Abstract

   Type 2 diabetes mellitus (T2DM) is a common comorbidity among
   Parkinson’s disease (PD) patients. Yet, little is known about
   dysregulated pathways that are unique in PD patients with T2DM. We
   applied high-resolution metabolomic profiling in serum samples of 636
   PD and 253 non-PD participants recruited from Central California. We
   conducted an initial discovery metabolome-wide association and pathway
   enrichment analysis. After adjusting for multiple testing, in positive
   (or negative) ion mode, 30 (25) metabolic features were associated with
   T2DM in both PD and non-PD participants, 162 (108) only in PD
   participants, and 32 (7) only in non-PD participants. Pathway
   enrichment analysis identified 17 enriched pathways associated with
   T2DM in both the PD and non-PD participants, 26 pathways only in PD
   participants, and 5 pathways only in non-PD participants. Several amino
   acid, nucleic acids, and fatty acid metabolisms were associated with
   T2DM only in the PD patient group suggesting a possible link between PD
   and T2DM.

   Subject terms: Prognostic markers, Parkinson's disease

Introduction

   Parkinson’s disease (PD) is a chronic neurodegenerative disease
   affecting roughly 8 million people globally^[46]1. Comorbidities
   frequently occur in PD patients and at a rate higher than similar age
   controls, which complicates the management of the disease^[47]2–[48]4.
   This includes a variety of health conditions, including type 2 diabetes
   mellitus (T2DM)^[49]2,[50]5. PD patients with comorbidities are an
   especially vulnerable subpopulation among all PD patients, as these
   patients often have worse prognosis, reduced quality of life, and
   increased medical costs^[51]5–[52]7.

   Previous research has indicated that T2DM, characterized as insulin
   resistance that results in impairment of glucose regulation and
   metabolism, may contribute to the onset of neurodegenerative diseases,
   such as PD, and also influence the progression of these
   conditions^[53]8,[54]9. Research points toward dysregulation of shared
   pathophysiologic pathways between T2DM and PD, including insulin
   resistance, mitochondrial dysfunction, inflammation, or metabolic
   dysregulation^[55]9,[56]10. Few epidemiologic studies, however, have
   analyzed metabolic profiles associated with T2DM among PD patients to
   help identify physiologic responses that are similar or different in
   those suffering from both medical conditions. To date, no published
   study has investigated metabolic features associated with T2DM in PD
   patients using untargeted metabolomics.

   High-resolution metabolomics (HRM)^[57]11 has emerged as a useful tool
   that can profile thousands of small molecules produced from metabolism
   (metabolites) in different biospecimens. Previous research has
   described metabolomic profiles in PD^[58]12–[59]14, including our
   own^[60]15, and T2DM^[61]16 separately. Two previous studies including
   both targeted and untargeted metabolomics found that lower levels of
   low-density lipoprotein cholesterol, higher level of fibrinogen, and
   lipid metabolic dysregulation were associated with a higher risk of
   dementia in PD patients with T2DM than without T2DM^[62]17,[63]18. The
   current study builds on this previous work, specifically assessing the
   metabolic profile of T2DM among PD patients. We applied untargeted HRM
   to serum samples of 636 PD patients of whom 96 also suffered from T2DM
   and 253 older adults without PD, among whom 36 were diagnosed with
   T2DM. The goal of the study is to describe shared and unique metabolic
   features in PD patients with and without T2DM and shed light on unique
   dysregulated metabolic pathways in PD with T2DM. Given the aging of
   populations and the projected increase of prevalence in both PD^[64]19
   and T2DM^[65]20, understanding the links between PD and T2DM is a
   public health priority^[66]21,[67]22.

Results

Study population

   As shown in Table [68]1, 15% of the 636 PD patients had T2DM and the
   mean age of PD diagnosis was similar in PD patients with T2DM (68 y,
   SD = ±10 y) and without T2DM (67 y, SD = ±10 y). PD patients with and
   without T2DM were also similar in terms of the percentages of men,
   people with ≥12 years of education, and have never smoked. However,
   more of the PD patients with T2DM reported non-European ancestry than
   those without T2DM. Race and ethnicity of the study population is shown
   in Supplementary Table [69]1. In total, 253 non-PD participants were
   included in the analysis, with 36 (14%) having T2DM. The patterns of
   age, lifestyle, and ethnicity were also similar to T2DM (see Table
   [70]1).

Table 1.

   Parkinson’s disease (PD) patients’ and non-PD participants’
   characteristics by diabetes among those with metabolomics data
   Characteristic Statistics PD Patients (N = 636) Non-PD participants
   (N = 253)
   Diabetes (N = 96) No diabetes (N = 540) Diabetes (N = 36) No diabetes
   (N = 217)
   Age^a Mean (SD) 68.03 (9.83) 66.62 (10.44) 68.92 (11.47) 65.39 (12.93)
   Male gender n (%) 65 (68%) 341 (63%) 22 (61%) 97 (45%)
   Years of education ≥12 years^b n (%) 75 (78%) 455 (84%) 30 (83%) 195
   (90%)
   Past or current smoker^b n (%) 45 (47%) 252 (47%) 17 (47%) 117 (54%)
   Non-European ancestry^b n (%) 40 (42%) 117 (22%) 11 (31%) 38 (18%)
   Study wave
   PEG1 n (%) 33 (34%) 248 (46%) 27 (75%) 163 (75%)
   PEG2 n (%) 63 (66%) 292 (54%) 9 (25%) 54 (25%)
   [71]Open in a new tab

   ^aAge refers to age of PD diagnosis for PD patient group and age of
   enrollment in the study for non-PD participants.

   ^b7 PD patients with imputed education, 5 non-PD participants with
   imputed education, and 4 non-PD participants with imputed smoking and
   ethnicity data.

Metabolome-wide analysis and annotation

   For untargeted, high-resolution metabolomics, we analyzed serum samples
   using both hydrophilic interactions (HILIC) chromatography with
   positive electrospray ionization (ESI) and C18 chromatography with
   negative ESI. After feature alignment and quality control, we included
   2913 metabolic features detected in the positive ion mode (HILIC) and
   2222 metabolic features from the negative ion mode (C18) in our MWAS
   analysis. MWAS results are shown in Figs. [72]1 and [73]2 and complete
   summary statistics for the metabolic features associated with T2DM are
   available through figshare (10.6084/m9.figshare.22589464). After
   adjusting for multiple testing and considering a significance level of
   FDR < 0.05, 192 metabolic features from the HILIC column and 133 in the
   C18 column were associated with T2DM among the PD patients. When
   considering only replication of the T2DM MWAS metabolites discovered in
   this PD population, after multiple testing correction, 49 of the 192
   metabolic features associated in HILIC and 38 out of 133 metabolic
   features in C18 were also associated with T2DM at a replication
   FDR < 0.05 in the non-PD participants (Supplementary Table [74]2). The
   coefficients for T2DM association across metabolic features from the
   MWAS among the PD patient group and non-PD participant group were
   moderately correlated (HILIC: r = 0.4, p < 2.2e−16; C18: r = 0.45,
   p < 2.2e−16). This is displayed in Fig. [75]2, which plots metabolite
   feature coefficients from the T2DM MWAS in PD and non-PD participants,
   colored features by association status (i.e., associated with T2DM in
   PD patient group, non-PD participant group or both). Most features
   showed concordance in the direction of association with T2DM among both
   the PD and non-PD participants.

Fig. 1. Manhattan plots of results of metabolome-wide association study for
diabetes mellitus (DM).

   [76]Fig. 1
   [77]Open in a new tab

   a Among Parkinson’s disease (PD) patients in HILIC column, b among PD
   patients in C18 column, c among non-PD participants in HILIC column,
   and d among non-PD participants in C18 column.

Fig. 2. Correlations between coefficients of metabolic features from
metabolome-wide association study for diabetes mellitus (DM) among
Parkinson’s disease (PD) patients and non-PD participants.

   [78]Fig. 2
   [79]Open in a new tab

   a HILIC column and b C18 column colored by significance level adjusting
   for multiple testing.

   Annotations for all metabolic features using the in-house library and
   xMSannotator are included in Supplementary Tables [80]3 and [81]4.
   Among the T2DM-associated metabolic features, 9 metabolites were
   uniquely annotated at high confidence, while 9 features were annotated
   at high confidence to multiple metabolites based on both the in-house
   library and xMSannotator (see Table [82]2). For example, the top
   feature (mz = 215.0328, retention time = 30.913) associated with T2DM
   among PD patients (OR = 1.80 per SD, 95% CI = 1.46, 2.25,
   FDR = 1.30e−05) and non-PD patients (OR = 3.62, 95% CI = 2.22, 5.91,
   FDR = 1.33e−04) was annotated at high confidence to glucose. Similarly,
   another top feature (mz = 179.0562, retention time = 36.803) in both
   groups (PD patients, OR = 1.75, 95% CI = 1.41, 2.15, FDR = 2.81e−05;
   non-PD participants, OR = 2.80, 95% CI = 1.84, 4.26, FDR = 2.83e−04)
   was annotated to simple sugars (i.e., glucose, fructose and mannose).
   The amino acid metabolite citrulline (mz = 174.088, rt = 30.043) was
   inversely associated with T2DM in both PD patients (OR = 0.54, 95%
   CI = 0.43, 0.68, FDR = 2.35e−05) and non-PD participants (OR = 0.48,
   95% CI = 0.33, 0.70, FDR = 1.07e−02).

Table 2.

   Annotation of metabolic features associated with diabetes mellitus
   among Parkinson’s disease (PD) patients and non-PD participants
   PD patients Non-PD participants
   Chemical name Library Indicator of Match Observed mass to charge ratio
   Observed retention time Column OR [95%CI] p value FDR p value OR
   [95%CI] p value FDR p value
   D-Galactose/D-Glucose xMSannotator Multiple 215.033 30.913 C18 1.81
   [1.46, 2.25] 0.000 0.000 3.62 [2.22, 5.91] 0.000 0.000
   Citrulline in-house Unique 174.088 30.043 C18 0.54 [0.43, 0.68] 0.000
   0.000 0.48 [0.33, 0.7] 0.000 0.011
   In-house Unique 176.103 102.372 HILIC 0.61 [0.48, 0.78] 0.000 0.004
   0.47 [0.32, 0.7] 0.000 0.014
   myo-Inositol/Allose/Sorbitol/D-Fructose/L-Sorbose/Alpha-D-Glucose/D-Gal
   actose/D-Tagatose/D-Mannose In-house Multiple 179.056 36.803 C18 1.75
   [1.41, 2.15] 0.000 0.000 2.8 [1.84, 4.26] 0.000 0.000
   L-Gulonolactone In-house Unique 177.040 33.332 C18 1.72 [1.36, 2.19]
   0.000 0.001 1.24 [0.84, 1.83] 0.283 0.786
   Argininic acid/Citrulline xMSannotator Multiple 198.085 100.113 HILIC
   0.58 [0.46, 0.73] 0.000 0.001 0.47 [0.31, 0.69] 0.000 0.013
   1-(Hydroxymethyl)-55-dimethyl-24-imidazolidinedione xMSannotator Unique
   159.076 102.224 HILIC 0.59 [0.46, 0.75] 0.000 0.002 0.48 [0.33, 0.71]
   0.000 0.015
   Glyceraldehyde/L-Lactic acid In-house Multiple 89.024 31.295 C18 1.57
   [1.26, 1.95] 0.000 0.003 1.57 [1.09, 2.27] 0.016 0.326
   Indoleacetaldehyde In-house Unique 158.061 87.220 C18 0.61 [0.48, 0.78]
   0.000 0.004 0.76 [0.52, 1.11] 0.150 0.650
   Sarcosine/L-Alanine/D-Alanine/beta-Alanine In-house Multiple 90.055
   84.601 HILIC 1.53 [1.21, 1.92] 0.000 0.011 1.36 [0.93, 2] 0.114 0.533

   2-(Methylthio)methyl-2-butenal/2-Ethyldihydro-3(2H)-thiophenone/S-2-Pro
   penyl propanethioate

   (R)-3-Hydroxybutyric acid/Alpha-Hydroxyisobutyric acid/2-Hydroxybutyric
   acid
   In-house Multiple 88.040 36.442 C18 1.3 [1.05, 1.6] 0.017 0.142 1.18
   [0.81, 1.71] 0.397 0.869
   In-house Multiple 103.040 32.211 C18 1.49 [1.2, 1.84] 0.000 0.012 1.44
   [1, 2.06] 0.049 0.452
   2-(Methylthio)methyl-2-butenal/2-Ethyldihydro-3(2H)-thiophenone/S-2-Pro
   penyl propanethioate xMSannotator Multiple 148.080 94.797 HILIC 0.66
   [0.53, 0.83] 0.000 0.012 0.83 [0.6, 1.16] 0.280 0.744
   1-Pyrroline-4-hydroxy-2-carboxylate/5-Oxoprolinate/dimethadione/N-Acryl
   oylglycine/Pyroglutamic acid/Pyrrolidonecarboxylic acid/Pyrroline
   hydroxycarboxylic acid xMSannotator Multiple 131.053 97.331 HILIC 0.7
   [0.56, 0.88] 0.003 0.042 0.84 [0.6, 1.16] 0.289 0.751
   L-Proline In-house Unique 114.056 36.877 C18 1.48 [1.19, 1.84] 0.000
   0.016 1.09 [0.75, 1.59] 0.646 0.957
   1-Pyrroline-4-hydroxy-2-carboxylate/5-Oxoprolinate/N-Acryloylglycine/Py
   roglutamic acid/Pyrrolidonecarboxylic acid/Pyrroline hydroxycarboxylic
   acid/dimethadione xMSannotator Multiple 130.050 94.746 HILIC 0.67
   [0.53, 0.85] 0.001 0.018 0.93 [0.66, 1.3] 0.666 0.918
   L-Glutamic acid In-house Unique 148.060 89.479 HILIC 1.47 [1.18, 1.84]
   0.001 0.018 0.93 [0.64, 1.35] 0.697 0.932

   Biliverdin

   (S)-N-(45-Dihydro-1-methyl-4-oxo-1H-imidazol-2-yl)alanine
   In-house Unique 581.240 42.898 C18 0.71 [0.58, 0.87] 0.001 0.024 0.86
   [0.58, 1.28] 0.471 0.909
   xMSannotator Unique 220.049 30.642 C18 0.68 [0.53, 0.86] 0.002 0.034
   0.49 [0.33, 0.72] 0.000 0.024
   6-Chloro-N-(1-methylethyl)-135-triazine-24-diamine/Indoleacrylic acid
   xMSannotator Multiple 188.071 59.806 HILIC 0.68 [0.54, 0.87] 0.002
   0.034 0.86 [0.58, 1.26] 0.442 0.828
   3-Methyldioxyindole In-house Unique 162.056 40.608 C18 0.67 [0.53,
   0.87] 0.002 0.040 0.87 [0.6, 1.27] 0.477 0.911
   Mannitol/Galactitol In-house Multiple 181.071 37.306 C18 1.38 [1.12,
   1.72] 0.003 0.050 1.3 [0.91, 1.86] 0.147 0.646
   ()-Tryptophan/3-Hydroxymethylantipyrine/4-Hydroxyantipyrine/D-Tryptopha
   n/Ethotoin/L-Tryptophan/Nirvanol/S-nirvanol xMSannotator Multiple
   205.097 59.833 HILIC 0.7 [0.55, 0.89] 0.003 0.051 0.84 [0.57, 1.24]
   0.377 0.803

   ()-Tryptophan/3-Hydroxymethylantipyrine/4-Hydroxyantipyrine/D-Tryptopha
   n/Ethotoin/L-Tryptophan/Nirvanol/S-nirvanol

   3-(Carboxymethylamino)propanoic acid/D-Glutamic
   acid/DL-Glutamate/L-4-Hydroxyglutamate semialdehyde/L-Glutamic
   acid/N-Acetylserine/N-Methyl-D-aspartic acid/O-Acetylserine
   xMSannotator Multiple 249.061 60.032 HILIC 0.72 [0.57, 0.91] 0.005
   0.067 0.76 [0.52, 1.11] 0.151 0.583
   xMSannotator Multiple 192.024 88.823 HILIC 1.38 [1.1, 1.73] 0.006 0.070
   1.47 [0.99, 2.18] 0.055 0.386

   Uridine

   Dihydrothymine/L-Cyclo(alanylglycyl)/Squamolone
   In-house Unique 243.062 33.439 C18 1.38 [1.09, 1.73] 0.006 0.075 1.04
   [0.71, 1.51] 0.858 0.989
   xMSannotator Multiple 129.066 86.264 HILIC 0.69 [0.53, 0.91] 0.008
   0.088 0.6 [0.38, 0.96] 0.033 0.309
   L-Fucose/Rhamnose xMSannotator Multiple 173.030 86.515 HILIC 0.75
   [0.57, 0.98] 0.032 0.206 0.59 [0.37, 0.92] 0.021 0.259
   L-Fucose/Rhamnose In-house Multiple 163.061 33.325 C18 0.75 [0.61,
   0.93] 0.008 0.088 0.59 [0.42, 0.84] 0.003 0.126
   Hypoxanthine In-house Unique 137.046 56.412 HILIC 0.76 [0.61, 0.94]
   0.013 0.118 1.17 [0.78, 1.75] 0.437 0.827
   L-Aspartic acid/D-Aspartic acid In-house Multiple 134.045 102.083 HILIC
   1.36 [1.06, 1.74] 0.015 0.129 0.98 [0.68, 1.41] 0.904 0.985
   L-Serine In-house Unique 104.035 30.101 C18 0.75 [0.59, 0.94] 0.015
   0.132 0.62 [0.42, 0.93] 0.020 0.374
   L-Cystine xMSannotator Unique 241.031 231.655 HILIC 1.3 [1.05, 1.63]
   0.019 0.146 1.1 [0.72, 1.67] 0.660 0.918
   Oxoglutaric acid In-house Unique 145.014 28.164 C18 1.3 [1.04, 1.63]
   0.022 0.166 1.31 [0.9, 1.9] 0.160 0.665
   trans-Aconitic acid In-house Unique 173.009 29.367 C18 1.33 [1.04, 1.7]
   0.022 0.166 1.06 [0.71, 1.57] 0.775 0.989
   Pantothenic acid In-house Unique 220.118 41.476 HILIC 1.31 [1.04, 1.64]
   0.023 0.167 1.12 [0.77, 1.63] 0.558 0.876
   L-Threonine/L-Homoserine/L-Allothreonine In-house Multiple 120.066
   87.232 HILIC 0.77 [0.61, 0.97] 0.027 0.183 0.89 [0.61, 1.3] 0.550 0.874
   gamma-Aminobutyric acid/2-Aminoisobutyric acid/3-Aminoisobutanoic acid
   In-house Multiple 102.056 29.529 C18 1.28 [1.02, 1.61] 0.031 0.202 1.1
   [0.76, 1.59] 0.623 0.952
   Mevalonic acid In-house Unique 147.066 27.018 C18 1.27 [1.02, 1.59]
   0.033 0.211 1.07 [0.75, 1.52] 0.714 0.978
   3-Mercapto-2-methyl-1-butanol/3-Mercapto-3-methyl-1-butanol/4-(Methylth
   io)-1-butanol/4-(Methylthio)-2-butanol/xi-2-Mercapto-3-methyl-1-butanol
   xMSannotator Multiple 121.069 87.491 HILIC 0.79 [0.63, 0.99] 0.043
   0.243 0.62 [0.42, 0.92] 0.018 0.246
   L-Cystathionine In-house Unique 223.075 221.449 HILIC 1.28 [1.01, 1.63]
   0.045 0.247 1.18 [0.79, 1.75] 0.413 0.821
   1-(2-Thienyl)-1-heptanone xMSannotator Unique 219.083 108.587 HILIC
   0.79 [0.62, 1] 0.046 0.252 1.12 [0.76, 1.65] 0.566 0.878
   N-Acetylneuraminic acid In-house Unique 308.098 30.995 C18 1.23 [1,
   1.51] 0.050 0.273 1.46 [1.02, 2.09] 0.038 0.436
   (S)-N-(45-Dihydro-1-methyl-4-oxo-1H-imidazol-2-yl)alanine xMSannotator
   Unique 184.073 35.977 C18 0.8 [0.63, 1.01] 0.065 0.313 0.54 [0.37,
   0.78] 0.001 0.079
   L-Histidine In-house Unique 154.062 38.677 C18 0.84 [0.67, 1.06] 0.144
   0.475 0.47 [0.29, 0.76] 0.002 0.105
   246-Octatriyn-1-ol/357-Octatriyn-1-ol/Benzofuran/xi-23-Octadiene-57-diy
   n-1-ol xMSannotator Multiple 119.049 71.599 HILIC 0.86 [0.68, 1.08]
   0.194 0.542 0.67 [0.46, 0.99] 0.042 0.343
   Phenylpyruvic acid In-house Unique 165.055 72.205 HILIC 0.87 [0.69,
   1.09] 0.228 0.581 0.67 [0.45, 0.98] 0.037 0.327
   Stearic acid In-house Unique 283.264 264.643 C18 1.14 [0.91, 1.43]
   0.241 0.608 1.55 [1.07, 2.25] 0.020 0.374
   D-Arabitol/D-Xylitol/L-2-(Hydroxymethyl)-1234-butanetetrol/L-Arabitol/R
   ibitol xMSannotator Multiple 197.041 89.659 HILIC 0.89 [0.71, 1.12]
   0.324 0.682 0.68 [0.48, 0.97] 0.033 0.309
   xMSannotator Multiple 153.077 90.009 HILIC 1.03 [0.82, 1.29] 0.796
   0.930 0.62 [0.42, 0.9] 0.011 0.202
   O-Phosphoethanolamine In-house Unique 140.011 27.424 C18 0.91 [0.74,
   1.11] 0.360 0.702 0.66 [0.47, 0.91] 0.012 0.299
   Petroselinic acid/Oleic acid/Elaidic acid In-house Multiple 281.248
   242.644 C18 1.11 [0.87, 1.4] 0.397 0.713 1.5 [1.01, 2.23] 0.044 0.440
   Benzylamine In-house Unique 108.081 30.716 HILIC 0.94 [0.75, 1.18]
   0.581 0.849 0.61 [0.39, 0.94] 0.024 0.274
   [83]Open in a new tab

   Most were associated with T2DM only among PD patients, though many
   showed similar trends among the non-PD participants. Metabolites with
   unique annotation matches included several amino acids (cystathionine,
   cystine, glutamic acid, gulonolactone, proline, aspartic acid, and
   indoleacetaldehyde), nucleic acids (uridine, hypoxanthine), and
   tricarboxylic acid (TCA) cycle/coenzyme A (CoA) related metabolites
   (oxoglutaric acid, pantothenic acid, trans-aconitic acid, and mevalonic
   acid).

Pathway enrichment analysis

   We identified 55 metabolic pathways enriched (p < 0.05) among the
   features associated with T2DM in PD patients and 28 metabolic pathways
   from the features associated with T2DM among non-PD participants (see
   Table [84]3). After removing duplicate pathways identified in both
   HILIC and C18 in either group, 17 metabolic pathways remained
   associated with T2DM (p < 0.05) in both PD patients and non-PD
   participants, 26 pathways were only enriched in PD patients, and 5 only
   in non-PD participants. Figure [85]3 shows these pathways, along with
   each pathway’s biochemical classification and functional group.

Table 3.

   Metabolic pathways associated with diabetes mellitus (DM) using
   metabolites captured by positive ion mode (HILIC) and negative ion mode
   (C18) among Parkinson’s disease (PD) patients and non-PD participants
   pathway Column PD Patients Non-PD participants
   Overlap size Pathway size p-value Overlap size Pathway size p-value
   Metabolic pathways associated with DM only among PD patients
   Alanine and aspartate metabolism C18 6 10 0.0211 2 10 0.2405
   HILIC 6 7 0.0003 1 7 0.2601
   Arginine and proline metabolism C18 8 13 0.0073 3 13 0.1682
   HILIC 6 14 0.0118 1 14 0.3388
   Ascorbate (Vitamin C) C18 7 14 0.0385 0 14 0.3702
   Aldarate metabolism C18 7 14 0.0385 0 14 0.3702
   Aspartate and asparagine metabolism C18 11 17 0.0015 4 17 0.1385
   Beta-Alanine metabolism C18 5 7 0.0124 2 7 0.1519
   Butanoate metabolism C18 5 5 0.0014 2 5 0.0888
   CoA Catabolism HILIC 2 2 0.0118 0 2 0.3738
   Glutamate metabolism C18 5 5 0.0014 1 5 0.2759
   HILIC 3 5 0.0186 0 5 0.3738
   Glutathione metabolism HILIC 3 5 0.0186 0 5 0.3738
   Glycine, serine, alanine and threonine metabolism C18 10 15 0.0018 3 15
   0.214
   Histidine metabolism C18 3 4 0.0367 1 4 0.2443
   Lysine metabolism C18 5 6 0.005 1 6 0.3025
   Methionine and cysteine metabolism C18 6 9 0.0102 1 9 0.3451
   Mono-unsaturated fatty acid beta-oxidation C18 2 2 0.0405 0 2 0.3702
   Nitrogen metabolism HILIC 3 3 0.0029 0 3 0.3738
   Pentose and glucuronate interconversions C18 4 5 0.0135 0 5 0.3702
   Phosphatidylinositol phosphate metabolism HILIC 4 8 0.0177 2 8 0.0858
   Porphyrin metabolism C18 5 7 0.0124 0 7 0.3702
   Purine metabolism HILIC 5 11 0.0157 2 11 0.1524
   Pyrimidine metabolism HILIC 6 14 0.0118 3 14 0.0778
   C18 9 14 0.003 4 14 0.0843
   Selenoamino acid metabolism C18 2 2 0.0405 1 2 0.1441
   TCA cycle C18 5 9 0.0426 0 9 0.3702
   Tryptophan metabolism C18 9 18 0.0255 3 18 0.2708
   Urea cycle/amino group metabolism C18 12 21 0.0029 3 21 0.3142
   Vitamin B3 (nicotinate and nicotinamide) metabolism HILIC 4 7 0.0082 0
   7 0.3738
   Vitamin B5 - CoA biosynthesis from pantothenate HILIC 2 3 0.0339 0 3
   0.3738
   Metabolic pathways associated with DM only among non-PD participants
   Glycerophospholipid metabolism C18 5 18 0.368 7 18 0.0066
   Glycosphingolipid metabolism C18 3 9 0.2925 4 9 0.0177
   Keratan sulfate degradation C18 3 5 0.0722 3 5 0.0129
   HILIC 2 4 0.0623 2 4 0.019
   N-Glycan biosynthesis C18 2 3 0.112 2 3 0.0291
   Vitamin B2 (riboflavin) metabolism HILIC 0 1 0.4588 1 1 0.0329
   Metabolic pathways associated with DM among both PD patients and non-PD
   participants
   Aminosugars metabolism HILIC 3 5 0.0186 1 5 0.2012
   C18 4 7 0.055 3 7 0.034
   Caffeine metabolism C18 4 5 0.0135 3 5 0.0129
   HILIC 2 3 0.0339 1 3 0.1257
   Chondroitin sulfate degradation HILIC 2 3 0.0339 2 3 0.0093
   C18 4 4 0.0038 1 4 0.2443
   Fructose and mannose metabolism C18 5 5 0.0014 5 5 0.0002
   HILIC 3 4 0.0066 2 4 0.019
   Galactose metabolism C18 6 8 0.005 5 8 0.0016
   HILIC 5 5 0.0003 2 5 0.0297
   Glycolysis and gluconeogenesis C18 6 7 0.0018 4 7 0.0066
   Glycosphingolipid biosynthesis—ganglioseries HILIC 2 3 0.0339 2 3
   0.0093
   Glycosphingolipid biosynthesis—globoseries HILIC 2 3 0.0339 2 3 0.0093
   Heparan sulfate degradation HILIC 2 3 0.0339 2 3 0.0093
   C18 4 4 0.0038 1 4 0.2443
   Hexose phosphorylation C18 4 5 0.0135 3 5 0.0129
   HILIC 4 5 0.0019 2 5 0.0297
   N-Glycan degradation HILIC 2 2 0.0118 2 2 0.0023
   C18 2 3 0.112 3 3 0.0017
   Pentose phosphate pathway HILIC 1 2 0.1668 2 2 0.0023
   C18 7 10 0.0046 3 10 0.0993
   Propanoate metabolism C18 2 2 0.0405 2 2 0.011
   Sialic acid metabolism C18 5 8 0.0268 5 8 0.0016
   HILIC 4 5 0.0019 2 5 0.0297
   Starch and sucrose metabolism C18 3 4 0.0367 3 4 0.0066
   HILIC 3 4 0.0066 1 4 0.1665
   Valine, leucine and isoleucine degradation C18 5 7 0.0124 3 7 0.034
   Xenobiotics metabolism HILIC 5 12 0.0219 4 12 0.0093
   [86]Open in a new tab

Fig. 3. Pathway enriched among Parkinson’s disease (PD) patients and non-PD
participants.

   [87]Fig. 3
   [88]Open in a new tab

   a Annotation of pathways by chemical groups, b by functional
   proporties.

   The pathways enriched for T2DM-associated metabolites in both PD and
   non-PD participants included carbohydrate-associated metabolic pathways
   (e.g., simple sugar metabolism, such as fructose, mannose, galactose
   metabolism and glycolysis, and complex sugar metabolism, such as starch
   and sucrose metabolism, n-glycan pathway, amino sugars metabolism) and
   glycosphingolipid metabolism.

   The T2DM-associated pathways enriched only among PD patients included
   multiple amino acid metabolism pathways (e.g., alanine, aspartate,
   arginine, proline, glutamate, and glutathione metabolism), nucleic acid
   metabolism pathways (e.g., purine metabolism and pyrimidine
   metabolism), and fatty acid metabolisms (e.g., butanoate metabolism and
   mono-unsaturated fatty acid beta-oxidation). In addition, the
   T2DM-associated metabolites from PD patients were also enriched for
   urea cycle and TCA cycle-related pathways, including CoA catabolism and
   vitamin B5 - CoA biosynthesis from pantothenate.

   Full pathway analysis results are included in Supplementary Table
   [89]5. The pathway classifications and supporting references are