I’d suggest to unmarry fitness and nutrition for now.
In nutrition, you can basically do a hierarchy as so often. High to low impact:
Net Caloric Intake < Macronutrient Composition < Micronutrient Composition < Intake Timing
You can lose weight on a twinkie diet and you can gain on an IF raw vegan diet. However, that will usually bring you to some kind of nutrient deficiency. (Workouts impact net caloric intake but you can’t outrun a bad diet. A marathon should burn around 2000 calories, so 2 big macs fries and a 2L coke will easily offset that.)
Usually it’s a good idea to move away from “good food bad food” and more towards IIFYM (If it fits your macros). Is a burger bad? Maybe it has too many carbs, but maybe it fits today’s macros and actually would not impact you negatively.
People recommend cooking yourself, because that usually means you’ll have less processed foods and a better control of carbs and fats. Processed foods tend to contain more carbs and fats, because these tend to make food yummy (Chocolate anyone?). Another way to think about it is calorie density. Brokkoli? Huge volume, few calories. Cheese? Dense block of fat. (100g cheddar : 400kcal : 1150g Brokkoli)
Ranging on the caloric intake Jan T is obviously doing better than Russ F. Russ F may be doing better on the Macro and Micro nutrients, however, with a standard diet, you will most likely not become nutrient deficient, turns out you could survive on potatoes pretty much. The health impact of abdominal fat, unfortunately, is pretty high (cardiovascular, inflamation scores, longevity). Ironically, that is often a problem, not every fat person digs into the golden M for 3 meals a day. They eat a nice salad, but spice it up with salmon and mozzarella, which, depending on the amount, can bring a salad to over 1000kcal easily. (With 2k kcal being the “average adult male” intake.) If you then “treat yourself because you had just a salad for dinner”, weight easily increases.
Just a ‘quick’ note on fitness (cardiovascular etc) of thin and fat people. It’s basically impossible to see if a person works out or not. Granted, an overweight person should probably not run said marathon due too the impact on joints (mostly knees, bit feet and hip), but you don’t know if they regularly do Crossfit, lift a ton or are a couchpotato. On thin people it’s often a bit easier to see if they’re training weights, due to fat not “masking” the sweet sweet biceps. But do they run? Aerial acrobatics? Climbing? So many climbers look like they are just thin, then scale a window pane without breaking a sweat.