This sounds a bit like The Forever War by Joe Haldeman.
The main character goes on a time dilated journey into space to fight a war, and returns to the far future of an Earth where the Ration War was fought over food scarcity, starting with the implementation of calorie rationing and a basic allowance of calories:
'Let me do some history to you. I guess the really important thing that happened after you left was the Ration War.
'That was 2007. A lot of things happened at once. Locust plague in North America, rice blight from Burma to the South China Sea, red tides all along the west coast of South America: suddenly there just wasn't enough food to go around. The UN stepped in and took over food distribution. Every man, womand, and child got a ration booklet, allowing thim to consume so many calories per month. If tha went over ther montly allotment, tha just went hungry until the first of the next month.'
This war ended up making calories the currency of Earth:
When the world went on a single currency, they'd tried to coordinate it with the food rationing in some way, hoping to eventually eliminate the ration books, so they'd made the new currency K's, kilocalories, because that's the unit for measuring the energy equivalent of food. But a person who eats 2,000 kilocalories of steak a day obviously has to pay more than a person eating the same amount of bread. So they instituted a sliding 'ration factor,' so complicated that nobody could understand it.
People trade this currency for actual food:
Food prices were astonishing, except for grains and legumes. I insisted on splurging on some good red meat: 1500 calories' worth of ground beef, costing 1730K. The same amount of fakesteak, made of soy beans, would have cost 80K.
You do have to work if you want to get more calories, but finding a job isn't very easy, which has created a black market of jobs where workers outsource their work to other people.
'The government, they tell us, handles the distribution of all natural resources. And there aren't many resources more valuable than empty jobs.'
Jobs are assigned on a basis of need. And you've got to be living on rice and water before the Employment Board considers you needy.'
'Well, hell, it's a bureaucracy – there must be somebody I can pay off, slip me into a good–'
'No. Sorry, that's one part of the UN that's absolutely incorruptible. The whole shebang is cybernetic, untouched by human souls. You can't–'
'But you said you had a job!'
'I was getting to that. If you want a job badly enough, you can go to a dealer and sometimes get a hand-me-down.'