Weight Loss with Tyrosinaemia Type 2 in South Africa

Tyrosinaemia Type 2, also called Richner-Hanhart syndrome, is a rare inherited metabolic disorder caused by mutations in the TAT gene. Unlike Tyrosinaemia Type 1 — which primarily attacks the liver — Type 2 is defined by extremely high tyrosine levels in the blood and tissues, causing painful crystal deposits in the corneas of the eyes, thickening of the skin on the palms and soles, and in some patients, intellectual disability. The dietary treatment — a combined restriction of both phenylalanine and tyrosine — is strict and lifelong. If you have Tyrosinaemia Type 2 and want to manage your weight safely, this guide explains the biochemistry, the dietary rules, and how to achieve a caloric deficit without disrupting the careful amino acid balance your metabolic team has established.

What Is Tyrosinaemia Type 2?

Tyrosine is an amino acid synthesised in the body from phenylalanine. It is a precursor to several critical molecules — dopamine, adrenaline, noradrenaline, thyroid hormones, and melanin. Under normal circumstances, excess tyrosine is broken down via a multi-step pathway. The first step in tyrosine catabolism is carried out by the enzyme tyrosine aminotransferase (TAT), encoded by the TAT gene on chromosome 16q22.2.

In Tyrosinaemia Type 2, mutations in both copies of the TAT gene produce a non-functional or severely reduced TAT enzyme. Without this first catabolic step, tyrosine cannot be broken down efficiently. Blood tyrosine concentrations rise dramatically — typically to 600–2000 micromol/L, compared with the normal range of roughly 40–100 micromol/L. At these concentrations, tyrosine precipitates out of solution and forms needle-shaped crystals in tissues that are rich in tyrosine:

• Corneal epithelium — causing painful dendritic (branching) ulcers, photophobia, redness, and tearing. This can be confused with herpes simplex keratitis and is sometimes misdiagnosed for years before the correct metabolic diagnosis is made.
• Palmar and plantar skin — causing painful hyperkeratosis (thickening) on the palms of the hands and soles of the feet. The hyperkeratotic plaques may crack, bleed, and interfere with walking or fine motor tasks.
• The brain — in a subset of patients (roughly 60%), intellectual disability ranging from mild to moderate is present. The mechanism is not fully understood but is thought to involve tyrosine or its metabolites interfering with neurotransmitter synthesis and neural development. Prompt dietary treatment in infancy dramatically reduces the risk of neurocognitive impairment.

Critically, Tyrosinaemia Type 2 does not cause liver failure, liver cancer, or kidney tubular damage — the features that define Tyrosinaemia Type 1 (caused by FAH gene mutations). The two forms share the name "tyrosinaemia" because both produce elevated blood tyrosine, but the underlying enzymes, affected organs, and treatments differ substantially. Type 1 requires the drug nitisinone (NTBC) plus dietary restriction. Type 2 is managed by diet alone.

Why Both Phenylalanine and Tyrosine Must Be Restricted

In the body, phenylalanine is converted to tyrosine by the enzyme phenylalanine hydroxylase (PAH) — the same enzyme that is deficient in Phenylketonuria (PKU). In Tyrosinaemia Type 2, there is no problem with this conversion step. Phenylalanine is converted normally to tyrosine. However, because TAT is absent, tyrosine then accumulates.

This means that restricting only tyrosine is insufficient. If phenylalanine intake is unrestricted, the body will continue converting dietary phenylalanine into tyrosine endogenously, maintaining high tyrosine levels regardless of how much dietary tyrosine is avoided. Therefore, both amino acids must be simultaneously restricted:

• Dietary phenylalanine is reduced to limit the substrate supply for tyrosine synthesis.
• Dietary tyrosine is minimised to limit direct tyrosine loading.

The practical result is that almost all high-protein foods are restricted, because all natural proteins contain both phenylalanine and tyrosine. Meat, fish, poultry, dairy products, eggs, legumes, soya, and nuts are all limited or excluded. Patients are maintained on specialised phenylalanine-free and tyrosine-free amino acid formula to meet protein and essential amino acid requirements. Low-protein starchy foods — certain bread substitutes, pasta substitutes, and low-protein rice — provide carbohydrate and energy without amino acid loading.

Blood tyrosine and phenylalanine levels are monitored regularly (typically every 1–4 weeks during childhood, and 1–3 monthly in stable adults) to ensure the diet is sufficiently restrictive. The target is to bring blood tyrosine below 400 micromol/L, with most metabolic centres aiming for 200–350 micromol/L. On correct dietary management, corneal lesions typically resolve within weeks to months, and skin hyperkeratosis improves substantially over months to years.

The Weight Challenge in Tyrosinaemia Type 2

Patients with Tyrosinaemia Type 2 face several weight-related challenges:

• High carbohydrate load: Because natural protein is restricted, energy intake tends to shift toward carbohydrates and fats. Low-protein specialist foods are typically starchy and calorie-dense. Weight gain, particularly in the form of fat mass, is common in adult patients who are not carefully monitoring total caloric intake.
• Limited satiety from protein: Protein is the most satiating macronutrient. With natural protein severely restricted, hunger can be harder to control, particularly between meals. The amino acid formula provides amino acids but often has a different satiety profile compared with food-based protein.
• Metabolic formula as caloric source: Amino acid formula often contains significant energy. Patients who drink large volumes of formula above their metabolic requirements can inadvertently over-consume calories.
• Pain limiting exercise: Plantar hyperkeratosis makes walking and standing painful. Corneal pain causes light sensitivity. In patients with inadequately controlled disease, physical activity is limited by pain — reducing caloric expenditure.

Safe Weight Loss Strategies for Tyrosinaemia Type 2

Weight loss in Tyrosinaemia Type 2 must be coordinated with your metabolic dietitian. Any significant caloric restriction changes the relative proportions of macronutrients and affects how much amino acid formula is needed to meet protein requirements. Never attempt aggressive caloric restriction without professional guidance — undereating protein formula below your prescribed amount risks amino acid deficiency and metabolic imbalance.

Within those constraints, practical approaches include:

• Target a modest deficit of 300–500 kcal/day: This produces gradual, sustainable weight loss of approximately 0.3–0.5 kg per week without compromising amino acid formula intake. Aggressive restriction is not appropriate.
• Audit low-protein specialty foods: Low-protein pasta, bread substitutes, and flour alternatives are useful tools, but serving sizes are often larger than necessary. Portion control of these starchy foods is typically the most effective lever for reducing caloric intake without changing the amino acid formula protocol.
• Prioritise fat reduction over carbohydrate reduction: Because carbohydrates from low-protein foods serve as a key energy source, cutting fat intake (cooking oils, spreads, cream-based sauces) is generally a more metabolically neutral way to reduce calories than cutting carbohydrates further.
• Low-phenylalanine, low-tyrosine vegetables for volume: Most vegetables are low in protein and therefore low in phenylalanine and tyrosine, and can be eaten in generous quantities (within the allowed limits prescribed by your dietitian). Courgettes, green beans, cabbage, cucumber, lettuce, and similar low-protein vegetables add bulk and micronutrients with relatively few calories.
• Optimise formula timing for satiety: Spreading amino acid formula doses throughout the day — rather than consuming all at once — can improve satiety and reduce the temptation to snack on allowed starchy foods between meals.

Exercise and Tyrosinaemia Type 2

Unlike fatty acid oxidation disorders (such as VLCAD or CPT2 Deficiency), Tyrosinaemia Type 2 does not carry a risk of exercise-induced muscle breakdown or metabolic crisis during physical activity. The metabolic block is in amino acid catabolism, not in energy production pathways. Exercise is therefore beneficial and encouraged — provided it is physically tolerable given any existing corneal or plantar symptoms.

• On good dietary control (tyrosine well-controlled, skin and eyes manageable): Most forms of exercise are safe. Swimming is particularly well-suited because it is non-weight-bearing (eliminating plantar pressure), performed in low-light environments (reducing photophobia issues), and provides excellent cardiovascular and caloric expenditure benefits.
• During suboptimal control (active corneal lesions or painful plantar hyperkeratosis): Focus on seated or supine exercise — upper body resistance training with light weights or resistance bands, seated cycling on a stationary bike with good footwear cushioning, or water-based exercise. Avoid high-impact activities (running, jumping) until skin lesions are better managed.
• Consistency matters more than intensity: Because physical pain may limit intense exercise, building a habit of moderate daily movement — 30–45 minutes of walking, swimming, or cycling — produces steady caloric expenditure and metabolic benefits without triggering flares.

Monitoring and Red Flags

Regular monitoring in Tyrosinaemia Type 2 includes blood tyrosine and phenylalanine levels, ophthalmology reviews for corneal status, and periodic nutritional assessment (micronutrients, bone density — low-protein diets can carry risks for calcium, zinc, and vitamin D status).

Contact your metabolic team promptly if:

• Eye pain, redness, or photophobia worsens — this may indicate corneal lesion recurrence from suboptimal dietary control.
• Plantar pain increases significantly — poorly controlled tyrosine causes worsening hyperkeratosis.
• You are losing weight rapidly (more than 1 kg per week) — rapid weight loss can cause muscle protein breakdown, releasing phenylalanine and tyrosine endogenously and disrupting biochemical control even on a restricted diet.
• You are pregnant or planning pregnancy — pregnancy markedly increases tyrosine requirements, and dietary management must be adjusted urgently.

South African Context

Tyrosinaemia Type 2 is rare in South Africa but has been reported across ethnic groups. Diagnosis is typically made via amino acid analysis (elevated plasma tyrosine on newborn screening or clinical investigation) and confirmed by molecular testing of the TAT gene. Metabolic dietetic support is available at major academic hospitals in Gauteng, the Western Cape, and KwaZulu-Natal — ask your metabolic physician for a referral if you are not already working with a metabolic dietitian.

Low-protein specialty foods (low-protein pasta, bread mixes, flour) are available through some pharmacies and online suppliers in South Africa, though they are expensive and not always covered by medical aid. Discuss with your metabolic team whether your medical aid scheme has a metabolic PKU/amino acidopathy dietary supplement benefit — some schemes that cover PKU formula also cover Tyrosinaemia Type 2 management.

Key Takeaways

• Tyrosinaemia Type 2 (Richner-Hanhart) is caused by TAT gene mutations — blocking the first step of tyrosine catabolism.
• Both phenylalanine AND tyrosine must be restricted — dietary phenylalanine converts to tyrosine endogenously.
• Diet is managed with amino acid formula (phe-free, tyr-free) plus low-protein specialty foods.
• Weight gain is common due to the high carbohydrate content of low-protein foods; a 300–500 kcal/day deficit is safe and effective.
• Exercise is not contraindicated — swimming is ideal when corneal or plantar symptoms are present.
• Rapid weight loss can destabilise biochemical control — gradual is always safer.
• Always involve your metabolic dietitian before making dietary changes.

This article is for educational purposes only and does not replace advice from your metabolic physician or dietitian. Dietary management of Tyrosinaemia Type 2 requires individual prescription based on regular blood monitoring.