Well, you said "survive", and not "thrive", so run-of-the-mill starvation will solve your problem nicely. Others have observed that muscle mass has high BMR, which is why atrophying muscle is one of the first things the body does in response to starvation. But the single hands-down winner for specific energy consumption is the brain. It provides less than 5% of the body's mass, but consumes more than 20% of its energy. It's a real hog. Anything that causes people's brains to be less active will reduce metabolic energy expenditure (think sedatives, barbiturates, etc.).
Batteries Already Included
In general, you don't need to take a pill to run on fewer calories: the body is already designed to respond to whatever level of calorie consumption is available, and has both short-term and long-term responses to starvation. For instance, people who survived World War I and II have epigenetic markers of starvation on their DNA, which has been passed down through multiple generations. The human genome is literally programmed to cope with inter-generational food shortages, and will typically respond by making people smaller and weaker (and probably more dimwitted, too).
What you might want to do is tailor the body's response to insufficient calories. Some folks might want to specialize in martial prowess and are willing to give up some gray matter in exchange for retained muscle mass. To give you an idea how much variability we have, consider that humans and chimps descended from a common ancestor not that long ago, but chimp muscles are about 2x as efficient as human muscles, pound for pound. That's why chimps can be less than a quarter our size and still beat the crap out of untrained humans.
Other folks may want more brainpower at the expense of muscle mass, and are walking twig eggheads. It would be like a parody of high school cliques, but for a scientifically driven reason. Except that you wouldn't have the good-looking, muscle-bound smarter-than-you-expected quarterback prom king, unless he was also getting twice as many calories as everyone else.
Some folks may prefer endurance to strength, and sacrifice fast-twitch for slow-twitch muscles. This can be done to some extent purely through physical training; but training just activates gene networks, so a sufficiently advanced technology should be able to shortcut the effect with a drug.
No Free Lunch
If you want hard science, you can't cheat thermodynamics and conservation of energy. Biological bodies are already pretty darned energy-efficient (except when they have a massive glut of energy, of course; c.f. obesity). The best you can do is reduce the energy demands via different sacrifices. Note that indigenous people are typically short and lightweight compared to well-fed folks in developed countries. Going from subsistence hunting/farming to factory farming can add a good 4-6" of height to the population average. Obviously, you can go the other direction, too.