Reduced plasma glycine concentration in healthy and chronically diseased older adults: a marker of visceral adiposity?

Background: Previous studies showed that a reduced plasma concentration of the amino acid glycine (Gly) is associated with intra-abdominal obesity, but the mechanism remains unclear. The study aimed to investigate whether lower Gly concentrations in older adults are independently associated with (visceral) adiposity, age, sex, presence of chronic disease, and glucose intolerance, and whether they are caused by a reduced whole-body production (WBP) and/or increased Gly disposal capacity.

Methods: We studied 102 older adults (47 males/55 females, 68.5 (SD 6.4) y) without comorbidities and 125 older adults with Chronic Obstructive Pulmonary Disease (COPD) (58 males/67 females, 69.7 (SD 8.6) yo). We assessed body composition and visceral adipose tissue (VAT) by dual-energy X-ray Absorptiometry, and muscle function by dynamometry. We measured postabsorptive plasma amino acid profile and glucose, followed by pulse administration of stable isotope labeled Gly ([2,2-²H₂]), and blood sampling was done to measure the WBP of Gly. Results were expressed as means [95% CI].

Results: We found a lower plasma Gly concentration in healthy and COPD males than in females (Healthy: 211 [193,230] vs 248 [225,271]; COPD: 200 [186,215] vs 262 [241,283];p<0.0001, respectively), with no difference between healthy and COPD groups. A negative relationship was found between unadjusted plasma Gly and VAT (R²:0.16, Slope -1.7 [-2.4,-1.2];p<0.0021), but not with total body fat or fasting glucose. The strong association between lower plasma Gly and increased VAT in older adults was independent of age, sex, and body weight lean mass or BMI), and the presence of COPD. Inclusion of these covariates increased the R² to 0.783. We found no relation between the VAT and WBP of glycine (p=0.35) or Gly clearance (p=0.187) when lean mass was taken into consideration.

Conclusions: Reduced plasma Gly in older adults can be viewed as a marker of visceral adiposity, independent of sex, age, body composition, presence of chronic disease, and whole body glycine production or clearance.

Clinical trial registry: In the present paper, we report the analysis of the baseline measurements from the following studies registered on ClinicalTrials.gov.