Table 1.

Reports of metformin’s effects on GDF-15 in different models.

Study, year	Models	Number	Dose of metformin	Change of GDF-15 by metformin
Animal studies
Day et al., 2019 (24)	Mice fed with chow diet and high fat diet	n = 6-7 per group	A single oral gavage of metformin (250 mg/ kg) or an equal volume of saline	Metformin significantly increased serum GDF-15 in both chow diet and high fat diet groups
Coll et al., 2020 (23)	Obese mice	Three groups: Vehicle, Metformin (300 mg/kg) Metformin (600 mg/kg), n = 7 per group,	Single oral dose of 300 or 600 mg/kg	300 mg/kg of metformin increased GDF-15 levels for at least 8 h. 600 mg/kg of metformin resulted in a six-fold increase in serum GDF-15 levels at 4 h and 8 h after the dose.
Cellular studies
Williams et al., 2013 (79)	MDA-MB-468 breast cancer cells	n = 3 per group	Cells were cultured for 48 h in the absence or presence of 1 mM metformin	GDF-15 gene expression was increased 25.61 fold in metformin group compared with control.
Zafarvahedian et al., 2017 (80)	Mesenchymal stem cells (MSCs)	n = 3 per group	MSCs were treated with 10, 50, and 100 mM metformin for 17 h	GDF-15 production was increased in a dose dependent manner. GDF-15 levels increased by dose up to 2-fold control group levels at 100 mM.
Day et al., 2019 (24)	Primary mouse hepatocytes	(1) n = 4 per group (2) n = 3-6 per group	(1) Cells were treated with 0.5 mM metformin for 24 h (2) 0-1,000 μM for 24 h	(1) Metformin treatment significantly increased GDF-15 mRNA levels. (2) Metformin increased GDF-15 release in a dose-dependent manner
Clinical trials
Gerstein et al., 2017 (81)	People with diabetes, impaired glucose tolerance, or impaired fasting glucose levels	8,401 participants (2,317 receiving metformin)	Various doses	Mean GDF-15 concentrations rose with metformin dose. GDF-15 was strongly linked to metformin, such that the odds of metformin use per standard deviation value increase in level varied from 3.73 (95% CI 3.40, 4.09) to 3.94 (95% CI 3.59, 4.33) depending on included variables.
Natali et al., 2018 (82)	Diabetic patients	644 (Metformin) vs 299 (Non-metformin)	Not mentioned	Metformin treatment was associated with a 40% rise in GDF-15 level, which was independent of the other major factors.
Coll et al., 2020-2 (23)	Overweight individuals	9 (placebo-controlled, double-blind crossover design)	Week 1: 500 mg twice daily; week 2: 1,000 mg twice daily.	After two weeks of metformin treatment, there was an increase of about 2.5-fold in mean circulating GDF-15.
Coll et al., 2020-3 (23)	Overweight or obese non-diabetic participants	86 (Metformin) vs 85 (placebo)	850 mg daily for 18 months	Metformin treatment was associated with significantly increased levels of circulating GDF-15 at all three time points (6, 12 and 18 months)
Isnard et al., 2020 (25)	Non-diabetic People living with HIV	Metformin	850 mg twice daily for 12 weeks.	Metformin treatment was associated with significantly increased levels of circulating GDF-15 at 12 weeks. Plasma GDF-15 levels went back to baseline levels 12 weeks after metformin discontinuation.