The purpose of government is not to take care of its citizens. Governments exist solely to ensure the rights of their citizens. The value of the freedom to be unhealthy far outweighs any government guarantee of health.
Consider the resulting scenario: If the government becomes sole arbiter over what constitutes physical health, anything could be deemed healthy or unhealthy for all citizens under the guise of guaranteeing health. The first to go would be smoking -- that one is under fire already, as is the freedom to eat trans fat food. But what would be next? Alcohol? Sweets? McDonald's? Shall we be forced to exercise as well as visit the doctor regularly?
Health is usually already defined as more than physical absence of disease. Consider if the government is empowered to decide what constitutes mental health? Or spiritual? Mandatory visits to a doctor presumably are not limited to a physician. What of a psychiatrist? If the government is deciding who is and who isn’t healthy, the government is choosing who can and can't be responsible for their own actions.
Don't be fooled. Behind any veil of security lies the ugly face of tyranny. Universal health care is no exception.