A 50-year-old man presents with feeling tired and unsteady on his feet. He has a poor appetite and has lost 10 lb. On examination, he appears cachectic, his heart and lungs are normal, but his liver span is 18 cm. His lab tests show a very low magnesium level (0.7 mEq/L). On further questioning, he reports drinking heavily since losing his job. Which of the following is the most likely explanation for his low magnesium level?