Myth: Keeping a food diary is a reliable way of monitoring and controlling what you eat.

Truth: Even when we're making an effort to be conscious about what we're putting into our bodies and how active we're being, we often give ourselves more credit than we deserve.

"People tend to overestimate their physical activity and underestimate how much food they eat," says Stanforth. "They consistently think they've worked out more and consistently think they've eaten less."