Can I eat some junk food while on a diet?

It depends what your definition of ‘some’ is. If some is a little than you can have some. If some is a lot, it will probably cause you to reverse your weight loss. Losing weight is about eating less calories than you burn.

Calories

If you are limiting yourself to 1,700 calories per day, it’s very easy to go over if you are eating junk food. You should be eating foods that are low in calories but will fill you up. Junk food is the complete opposite of what you are looking for: it has a lot of calories, and since it’s mostly made of sugar, does not fill you up at all.

Take a bagel and a doughnut. They both contain around 300 calories. Unlike the doughnut, the bagel is made up of mostly complex carbs which are digested slowly and fill you up. The doughnut is made up of simple carbs (sugar) and will not fill you up. When you are done with your 300 calories bagel, you will be satisfied for an hour or two. When you are done with your 300 calorie doughnut, you will probably go for another one.

While one doughnut here and another one there will not completely mess up your diet, making a habit out of picking junk over good quality food will.