The official answer (which I think is a little unhelpful!)
Q: How many Syns can I have each day?
A: For steady weight loss most people enjoy around 5 to 15 Syns a day, though this varies with height, weight and lifestyle. Members with more than 1 or 2 stones to lose, more physically active members and men tend to lose weight safely and well on more than 10 Syns as a basic daily allowance.