I am still working out the same amount (weight training, cardio, Pilates, yoga), but I can see the formation of belly fat. I have modified the Atkins diet to limit fat and to add carbs. What would be the best strategy now: return to Atkins since it worked when I was fat, try a low-fat type diet or what? Also, is any type of exercise more effective than another for this problem?
I guess what we need to do is define what type of low carb diet is most effective without causing some type of disease. Carbohydrate is essential for proper health. The type and quantity of carb is really the question. If you were eating a lot of refined carbs throughout the day, insulin levels remain high and fat deposits increase - unless you are a marathoner or adventure racer or some other athlete who moves for days without rest. These people need calories of any kind, any time. Of course, most of us are not human furnaces, so our calorie needs are minimal in comparison. The Harris-Benedict equation is the most effective guesstimate for calorie needs. In my opinion, what we are talking about is calories in versus calories out. It doesn't seem to matter what type of cals you eat. If you don't burn them, you get fat.
While I was in graduate school, Dr. Phillips, a super athlete and professor of Exercise Physiologist, told the class, "fat burns in a carbohydrate flame." This means that without sufficient levels of carbs, fat oxidation is retarded. How much carb is needed to fuel this flame is the ultimate question.
You mentioned an accumulation of waist fat. The first question is, have you changed your training program? If so, how? And if you have not, you should. There are several opinions on improving fat oxidation. They all deal with increasing energy expenditure. Two suggestions for expending calories is 1) perform the usual 50 percent VO2 pace (about one hour, three times/week) or 2) run or walk outside for 30 to 90 minutes. Carry some fluids (watered down Gatorade) and hit it. Run as fast and far as you like, and then walk as slow or fast and long as you feel. No set distance or pace. This is known as Fartlek training. It works well for most people and is used once or twice a week.