nicotinamide N-methyltransferase (NNMT) levels increase with age in the body
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Yes, nicotinamide N-methyltransferase (NNMT) levels and activity increase with age in the body, particularly in tissues such as skeletal muscle, liver, and adipose tissue
. This age-related increase is associated with several metabolic changes that are considered key factors in the aging process and age-related diseases. 
NNMT's function and its role in aging
NNMT is an enzyme that regulates metabolism by using S-adenosylmethionine (SAM) to methylate nicotinamide (NAM), a precursor for the coenzyme nicotinamide adenine dinucleotide (NAD+). As NNMT activity increases with age, it disrupts key metabolic pathways in the following ways: 

    Depletes NAD+: The methylation process consumes NAM, which reduces the amount available for the synthesis of NAD+. Lower NAD+ levels are linked to cellular aging and impaired mitochondrial function.
    Increases homocysteine: The NNMT enzymatic reaction also produces S-adenosyl-L-homocysteine (SAH), which is then converted into homocysteine (Hcy). Elevated homocysteine levels are a risk factor for age-related conditions like cardiovascular and neurodegenerative diseases.
    Contributes to sarcopenia: High NNMT expression is a key factor in the metabolic dysfunction of aging skeletal muscle. Studies in aged mice have shown that inhibiting NNMT can improve muscle regeneration and function. 

NNMT's connection to age-related diseases
The increase in NNMT levels and activity over time is associated with the onset and progression of several age-related conditions, including: 

    Diabetes: Elevated NNMT levels have been observed in the fat and liver tissues of diabetic animal models. Inhibiting NNMT has been shown to protect against diet-induced obesity and insulin resistance.
    Cardiovascular disease: NNMT is linked to conditions such as hypertension and high cholesterol, and its inhibition is being investigated as a therapeutic strategy.
    Neurodegenerative disorders: Increased NNMT expression has been found in the neurons of patients with Alzheimer's disease. NNMT's effect on cellular metabolism and the promotion of autophagy also links it to Parkinson's disease