How can someone put away 5K of calories a day and still be lean?
Interesting question - I'll pretend it wasn't rhetorical. The answer would have to be 1) thousands of kcal worth of exercise (=lots of regular fat & carb oxidation) + 2 very low fat, very high CHO diet. Probably only happens in extreme vegan athletes, lol. But for sure, one can build muscle and lose fat at the same time (see my other posts for references).
Overfeeding studies using similarly large kcal amounts (and little or no exercise) all show inceases in LBM and fat stores, with the most muscle and least fat being acheived via a very high carb, very low fat diet (see my other posts for more info than you may desire). The higher the percentage of kcal from fat, the greater the unwelcome shift in the LBM:F ratio.
A pair of related studies used a 4300 kcal hi-carb diet with 20% fat - since their activity level was very low the energy surplus was high: they gained about 1/2 lb of muscle per day (about 60 grams of actual protein out of 90 consumed - that's a 66% retention caused by elevated insulin levels via carbs). In addition they gained 70g fat per day, a near 1:1 recomposition ratio which is one of the best results in the literature (since most overfeeding studies are high in fat, the ratios are high, varying from 5:1 to up to 20:1, meaning each net gram of protein gained is accompanied by 5-20 grams of fat (yikes)! Thus the science shows us that the more we shift metabolism away from fat processing and towards carb processing, the more muscle and less fat is gained. For example another study showed that if you eat zero fat in a day, while meeting hunger (kcal maintenance) - you will still be burning over a gram of fat an hour. In other words it is possible to consume a surplus energy WITHOUT fat gain - but it is extremely rare because it doesn't take much fat intake to shift things in the wrong direction (of course exercise is another method for shifting in the right direction, as about half of energy used derives from fat. Personally, my program involves both very low fat and fasting (oxidizes about 4-6g fat per gram protein) - all in an effort to shift metabolism away from the highly efficient fat processing system!