Definition: According to Duffy, Gangster, and Pagon, 2002, the definition of social undermining in a workplace is, behavior intended to hinder over time and not allowing a person to establish or maintain positive interpersonal relationships. Undermining in the workplace may happen due to insecurities felt by the offender. The offender may disparage the ideas of others in an attempt to make up for her/ his own lack a creativity, or it may stem from a desire to feel powerful.

The article from which this passage is taken suggests that an employee receiving adverse comments from their colleagues may behave unkindly when at home. By mobilising more during their working day they will burn more calories which, in turn, will make them feel less irritable. Thereby reducing the negative effects of the undermining comments from their colleagues.

No idea. "Undermining" suggests some form of workplace bullying here. Maybe a bit of body shaming for an overweight 195 pound male?

The quote does not make sense. I suspect that what it is intended to mean is that sitting around all day in an office (which is what 'working,' means for many people) is bad for a person's health..... it could be said that 'working in an office undermines a person's physical health.' To undermine is a verb which means to reduce somebody's ability to do something or to reduce the effect of a key principle. So working in an office undermines a person's ability to live a healthy life. For example, if you are a boss and you have somebody in your team who is openly disrespectful of you in front of other members of the team, then you might say "This person's behaviour undermines me," because the way they are behaving reduces your ability to be a boss (or could reduce it) as other members of the team may lose respect for you. In fact, this is the true meaning of the term "workplace undermining," which is why it makes zero sense in the above context. Donald Trump's attempted bar (ban) on people claiming asylum in the United States of America can be said to "undermine both the letter and the spirit of the Geneva Convention," = this means that international law cannot have the effect that it is supposed to if one country opts out of the rules.