General Lack of Correlations between Age and Signs of the Metabolic Syndrome in Subjects with Non-diabetic Fasting Glucose Values

Circulating ALT Levels in Healthy Volunteers Over Life-Span: Assessing Aging Paradox and Nutritional Implications

Background: Two facts are generally recognized: (1) development of nonalcoholic fatty liver disease (NAFLD) is consistently linked to insulin resistance which has dietary implications and (2) circulating alanine aminotransferase (ALT) levels are reasonable markers predicting NAFLD status. In a recent cross-sectional study employing nondiabetic subjects, ALT values rose steadily within a normal range early in the life cycle but begin decreasing steadily around age 65 years.Objectives: Because of important nutritional implications, the association between ALT levels and aging in a significantly larger population of healthy volunteers was examined for corroborative purposes. A secondary goal was to gain further knowledge concerning mechanisms behind any age-related decline in ALT activity.Methods: Baseline data from over 10,000 physician-approved, nondiabetic subjects (age 21-84 years) of both genders who had volunteered for previous clinical investigations were assessed.Results: In this cross-sectional examination, the line of best fit (weighted) for average yearly circulating ALT levels displayed an upward surge from ages 21 to 64 years with a discernible steady downward decline around 65 years-mimicking earlier results. Examining linear lines of correlation in the younger and older age groups, the following calculations were determined: a significant positive slope for 21 to 64 years, r = 0.42, n = 44, p < 0.005, and a trending negative slope for 65 and beyond, r = -0.43, n = 20, p < 0.057. Using this same datum base, the correlations between age and fasting blood glucose (FBG) mimicked the ALT results by once more showing a similar upward rise in the younger and a steady decline the older group of volunteers.Conclusions: A paradoxical downward age-related (≥ 65 years) decline of circulating ALT coinciding with a comparable steady decrease in FBG levels was replicated in a larger population of volunteers. The close association of these two chemistries along with other findings suggest that altered glucose-insulin metabolism may participate via "survivor bias" in the ubiquitously found age-related decline of serum ALT-suggesting that nutritional measures could advance optimal health over the life-span.

Reflecting on Concepts Relevant to Contemplating the Relationships Between Glucose-Insulin Perturbations, Nutrition, and Aging

Background: Medical professionals attribute a crucial role in the development of several age-related, chronic health maladies to glucose-insulin perturbations - particularly, discernible insulin resistance (IR). However, little information is available concerning the degree to which relatively minor changes in IR participate over time in overall aging population, e.g., when circulating glucose and/or insulin concentrations are consistently within the higher end of normally accepted ranges. Objective: Obtaining precise knowledge is important, because proper nutritional management has the distinct capability of ameliorating the consequences. Methods: Five key concepts are suggested to underpin current thinking as to the applicable mechanisms and these are (1) the practical use of fasting blood glucose (FBG) levels as an estimate of IR, (2) the potential role of even slightly aberrant insulin regulation over time in the aging process, (3) the implementation of "continuum of risks" rather than checkpoints in considering prevention, (4) the presence and meaning of an aging paradox discovered in a recent study, and (5) the importance nutritional considerations in the "deadly triangle" as key factors in aging. Conclusions: Maintaining relatively low levels of FBG representing IR during aging via nutritional means has the potential to deliver a longer, more healthful lifespan as well as ameliorate many adverse manifestations of aging.

Examining the Relationship Between Nonalcoholic Fatty Liver Disease and the Metabolic Syndrome in Nondiabetic Subjects

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) is considered by some to be the hepatic manifestation of the metabolic syndrome (MS). However, others believe NAFLD is a distinct entity that actually initiates MS. Whichever is true, a definite linkage exists between both is generally accepted based upon the frequency of common occurrence and realization that insulin resistance (IR) is and realization that. The objective is to better understand the relationship between NAFLD and MS. Specifically, is there any concrete evidence that development of NAFLD precedes MS or vice versa? Another goal was to better comprehend capabilities of circulating aminotransferases (aspartate aminotransferase [AST] and alanine aminotransferase [ALT]) and their ratio used commonly for diagnosis of NAFLD.

METHODS

Data from 288 participants with fasting blood glucose (FBG) levels below the diabetic level (<125 mg/dL) and AST and ALT values in the normal range (<40 U/L) were examined. Correlations between ALT and AST and their ratio as independent variables with a variety of metabolic parameter were evaluated and compared.

RESULTS

Like FBG, many significant positive correlations among glucose-insulin indices, body composition, blood pressure, dyslipidemias, and inflammation were discovered using ALT, and less so with AST, as the independent variable. In some cases, even stronger correlations in a negative direction with IR and MS were found with the ratio AST/ALT. Corroboration occurred when values in the lowest and highest quartiles of ALT and AST/ALT readings showed appropriate statistically significant differences.

CONCLUSIONS

The findings here suggest that both NAFLD and the MS very early in development have a common inciting mechanism(s)-most likely IR. Accordingly, the early concurrent temporal results are consistent with the concept that NAFLD is a hepatic manifestation of the IR associated with the MS. They do not exclude the possibility that once some liver functional adjustments take place, several aspects of the MS are bolstered further, perhaps via intensified heightening of IR.

Longitudinal Examination of Links Between Risk Factors for the Metabolic Syndrome and Both Age and Fasting Glucose Levels in Nondiabetic Subjects

BACKGROUND

Previous evaluations in nondiabetic subjects revealed statistically significant correlations between fasting blood glucose (FBG) levels used as an estimate of insulin resistance and many components constituting the metabolic syndrome. Similar significant correlations were not found employing chronological age as the independent variable in the same nondiabetic individuals.

OBJECTIVE

The major purpose here was to replicate as well as corroborate the previous cross-sectional observations, emphasizing results obtained from data collected longitudinally.

METHODS

Material was assessed from 99 nondiabetic volunteers who had undergone 2 separate baseline measurements carried out over a minimum of 5 and up to a maximum of 20 years.

RESULTS

Findings from the starting baseline measurements mimicked many observations perceived in the earlier published cross-sectional material. The following correlations with elements constituting the metabolic syndrome using FBG as an independent variable were once more statistically significantly positive: scale weight, fat mass, circulating levels of triglycerides and high-sensitivity C-reactive protein (hsCRP). High-density lipoprotein (HDL) cholesterol was once again appropriately significant in a negative direction. In contrast, the same correlations were generally nonsignificant when age replaced FBG as the independent variable. Examining the 2 data sets over the 5-20-year intervals, FBG increased statistically significantly over time. However, the average increase clinically was relatively minor: -92.1 mg/dL ± 1.1 (SEM) vs 95.1 mg/dL ± 1.1 (SEM), p < 0.007. When the actual changes (delta) in the dependent parameters were correlated with the individual passages of time (intervals in years), only downward changes in aspartate aminotransferase (AST) levels were statistically significant. Fat-free mass showed a trend downward, whereas fat mass, trunk fat, and triglycerides merely demonstrated trends upward.

CONCLUSION

Current findings gathered over years are consistent with the original hypothesis that maintaining relatively low, stable circulating glucose levels during aging retards the development and intensity of many common manifestations of the metabolic syndrome.