Optimal Result: 8 - 74 micromol/g creatinine.

Cysteine (FMV Urine) is part of a group of markers that indicates the balance and sufficiency of the methylation/sulfation pathway substrates and co-factors.

Cysteine is required for the formation of coenzyme A, proteins with cross-linked polypeptide chains (eg. insulin), metallothionein, and enzymes with active sulfhydryl (SH-) groups (eg. glutathione peroxidase, Na/ K ATPase). Cysteine is the rate limiting amino acid for the formation of intracellular glutathione, which is one of the most important endogenous antioxidants and detoxifying (metals and chemicals) molecules in the body. 

What does the FMV stand for?

The FMV is a single urine collection upon waking. It provides a time-average for hormone spikes that may occur during the hours of sleep (approximately 8-hours), and includes the early morning peak excretion expected in a number of hormones. As a result, the time-average of the overnight collection paired with capturing the peak excretion provides an estimate of both the Total Output and Peak Output of measured hormones.

Clinical Relevance:
- Glutathione precursor
- Sulfation (detoxification)
- High affinity for mercury, lead, cadmium
- Antioxidant properties
- Collagen production
- Taurine precursor
- Sulfur-containing amino acid
- Formed from serine and methionine (sulfur donor)

What does it mean if your Cysteine (FMV urine) result is too low?

Cysteine levels may be low for a variety of reasons.

(1) Chronic exposure to sulfhydryl reactive metals (eg. mercury, cadmium, arsenic), or toxic chemicals, 
(2) Oxidative stress or insufficient antioxidants (vitamins E and C), 
(3) Inflammation, 
(4) Methionine insufficiency or impaired methionine metabolism (inadequate folic acid, B-12, B-6, magnesium), 
(5) Cysteinurea or hypertaurinurea. 
(6) Diet: Imbalanced or inadequate protein intake
(7) Gastrointestinal dysfunction:
    - Hypochlorhydria; gastric or small intestine resections may decrease absorption 
    - Pancreatic insufficiency (lack of digestive enzymes)
    - Evaluate GI function with "Comprehensive Stool Analysis"
(8) Methylation pathway defects:
    - Acquired enzyme defects
        - Environmental toxicant exposures
            - Cysteine is the rate limiting amino acid in the biosynthesis of glutathione
                - Evaluate Glutathione status with RBC Glutathione
            - Evaluate exposure to toxic metals Urine Toxic Elements, Whole Blood Elements, Urine Porphyrins or Hair Elements
    - Nutritional cofactors – RBC Elements
    - Inherited enzyme defects in cystathionine beta-synthase (CBS) or cystathionine gamma-lyase (CTH) 
        - SNPs
            - Evaluate DNA Methylation Pathway for CBS SNPs
        - Mutations
            - Some homozygous CBS mutations (present in 4 out of 10,000 people) may be vitamin B6-resistant, and result in congenital cystathioninemia. Most patients with congenital cystathioninemia are asymptomatic, unless dietary assimilation of cysteine is compromised and cysteine deficiency results.

Interventions, based on the cause, which may improve cysteine levels, include improved nutrition and supplements:
- Dietary sources of cysteine include undenatured whey protein, legumes and eggs
- Supplement cysteine directly as N-acetyl cysteine (N-AC)
- Supplement CBS cofactors pyridoxal-5’-phosphate (vitamin B6/P5P) and serine
    - Evaluate serine status with urine or plasma Amino Acids 
    - Evaluate heme (cofactor) synthesis with Urine Porphyrins

Supplementation with N-acetyl cysteine may be beneficial except in cystinurea, intestinal candidiasis or insulin-dependent diabetes.

What does it mean if your Cysteine (FMV urine) result is too high?

Cysteine levels may be elevated by a high protein diet, supplements (N-AC) or from enzymatic deficiencies on the pathways that convert cysteine to glutathione or taurine.

High levels of cysteine in circulation are considered toxic and increase oxidative stress. Low levels of glutathione may also result in increased oxidative stress.

Cysteine levels may be normalized by:
- Decreasing dietary cysteine and N-AC supplementation
- Evaluate Glutathione status with RBC Glutathione
- Evaluate taurine status with Plasma Amino Acids
- Support cysteine metabolic pathway with pyridoxal-5’- phosphate (vitamin B6/P5P)
- Evaluate iron status

Cysteine levels may also be elevated due to a genetic defect in cysteine dioxygenase (CDO). CDO is the pivotal enzyme that initiates the biochemical pathway from cysteine to taurine or glutathione.

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