It is not specifically, the Strength Training, that is causing Fat Gain. It is the CALORIE SURPLUS which is causing it…along with the TYPE OF CALORIES that you are consuming.
To be clear, Calorie Surplus is MANDATORY for you to grow. If you are in a Calorie Deficit status, you will NOT grow.
In a Calorie Surplus state, your body then distributes these Calorie Surpluses towards Muscle growth and Fat Storage etc… You cannot get only one to happen without the other. However, you can Skew this as, much more towards Muscle Growth and less towards Fat Storage.
Strength Training gives your Body a Reason to Grow Muscle. At that time, it is looking for something to Fuel that Growth. Therefore, you need to keep yourself in a Calorie Surplus state. If you are in a Calorie Deficit status however, then, no meaning growth will take place.
Note that MUSCLES NEED PROTEIN.
So, for sake of Calorie Surplus, if you eat JUNK FOOD a lot, then, proper fuel is NOT available in your body, for Muscle Growth.
Also, while Carbohydrates are Absolutely Essential, if you do NOT add Good Amounts of Quality Protein with your Meals, then again, proper fuel is NOT available for Muscle Growth.
So, the best advise would be,
- To keep up with your Strength Training (follow a proper, well-designed Training Plan for this, of course).
- Maintain a Calorie Surplus state
- Besides Good Carbohydrates and Good Fat, be sure to add a Good Amount of Quality Protein with ALL your meals. Around 1 gram Protein per POUND of your Body Weight is a GOOD START.