Human
In general, humans can survive for two to eight weeks without food, depending on stored body fat. Survival without water is usually limited to three or four days. About 36 million humans die every year from causes directly or indirectly related to starvation. Childhood malnutrition is also common and contributes to the global burden of disease. However global food distribution is not even, and obesity among some human populations has increased rapidly, leading to health complications and increased mortality in some developed, and a few developing countries. Worldwide over one billion people are obese, while in the United States 35% of people are obese, leading to this being described as an "obesity epidemic." Obesity is caused by consuming more calories than are expended, so excessive weight gain is usually caused by an energy-dense diet.
Space and survival
Many of the same existential risks to humanity would destroy parts or all of Earth's biosphere as well. And although many have speculated about life and intelligence existing in other parts of space, Earth is the only place in the universe known to harbor life. Eventually the Earth will be uninhabitable, at the latest when the Sun becomes a red giant in about 5 billion years. Mankind, or its intelligent descendants, then has to leave the Solar System to ensure survival of the human species.