A 65-year-old man who had a 25-lb weight loss over the previous 6 months is diagnosed with adenocarcinoma of the distal esophagus. He undergoes a transhiatal esophagectomy complicated by a cervical leak. He is receiving enteral feeds through a jejunostomy tube. After a week, his physicians wish to assess his nutritional resuscitation. Which of the following is the most accurate measure of adequacy of his nutritional suppo?