Understanding Your Blood Work: A Nutritional Perspective

Most doctors use standard reference ranges, which capture the lower 2.5% and upper 97.5% of the general population. That population includes people with chronic disease, poor nutrition, and metabolic dysfunction. People on medications that interfere with nutrient absorption. People eating processed food exclusively. You don't want to be normal. You want to be optimal.

Why normal range isn't optimal

Functional medicine practitioners and nutritionists use what's called 'functional range'. This is narrower than the standard reference range. It captures the zone where most healthy people with good nutrition, good energy levels, and absence of symptoms actually sit. The functional range represents not the absence of disease, but the presence of health.

For example, serum ferritin has a standard reference range of roughly 15 to 200 ng/mL. Within that range, you're technically normal. But functional practitioners look for ferritin between 50 and 100. Why? Because people with ferritin below 50 frequently report fatigue, poor exercise recovery, thinning hair, and poor temperature regulation, even though they're technically in the normal range. At 50 to 100, these symptoms tend to resolve.

This pattern repeats across markers. Vitamin D normal range is often cited as 20 to 100 ng/mL. Functionally, most people feel better and have better immune function between 50 and 70. TSH normal range is 0.4 to 4.0. Functionally, most people feel their best between 0.5 and 2.5. The numbers are different but the principle is the same: normal doesn't mean optimal.

Your blood work isn't just a snapshot of disease risk. It's a portrait of your nutritional status. Reading it through a nutritional lens gives you information your standard GP review might miss entirely.

Ferritin: the iron that matters

Ferritin is your iron storage marker. It tells you whether your body has adequate iron reserves. Low ferritin, particularly below 30 but functionally below 50, correlates strongly with fatigue, poor focus, hair loss, and cold intolerance. Women in particular can run chronically low on iron due to menstrual losses, even if they're eating well.

Most GPs test serum iron and TIBC (total iron-binding capacity) to diagnose anaemia. But you can have adequate haemoglobin and still be iron-depleted in storage. Your haemoglobin can be normal whilst your ferritin is dangerously low. That's when ferritin tells the real story. You're not anaemic by standard measures, but you're iron-depleted, and your body is compromising on performance to preserve what little iron you have.

Optimal ferritin is typically 50 to 100 ng/mL for most people.¹ Some research suggests even higher for athletes or people with high metabolic demands. If ferritin is low, the first step is determining whether it's from inadequate intake or excessive loss. Eat liver, beef, oysters regularly. If you menstruate heavily, address that first. If ferritin remains stubbornly low despite good food intake and normal menstruation, you might have poor absorption, in which case gut healing takes priority over iron supplementation.

Ferritin below 50 is common in people who feel perpetually exhausted, even if haemoglobin is technically normal.

B12 and methylation

B12 is measured two ways: serum B12 and methylmalonic acid (MMA). Serum B12 shows your circulating level. MMA shows whether your cells are actually using it effectively.² You can have adequate serum B12 and still be functionally deficient if MMA is elevated, which indicates your cells aren't properly utilising the B12 you have.

B12 is essential for methylation, the process that makes neurotransmitters, regulates gene expression, maintains DNA integrity, and supports immune function. Deficiency shows up as fatigue, poor mood, cognitive fog, neuropathy, and numbness in extremities. It's found almost exclusively in animal foods: liver, beef, fish, eggs, dairy. Plants don't contain usable B12.

Optimal serum B12 is above 500 pmol/L (or 700 pg/mL in US units). Some people feel symptoms of deficiency even at levels that standard medicine considers normal. If you're vegetarian or vegan, B12 deficiency is a genuine and unavoidable risk without supplementation. If you're eating animal foods and B12 is still low, investigate stomach acid production, intrinsic factor antibodies, and gut permeability.

Vitamin D: the forgotten hormone

Vitamin D is a steroid hormone, not a vitamin. It regulates immune function, mood, bone health, cardiovascular health, and metabolic function. Most blood tests show serum 25-hydroxyvitamin D.³ Standard range is 20 to 100 ng/mL, which is absurdly wide. Functional range is 40 to 60 ng/mL in winter and 50 to 70 in summer.

If vitamin D is below 30, mood disorders and immune dysfunction are common. Below 20, bone health begins to suffer meaningfully. But the relationship isn't perfectly linear. Some people genuinely thrive at 50 ng/mL. Others feel notably better at 70. The key is knowing your baseline and how you feel at different levels. Most people improve substantially when brought from 25 to 50.

Vitamin D is synthesised in your skin in response to UVB sunlight. If you live in the UK, supplementation or regular sun exposure is essential, particularly from October to March when the sun angle doesn't permit synthesis. Fatty fish, liver, and egg yolks contain some, but not enough to reach optimal levels without sun or supplementation. Mushrooms contain ergocalciferol, but conversion is limited.

Vitamin D below 40 ng/mL is associated with worse mood, worse immune function, and bone loss. Optimal for most people is 50 to 70.