Edited by: Xinhua Qu, Shanghai Ninth People’s Hospital, China
Reviewed by: Mario Ulises Pérez-Zepeda, Instituto Nacional de Geriatría, Mexico; Thomas W. Buford, University of Florida, USA; Giuseppe Calcagno, University of Molise, Italy
Specialty section: This article was submitted to Geriatric Medicine, a section of the journal Frontiers in Medicine
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Existing studies suggest that decreased branched-chain amino acid (BCAA) catabolism and thus elevated levels in blood are associated with metabolic disturbances. Based on such information, we have developed a hypothesis how BCAA degradation mechanistically connects to tricarboxylic acid cycle, intramyocellular lipid storage, and oxidation, thus allowing more efficient mitochondrial energy production from lipids as well as providing better metabolic health. We analyzed whether data from aged Finnish men are in line with our mechanistic hypothesis linking BCAA catabolism and metabolic disturbances.
Older Finnish men enriched with individuals having been athletes in young adulthood (
Out of the 593 participants, 59 had previously known type 2 diabetes, further 67 had screen-detected type 2 diabetes, 127 impaired glucose tolerance, and 125 impaired fasting glucose, while 214 had normal glucose regulation and one had missing glucose tolerance information. There were group differences in all of the BCAA concentrations (
The findings provided further support for our hypothesis by strengthening the idea that the efficiency of BCAA catabolism may be mechanistically involved in the regulation of fat oxidation, thus affecting the levels of metabolic disease risk factors.
High serum branched-chain amino acid (BCAA; isoleucine, leucine, and valine) concentrations have been shown to be predictors or markers of insulin resistance (
In sum, the existing metabolomic and transcriptomic studies suggest that decreased BCAA catabolism, and thus elevated levels in blood are associated with low physical activity, increased adiposity, and other risk factors for metabolic diseases. We have developed a mechanistic hypothesis (BCAA–FatOx hypothesis) that intertwines BCAA catabolism with the tricarboxylic acid (TCA) cycle and lipid metabolism allowing better lipid oxidation and better metabolic health (Figure
The participants were members of the cohort of male former Finnish elite athletes and their controls (
Of the surviving cohort members, 1183 were invited to a clinical study in 2008 (
Normal ( |
IGT or IFG ( |
T2D or ST2D ( |
||
---|---|---|---|---|
Mean ± SD | ||||
Age (years) | 71.6 ± 6.1 | 72.7 ± 5.9 | 72.9 ± 5.7 | 0.065 |
Height (cm) | 176 ± 7 | 175 ± 8* | 174 ± 8** | 0.010 |
Weight (kg) | 79.7 ± 10.8 | 81.6 ± 13.2 | 85.7 ± 16.6** | 0.004 |
Body mass index (kg/m2) | 25.6 ± 2.7 | 26.6 ± 3.5** | 28.2 ± 4.7*** | <0.001 |
Body fat (%) | 23.7 ± 4.9 | 25.3 ± 5.7** | 27.4 ± 6.4*** | <0.001 |
Leisure-time physical activity volume (MET-h/week) | 32.3 ± 29.7 | 28.3 ± 26.6* | 18.8 ± 19.3*** | <0.001 |
Alanine aminotransferase (U/L) | 23.7 ± 9.9 | 26.1 ± 11.2* | 30.7 ± 21.0** | 0.013 |
γ-glutamyltransferase (U/L) | 32.1 ± 19.4 | 36.5 ± 30.8 | 53.1 ± 56.2*** | <0.001 |
Glucose (mmol/L) | 4.31 ± 0.24 | 4.76 ± 0.32*** | 5.60 ± 1.42*** | <0.001 |
Serum total triglycerides (mmol/L) | 1.03 ± 0.33 | 1.09 ± 0.39 | 1.17 ± 0.59 | 0.064 |
Total fatty acids (mmol/L) | 9.91 ± 1.59 | 9.92 ± 1.81 | 10.03 ± 2.50 | 0.934 |
Total lipids in chylomicrons and extremely large VLDL (μmol/L) | 17 ± 13 | 19 ± 15 | 23 ± 34 | 0.168 |
Total lipids in very large VLDL (μmol/L) | 38 ± 36 | 45 ± 40* | 55 ± 74* | 0.029 |
Total lipids in large VLDL (μmol/L) | 162 ± 123 | 183 ± 141 | 214 ± 212 | 0.065 |
Mean diameter for VLDL particles (nm) | 35.96 ± 1.10 | 36.13 ± 1.11 | 36.32 ± 1.30** | 0.017 |
Mean diameter for HDL particles (nm) | 9.97 ± 0.24 | 9.94 ± 0.25 | 9.90 ± 0.24** | 0.007 |
Ratio of apolipoprotein B to apolipoprotein A-I | 0.565 ± 0.109 | 0.566 ± 0.118 | 0.557 ± 0.130 | 0.645 |
Glycoprotein acetyls, mainly α1-acid glycoprotein (mmol/L) | 1.39 ± 0.15 | 1.43 ± 0.16** | 1.48 ± 0.20*** | <0.001 |
Isoleucine (μmol/L) | 53 ± 10 | 55 ± 11 | 59 ± 15*** | 0.001 |
Leucine (μmol/L) | 78 ± 12 | 80 ± 14 | 87 ± 22*** | <0.001 |
Valine (μmol/L) | 179 ± 29 | 185 ± 29* | 196 ± 45*** | <0.001 |
The ethics committee of the Hospital District of Helsinki and Uusimaa approved the study, and all subjects have provided written informed consent.
Trained study nurses performed the physical examinations including assessment of body weight, height, body composition, and blood sampling in field survey laboratories around Finland.
Body composition was determined in light indoor clothing and without shoes and socks by a bioimpedance body composition device (InBody 3.0, Biospace, Seoul, South Korea): fat-free mass (FFM) to an accuracy of 0.1 kg and fat mass to an accuracy of 0.1 kg. Fat percent was calculated as fat mass divided by body weight and converted to percent. The body composition device measured body weight to an accuracy of 0.1 kg. If the participant had a pacemaker causing an exclusion from bioimpedance body composition analysis (
Venous blood samples were taken after 10 h fasting in a sitting position with a light stasis into a serum gel tube (Venosafe, Terumo Europe) for metabolomics, lipid, and liver enzyme assays into a serum tube (Venosafe) for insulin assay and into a fluoride–citrate tube (Venosafe) for glucose assay. The venous blood samples were centrifuged at the field survey sites. The plasma and sera were frozen immediately after separating and transferred in dry ice to the laboratory once a week for analyses. Lipids, alanine aminotransferase (Alat), aspartate aminotransferase (AST), γ-glutamyl transferase (Gt), insulin, and glucose measurements were performed on a clinical chemistry analyzer Architect ci8200 (Abbott Laboratories, Abbott Park, IL, USA) at the laboratory of National Institute for Health and Welfare (Helsinki, Finland). The following methods were used: enzymatic assay (Abbott Laboratories) for measuring serum total cholesterol, homogenous assay (Abbott Laboratories) for direct measurement of serum high density lipoprotein (HDL) cholesterol, enzymatic glycerol phosphate oxidase assay (Abbott Laboratories) for measuring serum triglycerides, International Federation of Clinical Chemistry (IFCC) method (Abbott Laboratories) for measuring ALT and AST, kinetic method (Abbott Laboratories) for measuring GT, chemiluminescent microparticle immunoassay (CMIA, Abbott Laboratories) for measuring insulin and enzymatic hexokinase assay (Abbott Laboratories) for plasma glucose. Low density lipoprotein (LDL) cholesterol was calculated by the Friedewald formula (
Participating men without a history of diabetes had a standard 2 h 75 g OGTT. A blood sample was drawn 2 h after the ingestion of the 300 mL solution, containing 75 g anhydrous glucose and 0.8 g citric acid. In this study, a definition of type 2 diabetes impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) was done according to WHO criteria from year 1999 (
Metabolic syndrome was defined according to criteria of the International Diabetes Federation: waist circumference >94 cm plus any two of the following factors: (i) TG >1.7 mmol/L or specific treatment for this, (ii) HDL <1.03 mmol/L or specific treatment for this, (iii) systolic BP ≥130 or diastolic BP ≥85 mm Hg or treatment of previously diagnosed hypertension, (iv) fasting plasma glucose ≥5.6 mmol/L or previously diagnosed type 2 diabetes (
Serum samples were kept at −80°C until analyzed with a NMR metabolomics platform, which provides quantitative information on lipoprotein subclass lipid distribution and particle concentrations, serum lipids including fatty acids, and low-molecular-weight metabolites such as amino acids (
Data have been reported as means ± SD. Group differences in BCAA levels were tested using Mann–Whitney
Out of the 593 participants, 59 had previously known type 2 diabetes, further 67 had screen-detected type 2 diabetes (ST2D), 127 IGT, and 125 IFG, while 214 had normal fasting glucose concentration and normal glucose tolerance (data missing for OGTT for one participant). Expectedly, the isoleucine, leucine, and valine concentration correlated highly with each other (
According to the International Diabetes Federation criteria, 323 participants had metabolic syndrome. Those participants with metabolic syndrome had higher BCAA concentrations compared to those without (
Occurrence of metabolic syndrome |
|||
---|---|---|---|
Yes ( |
No ( |
||
Mean (95% CI) | |||
Isoleucine, μmol/L | 58 (57–60) | 51 (50–53) | <0.001 |
Leucine, μmol/L | 85 (83–86) | 77 (75–79) | <0.001 |
Valine, μmol/L | 191 (188–195) | 178 (175–182) | <0.001 |
Our study is consistent with and extends the evidence for the hypothesis that BCAA catabolism and oxidative energy metabolism and metabolic disturbances are intertwined
In this study, we did not have diary-based food records to study how BCAA intake influences serum BCAA concentrations. Instead, we had food frequency questionnaire (
The use of BIA to measure body composition may be regarded as a limitation. BIA was used in order to provide comparable results between the study centers, since all centers did not have a possibility to use dual-energy x-ray absorptiometry (DEXA) that is considered one of the main methods to measure body composition. Another limitation is the cross-sectional nature of the study setup that does not allow causal interpretation. Furthermore, only males were included to the study so the results cannot be extrapolated to women.
Our results agree with earlier data showing that in rodents and humans with impaired glucose regulation, increased BCAA concentrations have been reported (
The findings are in line with our BCAA–FatOx hypothesis (
Interestingly, the associations of indicators of disturbed oxidative fat metabolism with valine were weaker than with isoleucine and leucine (see Table S1 in Supplementary Material). This may be due to the fact that valine is catabolized to succinyl-CoA, while degradation of isoleucine and leucine provides acetyl-CoA to the TCA cycle, which may be a more rate-limiting step.
We also calculated correlations with the indicators of impaired lipid metabolism and other serum amino-acid concentrations measured using the metabolomics platform (alanine, glutamine, glycine, histidine, phenylalanine, and tyrosine), but only three correlations showed a moderate correlation (
Of the BCAAs leucine and its catabolites, α-ketoisocaproic acid and β-hydroxy-β-methylbutyrate are known activators of mTOR pathway, thus regulating muscle protein synthesis (
Conception and design was carried out by UK and HK; all the authors were involved with the acquisition, analysis, and interpretation of data; UK and HK drafted the article, while critical revision of the manuscript was carried out by all authors. All the authors approved the final version of the paper to be published. UK is the guarantor of this work.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The study was funded by the Ministry of Education and Culture, the Juho Vainio Foundation, the Finnish Heart Research Foundation, Paavo Nurmi Foundation, the Finnish Cultural Foundation, the Academy of Finland (grants #265240 and 263278 to JK, and # 298875 to HK), and by a grant from Medical Society of Finland, Finska Läkaresällskapet.
The Supplementary Material for this article can be found online at