Question
I am trying to build my muscles and also burn fat, mainly from my stomach.
I am doing a weights session (with sit ups) every two days the the alternate days I am using the exercise bike for 30 minutes and also swimming 500 metres in the pool. I am resting 1 day per week, but I find I am not losing any fat from my stomach but rather from the muscles in my arms and chest instead. I am careful with my diet and avoid alcohol. Can you please advise?
Answer
Sounds like a good have a good fitness plan and protocol. Specifically the inclusion of the rest day, cardio and cross-training with swimming. You have not identified your age and it is also a factor as the older we get the more difficult it is to manage and maintain the middle section. It simply requires more work in all the variables (nutrition, cardio and strength conditioning).
That being said, while you say you are “careful with your diet,” 9 times out of 10, it is this component that will make or break the program. Clean healthy eating is crucial. For example, sugars and salt are easily hidden in most processed foods and can amount to excess calories and therefore extra work on the mid-section.
You should pay primary attention to your dietary habits (including your snacking habits) and continue the strength training. With strict adherence, that belly should fall into line.

