Sine qua non.

 A book just published is The Laws of Connection by David Robson - that describes the central importance of: a balanced diet, not smoking, limiting alcohol intake, not being overweight, regular exercise, a good night's sleep and not skipping breakfast. However as important is being socially connected. This is the sine qua non that underpines everything else. Not being or feeling lonely is necessary for good physical and mental health. Take note everyone as loneliness is a killer.