BACKGROUND: Iron deficiency (ID) is common in patients with chronic kidney disease (CKD) due to an inadequate dietary intake of iron, poor absorption from the gut and increased iron losses. In addition to preventing anaemia, iron is important for normal heart function, being involved in processes that generate a necessary continuous energy supply. Treatment with intravenous (IV) iron has been suggested to lead to improvement in heart function and well-being in people with ID and CKD. In the Iron and the Heart Study, we hypothesized that IV iron treatment will primarily improve exercise capacity and may secondarily impact the feeling of well-being in comparison to placebo over 3 months in non-anaemic CKD patients who have ID. METHODS: This was a prospective double-blinded explorative randomized, multi-centre study designed to compare the effects of IV iron supplementation and placebo in iron-deficient but not anaemic patients with established CKD stages 3b-5 on functional status, and in addition cardiac structure and function. The study included 54 adults with serum ferritin (SF) <100 µg/L and/or transferrin saturation <20%, randomized in a 1:1 ratio to 1,000 mg IV ferric derisomaltose or placebo. Following randomization, participants underwent baseline assessments and then received IV iron or placebo infusion. Each participant was followed up at months 1 and 3. At each visit, patients underwent clinical review, measurements of hematinics and haemoglobin (Hb), and assessments of physical function and well-being. The primary outcome was exercise capacity using the 6-minute walk test. Secondary objectives included effects on hematinic profiles and Hb concentration, changes in myocardial parameters assessed with speckle tracking echocardiography and change in patients' quality of life. RESULTS: Between October 2016 and April 2018, 55 from 326 individuals from 3 UK centres attended screening and were randomized. The mean (SD) age was 59.6 (11.7) years, 26 (48%) patients were male, the majority were Caucasians (42; 78%), and 32 (59%) were non-smokers. The mean (SD) body mass index was 30.3 (6.5); SF was 66.3 (44.1) µg/L, TS was 20.1 (7.4) % and Hb was 128.7 (10.1) g/L at randomization for the whole group. Mean (SD) serum creatinine was 186.7 (58.6) µmol/L, estimated glomerular filtration rate was 31.1 (9.6) mL/min/1.73 m2 and urinary albumin and protein/creatinine ratios 60.9 (133.3) and 83.8 (128.4) mg/mmol respectively. The mean (SD) C-reactive protein was 5.0 (4.4) mg/L and the mean (SD) 6-minute walk distance at baseline was 401.2 (120.2) m. CONCLUSION: The Iron and the Heart Trial will provide important information on the short-term effects of IV iron treatment in CKD patients with ID without anaemia on measures of exercise capacity, quality of life and mechanistic data on myocardial structure and function. TRIAL REGISTRATION: European Clinical Trials Database (No. 2014-004133-6; REC no. 14/YH/1209; Sponsor ref. R1766).