magnesium, phosphate | Low-moderate |
| Fish oil / omega-3 | Triglycerides (lowers chronically), bleeding time | Low |
| Glucosamine | Glucose tolerance in some studies | Low |
How to Get the Most Accurate Results
Stop biotin supplements 72 hours before testing - non-negotiable
Avoid heavy exercise for 24-48 hours pre-test
Avoid alcohol for 48-72 hours (longer for clean liver bloods)
Fast for 8-12 hours if any lipid, glucose or iron study is included
Stay hydrated - water is encouraged
Skip morning vitamin doses (especially iron and B12) until after the test
Book a morning slot - cortisol and testosterone peak in the morning
Tell the collector and your GP about EVERY supplement and recent activity
Avoid testing right after vaccination (alters inflammation markers for ~48h)
Avoid testing in the middle of an acute illness unless it is being investigated
Track Lifestyle vs Lab Trends
Upload your results - SmarterBlood charts your markers over months and years so you can see whether new habits actually moved the dial.