"Soup," he ordered. It was the cheapest item he could find.
The counterman stared at him, obviously spotting his Earth origin. "You adjusted to synthetics?"
Feldman nodded. Earth operated on a mixed diet, with synthetics for all who couldn't afford the natural foods there. But Mars was all synthetic. Many of the chemicals in food could exist in either of two forms, or isomers; they were chemically alike, but differently crystallized. Sometimes either form was digestible, but frequently the body could use only the isomer to which it was adjusted.
Martian plants produced different isomers from those on Earth. Since the synthetic foods turned out to be Mars-normal, that was probably the more natural form. Research designed to let the early colonists live off native food here had turned up an enzyme that enabled the body to handle either isomer. In a few weeks of eating Martian or synthetic food, the body adapted; without more enzyme, it lost its power to handle Earth-normal food.
The cheapness of synthetics and the discovery that many diseases common to Earth would not attack Mars-normal bodies led to the wide use of synthetics on Earth. No pariah could have been expected to afford Earth-normal.
Feldman finished the soup, and found a cigarette that was smokable. "Any objections if I sit in the waiting room?"
He'd expected a rejection, but the counterman only shrugged. The waiting room was almost dark and the air was chilly, but there was normal pressure. He found a bench and slumped onto it, lighting his cigarette. He'd miss the smokes—but probably not for long. He finished the cigarette reluctantly and sat huddled on the bench, waiting for morning.
The airlock opened later, and feet sounded on the boards of the waiting-room floor, but he didn't look up until a thin beam of light hit him. Then he sighed and nodded. The shoes, made of some odd fiber, didn't look like those of a cop, but this was Mars. He could see only a hulking shadow behind the light.
"You the man who was a medical doctor?" The voice was dry and old.
"Yeah," Feldman answered. "Once."