The 5:2 Diet
This diet is a form of intermittent fasting. You eat normally for 5 days and restrict your calories to between 500 and 600 for 2 days. You can choose which 2 days are restricted as long as there is at least 1 non-fasting day between them.
Concerns - There has been very little study done on this type of fasting. You can't just eat whatever you want on non-fasting days because if you binge on junk/processed food then chances are you won't lose any weight and might actually gain some. You also want to be careful when picking your fasting days as too much movement and activity could cause headaches, light-headedness and dizziness. You will also probably have to reduce your weekly amount of exercise to accommodate low levels of energy.
On a plus note, when done correctly (proper eating of whole foods on non-fasting days) you can improve insulin sensitivity and reduce leptin (hunger hormone) levels long term.