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Health condition

MCAD deficiency, picked up by newborn screening - and managed by avoiding fasting.

An inherited fatty acid oxidation disorder that used to be found too late. Today, the UK newborn blood spot programme identifies most affected babies before they ever become unwell.

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Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against BIMDG, NHS newborn screening standards and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK guidance including newborn blood spot screening, emergency regimens and specialist metabolic care.

Key facts

MCAD deficiency at a glance.

The essentials, in plain English - what it is, how it's found, and how it's managed in the UK today.

  • What it is

    An inherited fatty acid oxidation disorder - the ACADM gene mutation stops medium-chain fatty acids being broken down for energy, especially during fasting.

  • Inheritance

    Autosomal recessive - both parents carry one altered copy of ACADM, usually without symptoms themselves.

  • How it is found

    Part of the UK newborn blood spot screening programme - picked up via an acylcarnitine profile before most babies ever become unwell.

  • Key danger sign

    Hypoketotic hypoglycaemia - low blood glucose without the expected rise in ketones - during fasting or illness.

  • Foundation of care

    Avoiding prolonged fasting, with an individualised maximum fasting time set by a specialist metabolic dietitian.

  • Emergency plan

    A written sick day regimen and emergency letter so families and hospitals act fast when illness threatens a crisis.

Why this guide matters

A rare gene, a very ordinary daily routine.

MCAD deficiency sounds frightening on paper, but with screening and a clear fasting plan, most affected children live entirely normal lives. The three points below shape everything else on this page.

  • Screening changed everything

    The newborn blood spot programme finds most babies before a single crisis happens - this was not always the case.

  • Fasting is the trigger, not food

    MCAD deficiency is not about diet in general - it is specifically about how the body copes when fasting for too long.

  • Illness is the danger point

    Vomiting, poor feeding and infection are when crises happen - acting early with the emergency regimen prevents almost all of them.

How the diagnosis is made

From the heel-prick to a settled care plan.

The steps a UK metabolic team will normally follow, in order - so you know what to expect and why.

  1. 01

    Detecting

    Newborn blood spot screening

    Every baby in the UK has a heel-prick test around day five - the acylcarnitine profile looks for a raised octanoylcarnitine (C8), the signature of MCAD deficiency.

  2. 02

    Detecting

    Recall and specialist referral

    An abnormal screening result triggers urgent referral to a specialist commissioned metabolic paediatric centre, usually within days.

  3. 03

    Detecting

    Presenting unscreened or symptomatic

    Older children or those diagnosed abroad may present acutely - lethargy, vomiting, seizures or an encephalopathy-like illness after a fast or infection.

  4. 04

    Confirming

    Acylcarnitine profile and urine organic acids

    Confirms the biochemical pattern during an acute presentation, or repeats the newborn finding to confirm it was not transient.

  5. 05

    Confirming

    Genetic confirmation

    ACADM gene testing looks for pathogenic variants, most commonly the K304E mutation - carried out by specialist commissioned metabolic genetics.

  6. 06

    Building

    Family and sibling screening

    Genetic counselling explains the autosomal recessive pattern and offers testing to siblings and future pregnancies.

  7. 07

    Building

    MDT metabolic care set-up

    A specialist commissioned team - metabolic paediatrician, dietitian and family - builds the individualised fasting and emergency plan.

Typical timeline: screening result to a confirmed diagnosis and care plan within a few weeks.

Symptoms

What a crisis actually looks like.

Children with MCAD deficiency are usually completely well between episodes. The features below appear during prolonged fasting or intercurrent illness - and the last one is a genuine emergency.

  • Hypoketotic hypoglycaemia

    The defining biochemical feature - low blood glucose without the ketones the body should be making to compensate.

  • Lethargy and poor feeding

    Often the earliest sign during a fast or illness, before more serious features develop.

  • Vomiting and reduced intake

    A common trigger and a warning sign - illness that stops normal feeding can quickly precipitate a metabolic crisis.

  • Seizures

    Can occur as blood glucose falls further, reflecting the brain’s dependence on adequate fuel supply.

  • Encephalopathy-like presentation

    Reduced consciousness or confusion during a crisis - a medical emergency needing immediate assessment.

  • Hepatomegaly during crisis

    The liver may be enlarged while it struggles to meet the body’s energy demands through alternative pathways.

  • Well between crises

    Children with MCAD deficiency are typically entirely well when fed regularly - this is not a condition with constant symptoms.

  • Red flag - collapse or reduced consciousness

    Historically linked to sudden unexplained infant death before screening - any collapse during illness needs emergency IV dextrose without delay.

Treatment

How MCAD deficiency is managed in the UK.

There is no cure, but avoiding fasting and reacting quickly to illness prevents almost every crisis. Care follows British Inherited Metabolic Disease Group (BIMDG) guidance.

  • Avoid prolonged fasting

    The single most important measure - an individualised maximum fasting time by age, set and reviewed by a specialist metabolic dietitian.

  • Emergency regimen

    Frequent glucose polymer drinks during illness or reduced intake, following specialist commissioned sick day rules to prevent a fasting-driven crisis.

  • Acute crisis management

    Urgent IV dextrose in hospital if the child is unwell, vomiting or not taking fluids - families carry an emergency letter explaining this to any treating team.

  • L-carnitine supplementation

    Used selectively by specialist metabolic teams - the evidence base is mixed, and it is not given routinely to every child.

  • Regular meals and snacks

    Avoiding skipped meals and including a bedtime snack, guided by specialist dietetics, keeps energy supply steady day to day.

  • Vaccination uptake

    Kept fully up to date - infections are a common trigger for crises, so preventing illness matters more than usual.

  • Family education

    Specialist commissioned teams teach parents and carers to recognise early warning signs and act on the emergency regimen without hesitation.

  • Genetic counselling

    Specialist commissioned genetics support sibling testing and future family planning given the autosomal recessive inheritance.

What this guide is based on

The sources behind every claim on this page.

UK national screening standards and specialist metabolic society guidance, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your specialist metabolic team knows your child's history and can tell you which parts apply to them. If in doubt, get seen.

  • British Inherited Metabolic Disease Group (BIMDG). Management guidelines for medium-chain acyl-CoA dehydrogenase deficiency.

  • NHS Newborn Blood Spot Screening Programme. Standards and pathway for fatty acid oxidation disorders.

  • Public Health England / UK NSC. Newborn screening acylcarnitine profile guidance.

  • Metabolic Support UK. Family information on MCAD deficiency and emergency regimens.

Red flags

When MCAD deficiency needs urgent attention.

Day-to-day life with a fasting plan is usually calm. These are the situations that aren't - and where emergency care is needed without delay.

  • Collapse or reduced consciousness

    A medical emergency - call 999 or attend A&E immediately. Urgent IV dextrose is required without waiting for blood results.

  • Vomiting with reduced intake

    Any illness stopping normal feeding needs the emergency regimen started early - do not wait for symptoms to worsen before acting.

  • Prolonged unexplained fasting

    Missed feeds, long car journeys or illness-related sleep can tip a child into crisis - plan ahead and carry emergency supplies.

  • Seizure during illness

    Suggests significant hypoglycaemia - needs emergency assessment and glucose measurement straight away.

  • Emergency letter not recognised

    If a treating team is unfamiliar with the emergency letter, ask them to contact the specialist metabolic centre directly - do not delay treatment.

  • Missed newborn screening result

    Home births, early discharge or moving between areas can delay the heel-prick test - check with your health visitor if you are unsure it was done.

  • Sibling not yet tested

    Any full sibling of an affected child should be discussed with the metabolic genetics team, even if newborn screening appeared normal.

  • Pregnancy in a known carrier or affected adult

    Needs early specialist metabolic and genetic input to plan monitoring and reduce fasting risk.

  • Unexplained infant death in the family

    A history of sudden unexplained infant death in relatives should prompt discussion of fatty acid oxidation disorders with a geneticist.

Living with it

A manageable condition, with a clear routine.

Four things that make the biggest difference day to day - knowing the fasting limit, acting early on sick days, carrying the emergency letter and staying connected to support.

A quiet reminder

Preparation, not worry, is what keeps children safe.

Most families find that once the routine becomes habit, MCAD deficiency fades into the background of ordinary life.

  1. 01 Fasting

    Know the fasting limit

    Ask the metabolic team for the exact maximum fasting time for your child’s age and write it down somewhere visible.

  2. 02 Illness

    Act early on sick days

    Start the emergency regimen at the first sign of reduced feeding or vomiting - waiting is the most common cause of a crisis.

  3. 03 Carry it

    Keep the emergency letter close

    Take it to every hospital visit, GP appointment and when travelling - it tells any clinician exactly what to do.

  4. 04 Support

    Lean on specialist support

    Metabolic Support UK and your specialist dietitian are there for day-to-day questions, not just crises.

Frequently asked

Everything we get asked about MCAD deficiency.

Quick answers on screening, inheritance, fasting limits and emergency care.

  • What is MCAD deficiency?

    Medium-chain acyl-CoA dehydrogenase deficiency is an inherited disorder of fatty acid oxidation. A mutation in the ACADM gene means the body cannot break down medium-chain fatty acids for energy, which becomes a problem during fasting or illness when the body would normally rely on fat stores.

  • How is MCAD deficiency inherited?

    It is autosomal recessive. Both parents carry one altered copy of the ACADM gene, usually without any symptoms themselves, and each child has a one in four chance of inheriting both altered copies and being affected.

  • How is it picked up in the UK?

    Through the newborn blood spot screening programme. A heel-prick sample taken around day five is analysed for an acylcarnitine profile, and a raised octanoylcarnitine (C8) level is the characteristic finding that prompts urgent specialist referral.

  • What is hypoketotic hypoglycaemia and why does it matter?

    It is low blood glucose without the expected rise in ketones that would normally help the body cope. It is the key biochemical feature of a crisis in MCAD deficiency and, if untreated, can progress to seizures, coma or death - which is why emergency IV dextrose is given urgently in unwell children.

  • Has newborn screening changed outcomes?

    Significantly. Before screening, MCAD deficiency was sometimes only diagnosed after a sudden infant death. Identifying affected babies before they become symptomatic, and teaching families to avoid prolonged fasting and follow sick day rules, has prevented the vast majority of these crises.

  • Does my child need to avoid fasting forever?

    Yes, in the sense that an individualised safe fasting time - set by a specialist metabolic dietitian and reviewed as your child grows - should always be respected, particularly during illness. Most children lead entirely normal lives otherwise, as long as regular meals and the emergency regimen are followed.

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