There is currently little research on the effects of reduced left ventricular ejection fraction and altered nutritional status in patients with acute myocardial infarction. We therefore examined the interrelationship between the parameters of left ventricular dysfunction after acute myocardial infarction and changes in the Geriatric Nutrition Risk Index (GNRI) and the Nutrition Status Control Index (CONUT). Based on the evidence, frailty is considered to be an important factor affecting the prognosis of cardiovascular disease, so it is important to detect malnutrition early to prevent adverse cardiovascular events. This study was an observational, prospective study that included a total of 73 subjects who presented at the 3-month AMI follow-up. All subjects were subjected to laboratory tests and the groups were divided as follows: group 1, in which we calculated the CONUT score, (CONUT < 3 points, n = 57) patients with normal nutritional status and patients with moderate to severe nutritional deficiency (CONUT ≥ 3, n = 16). In group 2, the GNRI score was calculated and out of the 73 patients we had: GNRI ≥ 98, n = 50, patients with normal nutritional status, and GNRI < 98, n = 23, patients with altered nutritional status. The results of this study showed that we had significant differences between LVEF values at 3 months post-infarction where, in the CONUT group, patients with altered nutritional status had lower LVEF values (46.63 ± 3.27% versus 42.94 ± 2.54%, p < 0.001) compared to CONUT < 3. Also, in the GNRI group, we had lower LVEF values in patients with impaired nutritional status (46.48 ± 3.35% versus 44.39 ± 3.35%, p = 0.01). It can be seen that LVEF values are improved at 3 months post infarction in both groups, in patients with impaired nutritional status and in patients with good nutritional status. Patients with impaired nutritional status have lower ejection fraction and worse outcomes in both the CONUT and GNRI groups at 3 months post acute myocardial infarction.