diff --git a/src/app/api/nutrients/daily/route.ts b/src/app/api/nutrients/daily/route.ts
index dd0bfd4c6..382200b4b 100644
--- a/src/app/api/nutrients/daily/route.ts
+++ b/src/app/api/nutrients/daily/route.ts
@@ -22,6 +22,7 @@ import { prisma } from "@/lib/db";
 import { apiHandler, requireAuth } from "@/lib/api-handler";
 import { apiSuccess, returnAllZodIssues } from "@/lib/api-response";
 import { annotate } from "@/lib/logging/context";
+import { binaryReferenceSex } from "@/lib/profile/sex";
 import { requireModuleEnabled } from "@/lib/modules/gate";
 import {
   NUTRIENT_CATALOG,
@@ -72,8 +73,7 @@ export const GET = apiHandler(async (request: NextRequest) => {
     daySeries.push({ day: key, amount: sumByDay.get(key) ?? 0 });
   }
 
-  const sex =
-    user.gender === "MALE" || user.gender === "FEMALE" ? user.gender : null;
+  const sex = binaryReferenceSex(user.gender);
   const reference = resolveNutrientReference(nutrient, sex);
 
   annotate({
diff --git a/src/components/insights/nutrients/micronutrients-card.tsx b/src/components/insights/nutrients/micronutrients-card.tsx
index 7822d1811..998b15c91 100644
--- a/src/components/insights/nutrients/micronutrients-card.tsx
+++ b/src/components/insights/nutrients/micronutrients-card.tsx
@@ -12,6 +12,7 @@ import { useAuth } from "@/hooks/use-auth";
 import { useTranslations, useFormatters } from "@/lib/i18n/context";
 import { apiGet } from "@/lib/api/api-fetch";
 import { queryKeys } from "@/lib/query-keys";
+import { binaryReferenceSex } from "@/lib/profile/sex";
 import {
   NUTRIENT_CODES,
   isNutrientCode,
@@ -51,8 +52,7 @@ export function MicronutrientsCard() {
   const { t } = useTranslations();
   const fmt = useFormatters();
   const { user } = useAuth();
-  const sex =
-    user?.gender === "MALE" || user?.gender === "FEMALE" ? user.gender : null;
+  const sex = binaryReferenceSex(user?.gender);
 
   // A failed read left `data` undefined, `rows` empty, and the card rendered
   // its "no micronutrient data" EmptyState — the same output as an account that
diff --git a/src/lib/ai/insight-interpretation.ts b/src/lib/ai/insight-interpretation.ts
index 71ad3909e..1dbf420b0 100644
--- a/src/lib/ai/insight-interpretation.ts
+++ b/src/lib/ai/insight-interpretation.ts
@@ -27,6 +27,8 @@
  * The band position is computed HERE, server-side — never left to the model.
  */
 
+import { binaryReferenceSex, type BinaryReferenceSex } from "@/lib/profile/sex";
+
 /** How a band reads against health: the four-way valence the prose leans on. */
 export type BandValence = "favourable" | "neutral" | "caution" | "unfavourable";
 
@@ -54,7 +56,7 @@ export interface MetricInterpretation {
   /** Ascending bands when the metric is not sex-split. */
   bands?: InterpretationBand[];
   /** Sex-split bands; used only when the profile carries the sex. */
-  sexBands?: Record<"MALE" | "FEMALE", InterpretationBand[]>;
+  sexBands?: Record<BinaryReferenceSex, InterpretationBand[]>;
   /**
    * Optional honesty caveat surfaced in the prompt block — used where the KB
    * hedges the edges (consumer rating scales, proxy measurements).
@@ -300,7 +302,7 @@ const INTERPRETATION_REGISTRY: Record<string, MetricInterpretation> = {
  */
 export function resolveInterpretation(
   metricKey: string,
-  sex: "MALE" | "FEMALE" | null | undefined,
+  sex: BinaryReferenceSex | null | undefined,
 ): {
   unit: string;
   directionOfGood: DirectionOfGood;
@@ -312,8 +314,9 @@ export function resolveInterpretation(
   if (!entry) return null;
   let bands: InterpretationBand[] | undefined = entry.bands;
   if (entry.sexBands) {
-    if (sex !== "MALE" && sex !== "FEMALE") return null;
-    bands = entry.sexBands[sex];
+    const referenceSex = binaryReferenceSex(sex);
+    if (!referenceSex) return null;
+    bands = entry.sexBands[referenceSex];
   }
   if (!bands || bands.length === 0) return null;
   return {
@@ -398,7 +401,7 @@ export function classifyBandPosition(
  */
 export function interpretationBandEdges(
   metricKey: string,
-  sex?: "MALE" | "FEMALE" | null,
+  sex?: BinaryReferenceSex | null,
 ): number[] {
   const resolved = resolveInterpretation(metricKey, sex ?? null);
   if (!resolved) {
diff --git a/src/lib/analytics/effective-range.ts b/src/lib/analytics/effective-range.ts
index 2b78bcf06..f7be20816 100644
--- a/src/lib/analytics/effective-range.ts
+++ b/src/lib/analytics/effective-range.ts
@@ -15,6 +15,7 @@
  * consistently.
  */
 
+import { binaryReferenceSex } from "@/lib/profile/sex";
 import { getBpTargets } from "./bp-targets";
 import {
   getAgeFromDateOfBirth,
@@ -120,11 +121,6 @@ const GLUCOSE_DEFAULTS: Record<
   BEDTIME: { min: 90, max: 150 },
 };
 
-function normalizedGender(value: string | null): "MALE" | "FEMALE" | null {
-  if (value === "MALE" || value === "FEMALE") return value;
-  return null;
-}
-
 function toDate(value: Date | string | null): Date | null {
   if (!value) return null;
   return value instanceof Date ? value : new Date(value);
@@ -135,7 +131,7 @@ function defaultRange(
   profile: UserProfileForRange,
 ): TrafficRange | null {
   const age = getAgeFromDateOfBirth(profile.dateOfBirth ?? null);
-  const gender = normalizedGender(profile.gender ?? null);
+  const gender = binaryReferenceSex(profile.gender);
   const dateOfBirth = toDate(profile.dateOfBirth);
 
   switch (metric) {
diff --git a/src/lib/dashboard/snapshot.ts b/src/lib/dashboard/snapshot.ts
index 159ba501b..1dfd43431 100644
--- a/src/lib/dashboard/snapshot.ts
+++ b/src/lib/dashboard/snapshot.ts
@@ -101,7 +101,11 @@ import {
 import { getServerTranslator } from "@/lib/i18n/server-translator";
 import type { Locale } from "@/lib/i18n/config";
 import { getAgeFromDateOfBirth } from "@/lib/analytics/pulse-targets";
-import { toProfileSex, type ProfileSex } from "@/lib/profile/sex";
+import {
+  binaryReferenceSex,
+  toProfileSex,
+  type ProfileSex,
+} from "@/lib/profile/sex";
 
 /** Briefing freshness window — mirrors the 24 h TTL on the advisor cache. */
 const BRIEFING_TTL_MS = 24 * 60 * 60 * 1000;
@@ -1246,8 +1250,7 @@ export async function buildDashboardSnapshot(
   // never sinks the snapshot. The narrative block rides the warm phase's
   // healthScore; memory rides the briefing card. The locale defaults to English.
   const locale = options.locale ?? "en";
-  const narrativeSex: "MALE" | "FEMALE" | null =
-    user.gender === "MALE" || user.gender === "FEMALE" ? user.gender : null;
+  const narrativeSex = binaryReferenceSex(user.gender);
   const narrativeProfile = {
     ageYears: getAgeFromDateOfBirth(user.dateOfBirth),
     sex: narrativeSex,
diff --git a/src/lib/insights/metric-status.ts b/src/lib/insights/metric-status.ts
index 393563889..e1e085585 100644
--- a/src/lib/insights/metric-status.ts
+++ b/src/lib/insights/metric-status.ts
@@ -83,6 +83,7 @@ import {
 import { hashInsightSnapshot } from "@/lib/insights/snapshot-hash";
 import { returnTimeoutFallback } from "@/lib/insights/timeout-fallback";
 import { annotate } from "@/lib/logging/context";
+import { binaryReferenceSex } from "@/lib/profile/sex";
 
 export interface MetricStatusResult {
   hasProvider: boolean;
@@ -335,8 +336,7 @@ export async function generateMetricStatus(args: {
     : null;
 
   const ageYears = getAgeFromDateOfBirth(user?.dateOfBirth ?? null);
-  const sex =
-    user?.gender === "MALE" || user?.gender === "FEMALE" ? user.gender : null;
+  const sex = binaryReferenceSex(user?.gender);
 
   // v1.10.0 F1 — sharpen the coarse registry `normalRange` with the
   // age/sex reference-range enabler when one is available for this
diff --git a/src/lib/mcp/nutrients-read.ts b/src/lib/mcp/nutrients-read.ts
index 4c84f4010..6d4b457ac 100644
--- a/src/lib/mcp/nutrients-read.ts
+++ b/src/lib/mcp/nutrients-read.ts
@@ -20,6 +20,7 @@
  */
 import { prisma } from "@/lib/db";
 import { isModuleEnabled } from "@/lib/modules/gate";
+import { binaryReferenceSex, type BinaryReferenceSex } from "@/lib/profile/sex";
 import {
   NUTRIENT_CATALOG,
   NUTRIENT_CODES,
@@ -195,7 +196,7 @@ async function readOverview(
 async function readDaily(
   userId: string,
   userTz: string,
-  sex: "MALE" | "FEMALE" | null,
+  sex: BinaryReferenceSex | null,
   nutrient: NutrientCode,
   days: number,
 ): Promise<NutrientsDailyResult> {
@@ -268,9 +269,7 @@ export async function getNutrients(
     select: { timezone: true, gender: true },
   });
   const userTz = user?.timezone || DEFAULT_TIMEZONE;
-  const genderValue = user?.gender ?? null;
-  const sex =
-    genderValue === "MALE" || genderValue === "FEMALE" ? genderValue : null;
+  const sex = binaryReferenceSex(user?.gender);
 
   if (!args.nutrient) {
     const days = clampDays(args.days, OVERVIEW_DEFAULT_DAYS, OVERVIEW_MAX_DAYS);
diff --git a/src/lib/nutrients/catalog.ts b/src/lib/nutrients/catalog.ts
index dedf9dc0d..d69956529 100644
--- a/src/lib/nutrients/catalog.ts
+++ b/src/lib/nutrients/catalog.ts
@@ -36,6 +36,8 @@
  * its source string.
  */
 
+import type { BinaryReferenceSex } from "@/lib/profile/sex";
+
 export const NUTRIENT_CODES = [
   // 13 vitamins
   "vitamin_a",
@@ -331,7 +333,7 @@ export interface ResolvedNutrientReference {
  */
 export function resolveNutrientReference(
   code: NutrientCode,
-  sex: "MALE" | "FEMALE" | null,
+  sex: BinaryReferenceSex | null,
 ): ResolvedNutrientReference | null {
   const ref = NUTRIENT_CATALOG[code].reference;
   const value =
diff --git a/src/lib/profile/sex.ts b/src/lib/profile/sex.ts
index c9c9e96b0..b9b95ab3b 100644
--- a/src/lib/profile/sex.ts
+++ b/src/lib/profile/sex.ts
@@ -20,6 +20,13 @@
 /** Profile sex exactly as stored; `null` = the account gave no answer. */
 export type ProfileSex = "MALE" | "FEMALE" | "OTHER" | null;
 
+/**
+ * The two values a sex-split reference table actually has rows for. Named so
+ * a table's signature can say what it needs without re-listing the pair, and
+ * so the list has exactly one place to change.
+ */
+export type BinaryReferenceSex = "MALE" | "FEMALE";
+
 /**
  * Narrow a stored sex to the two values a sex-split reference table has
  * rows for. `OTHER` and "no answer" both resolve to `null`, and every
@@ -29,7 +36,7 @@ export type ProfileSex = "MALE" | "FEMALE" | "OTHER" | null;
  */
 export function binaryReferenceSex(
   sex: string | null | undefined,
-): "MALE" | "FEMALE" | null {
+): BinaryReferenceSex | null {
   return sex === "MALE" || sex === "FEMALE" ? sex : null;
 }