If you exercise first you're more of what you just ate. Amino acids from protein is a source of raw materials. It also acts as a signalling molecule, triggering the growth of new muscle.
In healthy adults, about 0.25 grams of protein per kilogram of body weight is ideal for a given meal. That target can be hit three to five times a day. If you exercise before you eat, your muscles become more sensitive to protein's signals.

Exercise first

... you're less of what you just ate. Factors that make muscles less sensitive to protein signalling is ageing. Much of the muscle loss takes place during short periods of inactivity, such as bed rest after a knee replacement.
A simple fix is never to feed someone in a hospital bed unless absolutely necessary. Instead, get them up and make them shuffle to their food. The proportion of protein eaten should be higher to ensure sufficient muscle synthesis signals.

If you’re inactive

You can mess up a lot more in one week than you can improve in six months of training.
Through a six-month strength-training program, one study showed older people could increase their muscle with an impressive 2.9 pounds. However, another study showed that bed-rest could make you lose 3.1 pounds of muscle within just one week.

Building muscle

Over 50 percent of the protein just eaten will make it into a person's circulation within five hours; the rest will be taken up by tissues in the gut or will not be absorbed. In the same period, 11 percent of ingested protein is incorporated into new muscle.
Overall, we break down and rebuilt up to 2 percent of muscle each day and completely rebuild ourselves every two to three months.

You are what you just ate

A study found that ground beef protein resulted in more tracer amino acid absorbed as well as it being absorbed more quickly. Another study showed that people who retained more of their own teeth tended to have more muscle.
Eating lying down will slow down protein digestion and possibly reduce the synthesis of new muscle protein.

Chew your food

The protein and muscle guru Luc van Loon wants you to bulk up—and keep what you've got.

Chew your food

4 Laws of Muscle

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The Science Behind Muscle Growth

#163 - Layne Norton, Ph.D.: Building muscle, losing fat, and the importance of resistance training