I know you've heard it before, the speech from me and many others about diet and exercise combo. Many people put a number on it saying weight loss is 80% diet 20% physical activity or whatever. I'm not going to put a number on it because there is no scientific research (that I am aware of) that can give an exact number. But I will tell you from personal experience, seeing it within my clients, and studying the research, diet is a large overall contributor to weight loss.
A little piece of advice, you need to understand your nutrition needs, (calories, food groups, vitamins) in order to see the weight loss or muscle growth changes you are working for.
It can't all be done in the gym.
If you are working out to the best of your abilities and have a relaxed diet (diet meaning the foods you habitually eat, not a restriction) you will not see the physical improvements you are working hard for in the gym.
My experience: I have always dreamed of a toned tummy. I would go to the gym for 2 hours a day and after a year I didn't see the progress I was hoping for. I wasn't eating entirely bad; I was actually eating my definition of healthy. A typical day would consist of having eggs and spinach for breakfast, snacks throughout the day, something consisting of ground turkey for lunch, chicken and brown rice with veggies and whole wheat toast for dinner, followed by a mid evening snack consisting of peanut butter and in a tortilla. Each weekend I would allow one day to eat whatever I wanted which was usually a burger or pizza. This wasn't working for me, although it sounds considerably healthy compared to many other things I could have been eating.
I started seeing the progress I was working so hard for when I finally got my diet lined up with my workouts. I think the barrier many of us unknowingly struggle with is thinking we are eating all the right things at the right times. For me, it was a matter of being uneducated.
3 things happened when I began to see progress:
1) I started to understand how many servings of each food group to eat for my caloric target.
2) I started corresponding my workouts with my diet. (Depending on my workout for the day I will eat more or less of a food group.)
3) What time throughout the day I should be consuming each food group.
Your nutrition plan (I choose to say nutrition plan instead of diet plan because of all the negative connotations that comes with "diet") should be set up along side your workout program. It doesn't need to be overly complicated. You do not need to weigh your food or count your macros. What made it easier on me when I was starting was counting my calories for a few weeks (with the app Fooducate) to ensure I was eating enough. Then I used portion control containers which allowed me to eat a number of colored containers per day corresponding with the food groups. After a few months of tracking and trial and error, my diet became a habit and I no longer track anything. I now just eat the foods I love and know to be healthy, at the right time of day, and coordinate my workouts with my meals for each day.
If there is one piece of advice you take away from my blogs, it is to get your diet on track with your exercise. The two go hand in hand.