Nutritional Assessment & Functional Lab Testing

Written by Empire Medical Training Editorial Team

Medically reviewed by Mark Tager, MD Integrative & Functional Medicine Physician; Author, ‘Feed Your Skin’

Updated June 2026 ·11 min read

Every patient sits somewhere on a continuum that runs from frank illness to optimal health, and no two start in the same place. Their genetics, environment, diet, medications and personal history are unique, which means the information you gather about them has to be interpreted for an N of one rather than against a population average. That is the premise of a precision nutrition assessment: you are not running a fixed panel and reading a printout, you are building a picture of one person from many overlapping signals.

This guide walks through how that picture comes together — what to ask, what to observe, and what the labs add — through the clinical lens Dr. Mark Tager teaches in Empire's Precision Nutrition Master Training. It is clinical education, not medical advice, and nothing here is a protocol, an ordering recipe, or a substitute for individualized clinical judgment.

Orienting the history to nutrition

A nutrition-aware history is a normal history with a few deliberate expansions. Alongside the chief complaint, skin concerns, past medical history and allergies, you gather lifestyle detail on sleep, stress, diet and movement, and you ask specifically about GI symptoms, mood and energy. The history is also the natural place to expand on basic dietary questions and to have the patient list every supplement they take — with the caveat that many patients genuinely do not know the specifics of what they are taking, or whether another practitioner is already guiding their nutrition.

Diet itself can be captured several ways: a 24-hour recall (prompting hard for the snacks, sodas and “little bites” people routinely omit), a food-frequency questionnaire, a forward-looking food diary, or simply open-ended behavioral questions about typical intake. In an aesthetic visit, Dr. Tager leans on the conversational version — opening with something like “great nutrition plays a real role in how your skin looks,” then gently working toward how the patient would describe their diet, how their GI tract is working, and what they take for hair, skin and nails. The goal is to make the patient connect their plate to their skin, which sets up every recommendation that follows.

Don’t skip the medication review

One high-yield step is screening for drug-nutrient depletions. Many commonly prescribed drugs quietly erode nutrient status: proton-pump inhibitors and H2 blockers impair B12, folate, iron, calcium and magnesium absorption; antibiotics disrupt the microbiome and B-vitamin and mineral uptake; oral contraceptives lower several B vitamins, magnesium and zinc; diuretics increase urinary loss of potassium, magnesium and calcium; and statins deplete CoQ10. None of this means stopping the drug — it means anticipating the deficiency and managing it. The full depletion map and how to act on it is part of the course.

What to observe: deficiency signs in skin, hair & nails

The physical exam is where a nutrition lens pays off, because the integument is a visible readout of nutrient status. Rather than handing the patient a mirror and cataloguing what they dislike, take a closer look at the hair, eyes, skin, nails and tongue — these often reveal deficiencies before serum does. Dr. Tager points clinicians to Dr. Michael Stone’s chapter on the skin in the Integrative and Functional Medical Nutrition Therapy text for the depth behind these associations. A few illustrative patterns from the course:

• Hair & scalp: dry hair can suggest a need for more essential fatty acids; thinning hair or alopecia may track with zinc, iron or protein; dandruff associates with essential fatty acids and zinc; dull hair with protein or iron; premature greying with copper and B12.
• Tongue: a white-coated tongue is characteristic of Candida and opens a conversation about sugar and gut health; scalloping points toward B vitamins, iron, niacin and riboflavin; a geographic tongue can flag zinc, iron, folate, B6 or B12.
• Eyes: red rings can signal food intolerance or zinc and B-vitamin deficiency; white spots on the conjunctiva raise the question of vitamin A.
• Skin & nails: pale or flaky skin suggests iron, vitamin C or B vitamins; dry skin points to a fatty-acid imbalance; cracked nails and slow capillary refill at the fingertip can indicate iron deficiency; the constellation of acne with leukonychia (white spots) and keratosis pilaris can point toward inadequate zinc, vitamin A and essential fatty acids.

These are clues that prompt a question, not diagnoses on their own. They tell you where to look next — in the history, in the diet, or in a confirmatory lab.

What basic labs reveal

Before any boutique panel, remember how much personalized nutrition information lives in the standard blood work you can order from Quest or LabCorp. A CBC can shed light on B6 and B12 status from the size and shape of red blood cells. A basic CMP shows electrolytes and can point to dehydration. Ferritin and serum iron matter enormously — especially in menstruating women — for iron stores, anemia, and one common driver of hair loss. An RBC magnesium level is often more accurate than serum magnesium, because the body shifts magnesium out of cells to defend the serum number; an RBC zinc reflects status over three to four months.

Several more workhorses round out the base layer: hemoglobin A1c as an entry point to discuss glycation, sugar and insulin resistance; hs-CRP for inflammation; homocysteine as a window onto B-vitamin status; and 25-hydroxy vitamin D, given how widespread low vitamin D status is. Where a beauty-from-within picture calls for it, a thyroid panel (TSH, free T3, free T4, TPO antibodies) and sex hormones are reasonable next steps — the patient with hair loss is a classic example. These hormone questions connect to the cluster page on hormones and skin nutrition and, for deeper endocrine work-up, to hormone replacement therapy.

Specialty labs I: food allergy, sensitivity & the IgG caveat

Food reactions are common — roughly a quarter to a third of people report reacting to some food — but the categories matter enormously, and this is where clinicians most often overclaim. A true food allergy is an IgE-mediated reaction: rapid, mast-cell driven, sometimes anaphylactic, and the province of an allergist or immunologist to diagnose and manage. That is fundamentally different from a delayed food intolerance or reaction, which can be enzymatic (lactose, fructose, histamine) or otherwise non-immune, and tends to show up hours to days later as bloating, fatigue, headache, brain fog or skin flares.

Here the evidence-honest point has to be stated plainly: IgG “food sensitivity” panels are not validated to diagnose food allergy and are not endorsed by major allergy bodies such as the AAAAI. A practical tell is that IgG panels frequently come back lit up across many foods at once — which usually signals increased intestinal permeability (“leaky gut”) rather than genuine reactivity to each food, so the right move is a gut protocol before chasing any single result. For that reason the elimination diet remains the practical gold standard: remove the likely offenders, then reintroduce them one at a time while watching for symptoms. The mechanics are covered in our companion guide on food sensitivities and elimination diets, and the gut-permeability picture connects to the broader gut health resource center.

Specialty labs II: GI & microbiome stool testing

For practices ready to go deeper, stool-based microbiome testing can add real value — particularly given the microbiome’s emerging role in skin conditions like acne, rosacea, atopic dermatitis and eczema. Whole-genome-sequencing stool panels profile microbial diversity and richness, the presence of pathogens, and the “keystone” commensal species doing the heavy lifting for health. Just as useful, they estimate whether the gut is producing helpful metabolites such as butyrate, and they characterize dysbiosis — the imbalance in which unhelpful organisms outnumber the beneficial ones — so recommendations can be targeted rather than generic.

A related family of tests, organic-acid and metabolomic urine panels, looks downstream at the metabolic end-products the body excretes, offering clues to macronutrient processing, mitochondrial function, micronutrient status, detoxification and microbial activity. These are genuinely informative, but interpreting them well requires more biochemistry and more dedicated patient time — they are not plug-and-play. Whether stool or organic-acid testing belongs in your practice depends on how comprehensive a wellness model you are building; the actual ordering and interpretation framework is taught in the course.

Specialty labs III: nutrigenomic testing

Nutritional genomics has two halves: nutrigenetics (how genetic variation shapes our response to nutrients) and nutrigenomics (how nutrients influence gene expression), both sitting inside the larger field of epigenetics. The practical unit is the SNP — a single-nucleotide polymorphism. We each carry millions of them; most are inconsequential, but some alter how we absorb, transport or metabolize a given nutrient, which is why two people eating the identical diet can end up with different nutrient needs. Caffeine metabolism is the everyday example; vitamin D activation and transport (via genes such as CYP2R1 and the vitamin-D binding protein) is a more clinically actionable one.

The critical framing — and the one most easily lost in marketing — is that these skin and nutrition gene tests are predictive, not diagnostic. A SNP is a predisposition, not a verdict. Because epigenetics means lifestyle can turn genes up or down, a patient with variants that favor collagen breakdown is not doomed; they simply need to be more diligent with sun protection, collagen substrate and the minerals collagen synthesis requires. Dr. Tager uses a GPS analogy: a SNP is a road closure your body can often route around. Used this way, nutrigenomics individualizes care and motivates behavior change — used carelessly, it breeds genetic fatalism. The personalization logic is expanded in our guide to nutrigenomics and personalized nutrition, and the downstream supplement decisions connect to micronutrients, vitamins and minerals.

Choosing tests wisely — and knowing the red flags

It is easy to feel overwhelmed by the menu of laboratory-developed tests, and easy to over-rely on them. Because most specialty panels are expensive and not reimbursed by insurance, Dr. Tager runs every order through a short mental checklist first. Ask whether the test will: identify a suboptimal, easily correctable nutrient; clarify a predisposition to disease; better predict the clinical course; guide a specific treatment recommendation; or improve counseling and compliance. If a test answers none of those, it is fishing — and you should not order it.

Two more guardrails matter. First, direct-to-consumer testing is proliferating; it can be empowering, but there is no substitute for professional interpretation, so ask in the history whether a patient has done DTC testing and needs help reading it. Second, and most important, nutrition supports but does not replace medical work-up. Genuine red flags — unintended weight loss, difficulty swallowing, GI bleeding, or severe deficiency signs — warrant medical evaluation, not a supplement plan. And because supplements can interact with medications (for example, with anticoagulants), the medication review is as much a safety step as a diagnostic one.

The synthesis Dr. Tager returns to is reassuring: start simple. A great history, a careful look, and a handful of basic labs form the foundation. Layer one specialty test at a time, only when it answers a real question, and you build a precision plan that is both honest and actionable.