According to Men's Fitness, fat burners, or thermogenics, refer to blends of stimulants and herbs that aid in the burning of calories. The supplements work by slightly increasing the temperature of the body.
Men's Fitness states that some types of fat burners only burn calories as heat, while others claim to enhance the release of adrenaline, suppress appetite and increase the rate of metabolism. According to Men's Fitness, fat burners increase the levels of cortisol, which is a stress hormone. Thus, if a person is suffering from anxiety, taking fat burners may worsen the condition. Most fat burners contain caffeine, which is why they are recommended to be taken in the morning. Taking fat burners after 2 p.m. may disrupt sleep patterns.