Keto Diet Blood Test: Markers to Track

The ketogenic diet fundamentally changes how your body generates energy — switching from glucose to fat as the primary fuel source. This metabolic shift produces measurable changes in blood markers that can look alarming on a standard blood test if your doctor isn't familiar with the expected patterns. Total cholesterol may jump by 30–50%, liver enzymes can rise in the first few weeks, and uric acid may spike enough to trigger a gout attack.

Not all of these changes are harmful. Some are genuinely beneficial. The challenge is knowing which is which, and that's where targeted blood testing becomes essential for anyone following a ketogenic diet.

The Lipid Panel: The Most Important Test on Keto

Cholesterol is the marker that causes the most anxiety for people following a ketogenic or high-fat diet. Total cholesterol and LDL cholesterol frequently rise — sometimes dramatically. It's not unusual to see LDL double within the first 3–6 months of strict keto. This is the so-called "lean mass hyper-responder" pattern, first characterised by researcher Dave Feldman, and it appears most pronounced in lean, metabolically healthy individuals.

However, the full lipid pattern matters far more than LDL in isolation. The classic keto lipid profile shows:

• LDL cholesterol — often elevated, sometimes significantly
• HDL cholesterol — typically increases by 10–20% (a positive change)
• Triglycerides — typically decrease by 20–40% (a strongly positive change)
• Triglyceride-to-HDL ratio — often improves to below 1.0 (associated with lower cardiovascular risk)

The triglyceride-to-HDL ratio is increasingly recognised as a more meaningful cardiovascular risk marker than LDL alone. A ratio below 1.0 (in mmol/L units) suggests a predominance of large, buoyant LDL particles rather than small, dense LDL — the latter being more atherogenic. If your LDL has risen on keto but your triglycerides are below 0.8 mmol/L and your HDL is above 1.5 mmol/L, the overall cardiovascular risk profile may actually be improved.

That said, very high LDL (above 5.0 mmol/L) warrants monitoring regardless of context. Consider an ApoB test for a more accurate count of atherogenic particles if your LDL is persistently elevated on keto. ApoB directly measures the number of potentially harmful lipoprotein particles, which is arguably a better risk predictor than LDL-C concentration alone.

Liver Function: Initial Stress That Should Resolve

Starting a ketogenic diet can temporarily elevate liver enzymes — particularly ALT and GGT. The liver is working harder during keto-adaptation: it's upregulating fatty acid oxidation, producing ketone bodies (beta-hydroxybutyrate, acetoacetate, and acetone), and processing significantly more dietary fat than before.

In most people, liver enzymes rise modestly in weeks 2–6 of keto and then normalise as the liver adapts. If ALT remains elevated beyond 3 months, or GGT is persistently above the upper reference limit, further investigation is appropriate — particularly if pre-existing fatty liver disease (NAFLD) was present before starting keto.

Ironically, ketogenic diets are one of the most effective dietary interventions for reversing NAFLD. Studies have shown that keto can reduce liver fat content by 40–50% within 6 months. But the initial adaptation period can transiently worsen liver markers before they improve. Blood testing at baseline (before starting keto), at 6 weeks, and at 3 months maps this trajectory clearly.

Kidney Function: Dehydration and Electrolyte Shifts

Ketosis is inherently diuretic. When carbohydrate intake drops below approximately 50g per day, glycogen stores deplete. Each gram of glycogen holds 3–4g of water, so the initial 1–3kg of weight loss on keto is almost entirely water. This water loss carries electrolytes — particularly sodium, potassium, and magnesium — with it.

Blood urea and creatinine can rise due to dehydration rather than genuine kidney impairment. eGFR may dip transiently. Testing kidney function in the first two weeks of keto will likely show misleadingly poor results. Wait until at least 4 weeks in, with consistent hydration, before interpreting kidney markers.

Long-term, higher-protein ketogenic diets (some versions emphasise protein alongside fat) naturally elevate creatinine and urea through increased protein metabolism. As with bodybuilders, eGFR is the more reliable indicator of actual kidney function than raw creatinine.

HbA1c: Where Keto Shines

Glycated haemoglobin (HbA1c) reflects average blood glucose over the preceding 8–12 weeks. Ketogenic diets typically produce significant improvements in HbA1c, often dropping it by 5–10 mmol/mol within 3 months. For people with type 2 diabetes or pre-diabetes (HbA1c 42–47 mmol/mol), keto can bring HbA1c into the normal range (below 42 mmol/mol) without medication.

Diabetes UK reports that approximately 4.3 million people in the UK are living with diabetes, with 90% having type 2. The NHS spends roughly £10 billion annually on diabetes management. For those using diet as a primary intervention, tracking HbA1c every 3 months provides objective evidence of whether the approach is working.

Fasting glucose on keto can sometimes appear paradoxically elevated — the "dawn phenomenon" is amplified when the liver is highly active in gluconeogenesis overnight. A fasting glucose of 5.5–6.0 mmol/L with an HbA1c of 34 mmol/mol is not a concern — the HbA1c confirms excellent average glucose control despite the single-point morning reading.

Uric Acid: The Gout Risk

Uric acid often spikes in the first 2–4 weeks of ketosis. Beta-hydroxybutyrate (the primary ketone body) competes with uric acid for excretion through the kidneys. When BHB levels are high during early keto-adaptation, uric acid clearance is reduced, and serum levels rise.

For people with a history of gout or hyperuricaemia, this can trigger an acute gout attack during the transition period. Uric acid levels above 420 µmol/L in men or 360 µmol/L in women indicate elevated risk. The good news is that uric acid typically normalises after 4–8 weeks of consistent ketosis as the kidneys adapt to handling both BHB and urate efficiently.

Staying well-hydrated and maintaining adequate sodium intake during the transition period helps mitigate the uric acid spike.

Electrolytes: Sodium and Magnesium

Electrolyte depletion is the most common cause of the "keto flu" — headaches, fatigue, muscle cramps, and irritability in the first 1–2 weeks. Sodium requirements on keto are higher than on a standard diet because insulin levels fall (insulin promotes sodium retention in the kidneys), and the diuretic effect of ketosis accelerates sodium excretion.

Many keto practitioners supplement 3,000–5,000 mg of sodium daily, compared to the general recommendation of no more than 2,300 mg. Blood sodium (serum sodium) rarely drops below normal even with significant sodium loss because the body tightly regulates serum levels by concentrating urine. However, symptoms of sodium depletion can occur well before serum sodium is flagged as low.

Magnesium is the more concerning electrolyte on keto. Serum magnesium below 0.7 mmol/L is clinically deficient, but symptoms (cramps, insomnia, anxiety, heart palpitations) can appear even within the "normal" range. Magnesium glycinate or citrate supplementation of 200–400 mg daily is commonly recommended for keto followers.

Thyroid Function: Very Low Carb Can Lower T3

Very low carbohydrate intake can reduce the conversion of T4 to T3 — the active thyroid hormone. This is sometimes called "euthyroid sick syndrome" or "low T3 syndrome." It's an adaptive response to reduced caloric intake or carbohydrate restriction, not true hypothyroidism.

TSH typically remains normal, free T4 is normal, but free T3 drops below the optimal range. Symptoms can include fatigue, cold hands and feet, and slight hair thinning — which overlap with the symptoms people often attribute to keto-adaptation itself. Testing thyroid function (TSH, free T4, free T3) at baseline and after 3 months on keto clarifies whether thyroid function is being affected.

If free T3 is suppressed, increasing carbohydrate intake to 50–75g per day (a more moderate low-carb approach) often resolves the issue while still maintaining ketosis for much of the day.

Recommended Blood Tests for Keto Dieters

The Cardiovascular Health test (£83) is the ideal starting point for anyone on keto — it covers the full lipid panel alongside HbA1c, providing the two most critical data points: how your cholesterol pattern is responding and whether your blood sugar control is improving.

For comprehensive monitoring including liver function, kidney function, thyroid, electrolytes, and uric acid, the Core Health 45 biomarker test (£120) covers all the key markers discussed in this article in a single panel.

Testing Schedule on Keto

Test at baseline before starting the diet, then again at 6–8 weeks (once keto-adapted but early enough to catch problems), and at 3–6 months to confirm long-term trends. After the first year, annual testing is generally sufficient unless markers are being actively managed.

Fast for 12–14 hours before your blood draw for the most accurate lipid and glucose results. Morning draws produce the most consistent readings across all markers.