20-MFI	20-item Multidimensional Fatigue Inventory
25OHD	25-hydroxyvitamin D
6MWT	6-Minute Walk Test
7-day food diary	Seven-day dietary intake recording tool
ADMA	Asymmetric Dimethylarginine
AEO	Enol-Oxaloacetate
BDI	Beck Depression Inventory
BioICOPER	Biopsychosocial Integrated COVID Pathway for Evaluation and Rehabilitation
BMI	Body Mass Index
BTT	Butanol Threshold Test
CDC	Centers for Disease Control and Prevention
CES-D	Center for Epidemiologic Studies Depression Scale
CFS	Chalder Fatigue Score
COMPASS 31	Composite Autonomic Symptom Score 31
COVID-19	Coronavirus Disease 2019
CRP	C-reactive protein
CT	Controlled Trial
DQQ	Dietary Diversity Score
DSM	Diagnostic and Statistical Manual of Mental Disorders
ED	Emergency Department
EQ-5D	EuroQol 5-Dimension Questionnaire
EQ-5D-5L	EuroQol 5-Dimension 5-Level Questionnaire
ESPEN	European Society for Clinical Nutrition and Metabolism
FACIT	Functional Assessment of Chronic Illness Therapy
FAS	Fatigue Assessment Scale
FFMI	Fat-Free Mass Index
FMI	Fat Mass Index
GAD-7	Generalized Anxiety Disorder—7 item scale
GLIM	Global Leadership Initiative on Malnutrition
HR	Hazard Ratio or Heart Rate (context-dependent)
HRQoL	Health-Related Quality of Life
ICD	International Classification of Diseases
ICU	Intensive Care Unit
IES-R	Impact of Event Scale—Revised
IL-6	Interleukin-6
IQR	Interquartile Range
IU	International Unit
LDH	Lactate Dehydrogenase
LDL	Low-Density Lipoprotein
MD	Mediterranean Diet
MDPI	Multidisciplinary Digital Publishing Institute
MeSH	Medical Subject Headings
MEDAS	Mediterranean Diet Adherence Screener
ME/CFS	Myalgic Encephalomyelitis/Chronic Fatigue Syndrome
MetS	Metabolic Syndrome
MHC-SF	Mental Health Continuum—Short Form
MLR	Monocyte-to-lymphocyte ratio
MMS	Multiple Micronutrient Supplement
MoCA-BLIND	Montreal Cognitive Assessment—Blind Version
MNA	Mini Nutritional Assessment
NHS	National Health Service
NICE	National Institute for Health and Care Excellence
NLR	Neutrophil-to-lymphocyte ratio
NNLM	National Network of Libraries of Medicine
OFS	Oral Food Supplement
Omega-3/6	Omega-3 and Omega-6 polyunsaturated fatty acids
ONS	Oral Nutritional Supplements
OR	Odds Ratio
PACS	Post-Acute COVID-19 Syndrome
PCC	Post-COVID-19 Condition
pCOVq	Post-COVID-19 Questionnaire
PCQ	Primary Care Questionnaire (or Patient Care Quality; context-dependent)
PHQ-9	Patient Health Questionnaire-9
PACSQ-14	Post-Acute COVID-19 Syndrome Questionnaire
PREDIMED	Prevención con Dieta Mediterránea / Prevention with Mediterranean Diet
PRISMA	Preferred Reporting Items for Systematic Reviews and Meta-Analyses
PRISMA-ScR	Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews
PROMIS	Patient-Reported Outcomes Measurement Information System
PSQI	Pittsburgh Sleep Quality Index
QoL	Quality of life
RCT	randomized controlled trial
SARC-F	Strength, Assistance with walking, Rise from a chair, Climb stairs, and Falls
SARS-CoV-2	Severe Acute Respiratory Syndrome Coronavirus 2
SF-36	36-Item Short Form Survey
SIM01	A specific probiotic strain/formulation (context-dependent)
TDR	Total Diet Replacement
UK	United Kingdom
VAS	Visual Analogue Scale
VDD	Vitamin D deficiency
WHO	World Health Organization
WHOQOL-BREF	World Health Organization Quality of Life—Brief Version