Jerry Maguire said:
To lose weight, or fat, you need to maintain a calorie deficit.
To gain weight, or muscle, you need to maintain a calorie surplus.
Hmmmmm.. who says?
Most articles/websites would have you thinking in 24hr periods. So, if you are in a calorie surplus by the end of the day, you gain weight (muscle and/or fat). If you're in a deficit by the end of the day, you lose weight (muscle and/or fat).
That's not true.. say you were in a calorie deficit the first half of the day through cardio/calorie restriction (plus low carbs to keep insulin levels low and fat burning alive) and you lose weight/fat/muscle that part of the day. Then you work out at noon, and eat a calorie surplus for the rest of the day. You gain muscle/weight/fat the second half of the day.
The net calorie result of this sytem may well be zero. I.e. you eat 500 calories under 'maintainance' for half the day, and the other half you eat 500 over 'maintainance'. But the net result is actually a small amount of muscle gained and fat lost (provided the food you eat is of the correct type and quantity).
Muscle gain and fat loss are down to nutrient availability and hormonal responses at certain times of day, not just at the end of the day. You're gaining (or losing) muscle every second of the day and losing (or gaining) fat every second of the day too.
A good strategy for people wanting to lose fat and gain muscle would be to keep calories high (and clean), meaning high protein, moderate fat and carbohydrates appropriate to their body types. Some body types do better on low carbs and high fat. Others mod carb mod fat. Depends on you. Everyone has their sweet spot.
Throughout the day your high protein intake, high vegetable intake (both protein and vegetables are thermogenic) plus lots of water, green tea and cardio/high activity levels keep your metabolism running hot, which in turn helps to mop up excess calories/potential fat gain caused by your eating, as well as more efficient nutrient partitioning and insulin sensitivity.
The net result of this (high clean calories suited to your body type with high protein and vegetables intake + cardio, green/white tea, high metabolic activity + heavy lifting) will be a change in body composition, muscle gained and fat lost.
Warning: this is a gradual change and won't give you incredible results overnight (nothing will) - if you want to lose 10lbs of fat and gain 10lbs this will work just fine. But if you want to gain (or lose) 50lbs then you'll most likely be frustrated as it'll take you a while to get there.
(and if your goal -IS- to gain 50lbs while losing fat at the same time (or keeping your abs crisp), reassess your goals..)