Background: No solution has yet been found for the nutritional issues of Parkinson’s disease (PD) patients with weight loss, which affects their prognosis. The objective of the present study was to investigate whether a defined-formula diet rich in ω3 fatty acids improved nutrition through the effect of ghrelin in PD patients with weight loss. Method: Weight, serum total protein, albumin, lipids, serum ghrelin, serum acylghrelin, leptin, and the Unified Parkinson’s Disease Rating Scale were monitored for approximately 3 months in 11 subjects given a defined-formula diet rich in ω3 fatty acids for 3 months and 5 control subjects who received no such treatment. Results: No significant changes from baseline in serum ghrelin, acylghrelin, and leptin levels were noted after administration of the nutritional product. Meanwhile, compared with the control, changes from baseline in the levels of albumin and weight were significant after the nutritional therapy rich in ω3 fatty acids (P < 0.05). Conclusion: Administration of a medium-chain fatty acid-rich product has few effects on motor functions and activities of daily living in PD patients with weight loss, but it is expected to offer effective ghrelin-mediated nutritional improvement.