The ketogenic diet is popular among those looking to optimise their health, stabilise energy levels or improve body composition. But how do you know if keto is actually working for you? The answer lies in your blood values. By testing before and after starting keto, you get hard data instead of guesswork.
This article walks through every relevant biomarker that changes on keto, what to expect, when to retest, and which red flags mean you should adjust your approach.
Why test before you start?
Without a baseline measurement, you have no reference point. You cannot tell whether a change in your blood values comes from the diet, a seasonal effect, more exercise, or something else entirely. A fasting blood test before starting gives you a personal baseline of your lipid profile, glucose regulation, liver values and thyroid function. It reveals existing risk factors that may influence your approach, and provides data to compare objectively after 3 months. This is not a luxury; it is the foundation of an evidence-based approach. You cannot optimise what you do not measure.
What typically changes on keto?
A ketogenic diet shifts your primary fuel source from carbohydrates to fats. This triggers a cascade of metabolic changes reflected in your blood values. The effects vary by individual depending on genetics, metabolic starting point and specific dietary choices.
LDL cholesterol: often rises
LDL cholesterol increases in many people on keto, sometimes substantially (30-50% increases are not uncommon). The degree varies by genetics (especially APOE gene polymorphisms), fat type in the diet, and metabolic starting point. Standard LDL measurements (Friedewald formula) report cholesterol content but not particle count or size. High LDL with mostly large buoyant particles (pattern A) may carry different risk than high LDL with many small dense particles (pattern B), which are more oxidation-prone and penetrate arterial walls more easily.
If your LDL rises sharply, consider testing ApoB (the best surrogate for atherogenic particle count). ApoB above 1.0 g/L is considered elevated risk by most guidelines, regardless of LDL value.
HDL cholesterol: usually rises
One of the most consistent positive effects of keto is a rise in HDL cholesterol. HDL particles transport excess cholesterol from arterial walls back to the liver. HDL above 1.5 mmol/L is considered protective. The combination of higher HDL and lower triglycerides is one of the most favourable changes you can see. The triglyceride/HDL ratio (in mmol/L: below 0.9 is excellent, below 1.5 is good) is a powerful proxy for insulin resistance and a better cardiovascular risk predictor than LDL alone.
Triglycerides: often drop
Triglycerides typically drop 20-40% on keto, with some people seeing a halving. By drastically reducing carbohydrate intake, your liver produces fewer VLDL particles (which transport triglycerides). Low triglycerides (below 1.0 mmol/L) combined with high HDL is one of the strongest indicators of a favourable metabolic profile, correlating with low insulin resistance and few small dense LDL particles.
Glucose and HbA1c: decline
Without a constant carbohydrate supply, fasting blood glucose drops and stabilises at a lower, steadier level. The spikes and crashes of a standard diet are flattened. HbA1c, reflecting your 2-3 month average, typically drops 0.3-0.5% after 3 months. Particularly relevant for those with insulin resistance or prediabetes. Note: type 1 diabetics or those on diabetes medications (especially insulin or sulfonylureas) need medical supervision due to hypoglycaemia and ketoacidosis risk.
Liver values: temporary rise possible
ALT may rise slightly in the first weeks as your liver ramps up fatty acid beta-oxidation to produce ketone bodies. This adaptation usually completes within 4-8 weeks. Long-term, keto may actually improve liver values, especially in those with non-alcoholic fatty liver disease (NAFLD). ALT that keeps rising after 8 weeks or exceeds 2x the upper limit (above approximately 90 U/L) is a reason to reassess, especially if you combine high fat intake with significant alcohol consumption.
Uric acid: may rise
Uric acid can increase initially because ketone bodies (especially beta-hydroxybutyrate) compete with uric acid for renal excretion via the same transporters. This is particularly relevant if you already have elevated uric acid or are gout-prone. For most people, uric acid normalises after 4-6 weeks as ketone production stabilises. Adequate hydration (2-3 litres daily) and avoiding excessive purine-rich foods (organ meats, shellfish) helps.
When to retest
- Baseline (week 0) - fasting blood test with full lipid panel, fasting glucose, HbA1c, liver values (ALT, AST, GGT), thyroid function (TSH) and uric acid
- 3 months - first real evaluation after metabolic adaptation
- 6 months - trend confirmation and decision point for long-term sustainability
- Then every 6 months - long-term monitoring, focusing on lipid profile and thyroid
Always test fasting (12 hours, water only). Avoid intense training the day before, as this can skew liver values, CK and inflammatory markers.
Red flags: when to adjust
Keto is powerful but not universally optimal. Consider adjusting if you see: LDL above 5.0 mmol/L or a rise exceeding 50% from baseline (especially with high ApoB), ALT continuing to rise after 8 weeks, TSH rising above 4.0 mU/L (possible thyroid suppression), uric acid staying above 420 mcmol/L after 6 weeks with gout symptoms, or feeling persistently worse beyond the 4-6 week adaptation phase (fatigue, brain fog, cold intolerance).
Adjusting does not have to mean quitting. A moderate low-carb approach (100-150g carbs/day) or cyclical keto (1-2 higher-carb days per week) often retains benefits without the downsides.
Keto and thyroid function
TSH may rise and T3 (the active thyroid hormone) may drop on very low carbohydrate intake because hepatic T4-to-T3 conversion requires insulin, which is structurally lower on keto. Some researchers consider a mild T3 drop on keto a normal adaptation, similar to the drop seen during calorie restriction.
Watch for signals of excessive thyroid suppression: declining energy despite stable ketosis, cold intolerance, weight stagnation or gain despite ketosis, hair loss, brittle nails, or constipation. Adding 1-2 higher-carbohydrate days per week (cyclical keto) may be sufficient to maintain T3 conversion while staying in ketosis the rest of the week.
Frequently asked questions
Is high LDL on keto dangerous?
Context matters. LDL rises on keto often coincide with higher HDL and lower triglycerides, making the overall risk profile more nuanced. The triglyceride/HDL ratio often improves and the pattern shifts towards larger, more buoyant LDL particles. However, LDL above 5.0 mmol/L, family history of cardiovascular disease (especially premature events before age 55), or other risk factors warrant medical consultation. Consider ApoB testing and possibly a coronary artery calcium score for a complete risk assessment.
Should I fast before testing on keto?
Yes, and this is extra important on keto. Your body continuously burns fat as fuel, making non-fasting lipid values unreliable: triglycerides and LDL can look very different after a meal. Fast 12 hours, drink only water, and avoid testing the day after a very high-fat meal or cheat day.
Which test gives the best picture?
At minimum: full lipid panel (total cholesterol, LDL, HDL, triglycerides and calculated ratios), fasting glucose, HbA1c, liver values (ALT, AST, GGT), thyroid function (TSH, preferably also free T4) and uric acid. If LDL rises more than 30% above baseline, add ApoB as the key additional marker. A comprehensive health panel combining all these markers in a single blood draw is the most efficient choice.
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