Implications of protein malnutrition and inflammatory disorders in the pathophysiology of Alzheimer's disease

Lean body mass (LBM) agglomerates the bulk of nitrogen (N)-containing molecules following well-identified age and sex evolutionary patterns best appraised in clinical practice using the serial measurement of plasma transthyretin (TTR). Methionine (Met), the sole essential amino acid bearing a sulfur (S) atom, presides at the initiation of protein synthesis while maintaining stable body tissue S:N molar ratios of approximately 1:14.5. In protein- depleted states, N- and Met-deficiencies operate as limiting factors for LBM protein synthesis and accretion, causing growth retardation and subnormal TTR plasma values. In inflammatory disorders, LBM is subjected to cytokine-induced tissue breakdown reflecting the S:N ratio found in healthy tissues whereas the liver secretion of TTR declines in proportion. Both malnutrition and inflammation are characterized by stepwise LBM downsizing and reduced bioavailability of Met body stores setting in motion molecular mechanisms safeguarding Met homeostasis at the expense of augmented homocysteine (Hcy) values in biological fluids. Divergent TTR and Hcy alterations indicate that rising Hcy values measured in plasma and cerebrospinal fluid should be regarded as the dark side of efficient compensatory processes. As a result, the neuroprotective activities normally exerted by TTR are weakened, whereas the oxidative burden generated by supranormal Hcy concentrations are strengthened. The combination of protein malnutrition and inflammatory disorders of any cause maximizes the risk of incurable neurodegenerative effects.