Table 1. List of clinical trials that evaluated the role of vitamin D supplementation in CHF patients.

CHF=Congestive Heart Failure, 6MWT=6 Minute Walk Test, MCS=Mechanical Circulatory Support, LVEF=Left Ventricular Ejection Fraction, HR=Hazard Ratio, PTH: Parathyroid Hormone, TNF-a=Tumour Necrosis Factor-alpha, IL=Interleukin, BNP=Brain Natriuretic Peptide, ANP=Atrial Natriuretic Peptide, f/b=Followed By, 25-OHD=25-Hydroxyvitamin D, IU=International Units

Study details (year)	Follow-up	Vit D dose	Results
1. Hosseinzadeh et al. 2020 [24]	8 weeks	50,000 IU weekly	No improvements were found in BP and 6MWT when compared to the placebo. Levels of vitamin D were increased significantly in the intervention group. With short-term no improvement in HF patients’ physical activity consistent with previous studies.
2. Zittermann et al. 2017 [15]	3 years	4000 IU daily	No significant effect was found with supplements in advanced HF patients. The study also concluded that group on vitamin D had a greater need for MCS implants in the intervention group with HR 1.96 (1.04–3.66). No difference was found between the intervention and the placebo group in terms of mortality with HR 1.09 (0.69-1.71). Data also suggests caution with prolonged supplementation of high doses.
3. Scragg et al. 2017 [26]	3.3 years	200 000 IU f/b 100 000 IU monthly	Monthly high dose vitamin D did not prevent cardiovascular disease with HR 1.02 (0.87-1.21). Results also suggest that there was no purpose of supplementation in relation to heart failure with HR of 1.19 (0.84-1.68). This study also stated that high dose Vit D can be less effective than weekly or daily supplements in preventing a cardiovascular event.
4. Turrini et al. 2017 [16]	6 months	300,000 IU f/b 50,000 monthly	Treatment of deficiency did not influence the final outcome when compared to the placebo group. Supplements improved functional capacity and PTH levels at 3 months but this was not observed at 6 months. Concludes baseline vitamin D levels did not affect the functional capacity.
5. Witte et al. 2016 [27]	1 year	4000 IU daily	Results showed a significant improvement of cardiac function showing a mean change of +6.7 % LVEF (3.20-8.95) on echocardiogram. 1-year supplementation of daily Vit D did not improve 6-minute walk distance. It also has beneficial effects on LV structure and function in patients on current standard medical therapy.
6. Dalbeni et al. 2014 [28]	6 months	4000 IU daily	Results showed that there was a significant improvement in EF of 6.71% in elderly patients with HF and vitamin D deficiency. Therapy also improved SBP after 6 months from 129.6 to 122.7 mm Hg, but no significant variations were found on other parameters.
7. Schroten et al. 2013 [29]	6 weeks	2000 IU daily	Short term supplementation improved the Vit D levels from 48 nmol/L to 80 nmol/L. Results showed Plasma renin activity decrease from 6.5 ng/mL per hour (3.8-11.2) to 5.2 ng/mL per hour (2.9-9.5) in 6 weeks. No significant changes were seen in natriuretic peptides (BNP, ANP) and other markers of fibrosis.
8. Boxer et al. 2013 [30]	6 months	50,000 IU Weekly	High dose Vit D improved serum 25-OH D levels from baseline 19.1 ± 9.3 ng/ml to 61.7 ± 20.3 ng/ml. Vitamin D supplements did not improve physical performance for patients with HF despite a good increase in serum 25-OHD
9. Zia et al. 2011 [31]	14 weeks	50,000 IU weekly for 8 weeks f/b 1400 IU daily	Although a small patient population, results suggest improvement in secondary hyperparathyroidism, oxidative stress, and ventricular function (LVEF). Serum PTH was reduced at 14 weeks from 104.8 to 73.8 pg/mL, Plasma 8-isoprostane a marker of oxidative stress was reduced at 14 weeks to 117.8 from 136.1 pg/ml. With baseline EF of 24.3 ± 1.7% at entry was improved to 31.3 ± 4.3%.
10. Witham et al. 2010 [17]	20 weeks	100 000 IU and in the 10th week	Supplementation did not improve functional capacity or quality of life in heart failure patients with vitamin D insufficiency. B-type natriuretic peptide levels decreased in the treatment group when compared with placebo
11. Schleithoff et al. 2006 [32]	9 months	1000 IU daily	Significant treatment effects were observed, parathyroid hormone levels were significantly decreased when compared to the baseline. Anti-inflammatory cytokine IL-10 was significantly higher in the intervention group at 9 months and pro-inflammatory cytokine TNF-a was increased in controls but remain constant in the therapy group. Vitamin D reduced the inflammatory state in CHF patients and might be a new anti-inflammatory agent in the future. But during follow-up at 15 months, the survival rate did not differ significantly.