Table 2.

The characteristics of the included studies.

Study	Register ID	Country	Sample size		Intervention		Relevant outcomes	Duration
			Trial group	Control group	Trial group	Control group
Panahi et al. [16–19]	IRCT201505301165N4	Iran	50	50	Curcuminoids (C3 Complex®) 1000 mg + 10 mg piperine	Placebo + 10 mg piperine	BMI, TG, TC, LDL-C, HDL-C, HOMA-IR, HbAlc, fasting blood glucose, fasting insulin, adiponectin adverse events	12 weeks
Na et al. [20, 21]	ISRCTN85826075	China	50	50	Curcuminoids 300 mg with no changes to patients' previous drug medication	Placebo with no changes to patients' previous drug medication	TG, TC, LDL-C, HDL-C, HOMA-IR, HbAlc, fasting blood glucose	12 weeks
Chuengsamarn et al. [11]	NCT01052597	Thailand	107	106	Curcumin 1500 mg with no oral antidiabetes or insulin injection	Placebo (starch) 1500 mg with no oral antidiabetes or insulin injection	BMI, TG, TC, LDL-C, HDL-C, HOMA-IR, HbAlc, fasting blood glucose, fasting insulin, adiponectin, adverse events	24 weeks
Khajehdehi et al. [22]	—	Iran	20	20	Turmeric 1500 mg with no changes to patients' previous drug medication	Placebo with no changes to patients' previous drug medication	TG, TC, LDL-C, HDL-C, HOMA-IR, HbAlc, fasting blood glucose, adverse events	8 weeks
Asadi et al. [31, 32]	IRCT20140413017254N5	Iran	40	40	Nanocurcumin 80 mg with no changes to patients' previous hypoglycemic drugs	Placebo with no changes to patients' previous hypoglycemic drugs	BMI, HbAlc, fasting blood glucose, adverse events	8 weeks
Thota et al. [33]	ACTRN12615000559516	Australia	15	16	Curcumin 1000 mg	Placebo (2000 mg corn oil)	HbAlc, TC, LDL-C, HDL-C, fasting blood glucose, adverse events	12 weeks
Adibian et al. [34]	NCT02529969	Iran	21	23	Curcumin 1500 mg with no changes to patients' previous drug medication	Placebo (rice flour) 1332 mg with no changes to patients' previous drug medication	BMI, TG, TC, LDL-C, HDL-C, fasting blood glucose, fasting insulin, adiponectin	10 weeks

Study	BMI		Mean age (years)		Inclusion criteria	Exclusion criteria
	Trial group	Control group	Trial group	Control group
Panahi et al. [16–19]	26.53 ± 2.32	27.33 ± 1.58	43 ± 8	41 ± 7	Diagnosis of T2DM based on fasting plasma glucose (FPG) ≥ 126 mg/dL, glycated hemoglobin (HbA1C) ≥ 6.5%, or the use of antidiabetic treatments	Pregnancy or breastfeeding, lack of possibility to give an informed consent, participation in a concomitant trial, presence of malignancies, chronic liver disease (alanine aminotransferase levels three times the upper limit of normal value range), renal failure (serum creatinine ≥ 2.0 mg/dL or being on dialysis), chronic inflammatory diseases such as rheumatoid arthritis and acute infections, endocrine diseases other than T2DM (e.g., hypothyroidism or hyperthyroidism), obsessive compulsive disorder, hyperglycemia due to secondary causes, receiving hormone therapy or other herbal medicines, hypersensitivity to the study medication, and lack of compliance with the study medication
Na et al. [20, 21]	27.12 ± 2.26	27.42 ± 3.04	55.42 ± 6.40	54.72 ± 8.34	(1) Aged 18–65 years, both male and female (2) Type 2 diabetes with fasting blood glucose greater than or equal to 7.0 mmol/L or postprandial blood glucose greater than or equal to 11.1 mmol/L (3) Current optimal therapeutic regimens lasting for at least 6 months	(1) A history of type 1 diabetes, malignancies, thyroid, or any other endocrine diseases likely to interfere with the study (2) Diabetic ketosis acidosis and infection in recent 3 months (3) Pregnancy or breastfeeding (4) Incomplete information or unwillingness to attempt to comply with the intervention
Chuengsamarn et al. [11]	27.09 ± 0.52	26.84 ± 0.42	59.16 ± 11.04	59.58 ± 10.71	(1) Diabetic patients aged 35 years or older and did not use insulin during the first 5 years of treatment after being diagnosed (with or without symptoms listed in the following inclusion criteria) (2) Patients with hyperlipidemia (cholesterol ≥ 200 mg/dl, TG ≥ 150 mg/dl, LDL ≥ 100 mg/dl, and HDL ≥ 35 mg/dl) (3) Patients with hypertension (blood pressure ≥ 130/85 mmHg or take hypertensive drugs) (4) Obesity (BMI ≥ 25)	(1) Current diagnosis of secondary peripheral arterial disease (PAD) (except listed in the inclusion criteria item 1–4) (2) Current diagnosis of cardiovascular disease, i.e., coronary arterial disease and cerebrovascular disease (3) Current diagnosis of end-stage renal function with serum creatinine >2.0 mg/dl or on renal dialysis (4) Current diagnosis of cirrhosis with ALT ≥3 times the normal range
Khajehdehi et al. [22]	—	—	52.9 ± 9.2	52.6 ± 9.7	Patients with overt type 2 diabetic nephropathy (proteinuria ≥500 mg/day), who had normal kidney function and well-controlled blood pressure by ACE-I and/or angiotensin receptor blockers	Patient with recurrent or relapsing urinary tract infection, bacteriuria, pyuria and/or haematuria, or who failed to sign a written informed consent when risks associated with the trial was carefully outlined for them
Asadi et al. [31, 32]	31.1 ± 4.2	30.8 ± 3.8	53.3 ± 6.5	54.6 ± 6.2	Age between 30 and 60 years, desire to participate in the study, body mass index between 25 and 39.9, noninsulin type 2 diabetic patients, detection of mild sensorimotor polyneuropathy by using the Toronto questionnaire (score 6–8)	Follow a special diet during last month, sensitivity to curcumin, pregnancy and lactation, took any nutritional supplement, vitamin and mineral supplement last month, history of gastrointestinal ulcer and bile duct, neuropathies other than the sensory-motor polyneuropathy diabetic diagnosed by a neurologist, taking gabapentin and any medication, diseases such as cancer, liver, kidney, autoimmune diseases, inflammatory, thyroid and nervous and cardiovascular diseases are diagnosed, and drug use associated with these diseases
Thota et al. [33]	30.9 ± 1.2	31.9 ± 1.7	55 ± 2.8	50 ± 2.5	(1) Age: 30–70; gender: both male and female (2) No participation in any clinical trial for at least 3 months (3) An HbA1c of 5.7%–6.4% (4) Impaired glucose tolerance (IGT) (5) 2-hour OGTT plasma glucose greater than or equal to 7.8 mmol/land <11.1 mmol/L (6) Impaired fasting glucose (IFG) (7) Fasting plasma venous glucose measurement 6.1–6.9 mmol/L (8) 12 or more score or high-risk individuals in AUSDRISK assessment tool (9) BMI between 25 and 45	(1) Pregnancy or lactation (2) Established type 2 diabetes (3) Allergic to sea foods (4) People with gall bladder problems (5) People currently on medication with erythropoietin (6) People with anaemia (7) People with pace maker implants (8) Currently on medication with aspirin and warfarin (9) History of severe neurological diseases or seizures (10) History of new investigational drug three months prior to this trial (11) Consuming more than 2 servings of oily fish per week (12) Taking regular dietary supplements known to influence blood glucose level (13) People taking regular vitamin C supplements (14) Unwilling to fast for 10 hr before obtaining blood sample. People currently on medication with clopidogrel, ibuprofen, naproxen, dalteparin, enoxaparin, and heparin
Adibian et al. [34]	—	—	Around 58	60 ± 7	(1) Tendency to participate (2) Age range of 40–65 (3) Suffering from diabetes type 2 (for 1 to 10 years) (4) BMI 18/5–30 (5) Patients with diabetes who administer oral hypoglycemic agents and who do not use them	(1) Patients with liver diseases (2) Patients with kidney diseases (3) Patients with inflammatory diseases (4) Patients with liver diseases (5) Administering herbal agents (6) Administering multivitamins and minerals in past 3 months