Metabolic syndrome is a shorthand for the unfavorable changes that result from eating too much, exercising too little, and packing on the pounds of visceral fat. These lifestyle choices alter the operation of your biology for the worse: in most people they will cut short life expectancy, boost chronic inflammation, and raise the risk of suffering all of the common disabling and fatal age-related conditions, such as dementia, diabetes, heart disease, cancer, and so forth. If you let things rust, don't be surprised when they fail and fall apart more readily.
Degeneration of the mind is perhaps the worst consequence of a lifestyle of fat and indolence. Metabolic syndrome brings with it fat-induced chronic inflammation that significantly increases the risk of suffering a range of neurodegenerative conditions, such as vascular dementia - destruction occurring in the brain as a result of damage and dysfunction in small blood vessels. Here is a paper from the Italian Longitudinal Study on Aging to back up that claim with data:
We investigated the relationship of metabolic syndrome (MetS) and its individual components with incident dementia in a prospective population-based study with a 3.5-year follow-up. ... A total of 2,097 participants from a sample of 5,632 65-84 year old subjects from the Italian Longitudinal Study on Aging were evaluated. MetS was defined according to the Third Adults Treatment Panel of the National Cholesterol Education Program criteria. Dementia, Alzheimer's disease (AD), and vascular dementia (VaD) were classified using current published criteria.
MetS subjects compared with those without MetS had an elevated risk of VaD ... Moreover, those with MetS and high inflammation had a still further higher risk of VaD [compared] with those without MetS and high inflammation. On the other hand, those with MetS and low inflammation compared with those without MetS and low inflammation did not exhibit a significant increased risk of VaD.
It is interesting to find a population with metabolic syndrome and low levels of inflammation: I would venture a guess that those people have a more active lifestyle rather than being blessed with better genes. It does indicate that the real bugbear behind the curtain is inflammation - but the existence of fortunate people doesn't make it smart for the rest of us to pile on the pounds assuming we'll be amongst the lucky few who retain lower risk profiles and lower levels of inflammation.
Solfrizzi V, Scafato E, Capurso C, D'Introno A, Colacicco AM, Frisardi V, Vendemiale G, Baldereschi M, Crepaldi G, Di Carlo A, Galluzzo L, Gandin C, Inzitari D, Maggi S, Capurso A, & Panza F (2009). Metabolic Syndrome and the Risk of Vascular Dementia. The Italian Longitudinal Study on Aging. Journal of neurology, neurosurgery, and psychiatry PMID: 19965842