Food

§ Cities need food to survive. The more food available, the larger the city can grow. Cities get food by working the lands around them (spaces that are both within the national Borders and within their City Radius.)
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Food Consumption

§ Each citizen (population point) in a City consumes two food per turn to survive.
§ Excess Food: If a city generates more food than its citizens consume, it stores the excess food. Once it has enough stored food, all of its stored food disappears and the city's population increases by one. As a city grows larger, it takes more stored food to increase its population. The Granary stores half of the food needed to grow to the next size, thus accelerating the speed at which the city's population increases.
§ Insufficient Food: If a city does not produce enough food to support its population, the city will begin to lose stored food. When a city loses all of its stored food, one of its citizens starves. Starvation will continue until the city is producing enough food to support the population.
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Health

§ A city's Health has a large effect on its food production; when a city slips into unhealthiness, it requires additional food to feed all of the citizens. See that section for more details.
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Working the Land

§ Each type of Terrain generates a specific amount of food, as does each Resource. You can often increase a square's food output by Improving it with your workers; for example, grassland tiles next to fresh water can be irrigated with farms to produce additional food.
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Buildings and Wonders

§ Some Buildings and Wonders can increase a city's food output (for example, the Lighthouse building adds more food to water tiles).
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Civics

§ The State Property civic increases the food output of certain improvements.
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Technology

§ Some Technological Advances allow your workers to construct improvements increasing your land's food output, while other advances make already-existing improvements more efficient.