First and foremost one area that most people miss when it comes to nutrient absorption is that our bodies on their own produce enzymes to break down food which starts in the mouth with chewing. That being said we are not all alike and the enzymes produced are not all the same for each individual so one of the keys to properly digesting your food is to pay attention to each meal, how that meal sits with you and your bodies response to follow. Following a meal outside of the satiety effect it provides you should not be feeling overly distended, gassy/bloated or bogged down from a meal. If you are your body is having a hard time digesting something that you are eating. (unless its just sheer volume, this is another issue in and of itself) Just to ease your mind you do not have to eat all "bodybuilder" foods in order to make progress. There are several variables that come into play with absorption or what we might think is an absorption issue. It might be a food sensitivity or our bodies might be having a hard time digesting a specific source which might be fats, carbs or proteins. Lets use carb sources for an example. Some of us simply cannot handle the amount of fiber that is associated with these foods and we need to be mindful of that. Oats, brown rice, potatoes are all dense in both soluble and insoluble fiber and if you are consuming these as your primary carb sources you might be left feeling bloated for a greater majority of the time. This does not mean off the bat that you have a sensitivity to these foods but more so that your body might not be able to handle the amount of fiber you are consuming as a result of the carbs you are choosing to eat.
Fats are also a macronutrient source that present issues for a good majority of individuals. Often times it is not that your body cannot handle a specific source of fat but more so that your body is not producing enough bile in order to break the sources of fat down. I for instance use a product called betaine HCL with my meals containing fats to help me better break down and utilize the fat sources. I do not have a food sensitivity to fats as a whole rather my body lacks the ability to naturally break them down on their own fully which leaves me feeling bloated.
Its important to have this knowledge when you are having issues with foods and be able to assess if it is the food itself or an issue with your body properly digesting the food which through proper supplementation it can overcome. This is why I am an advocate of having a set meal plan when you are dieting. There is not magic in eating the same foods every day but more so it just provides more clarity and its easier to assess progress and also any issues that arise if the nutrition is consistent on a daily basis. If every time you eat potatoes you feel bloated following that meal, remove the potatoes or try another type of potato or just add another carb source. There is no rule saying you have to eat potatoes in order to be a successful bodybuilder, sometimes it comes down to trail and error in finding what sits best with you. Through following a consistent diet its easier to pin point sources that might be causing you issues vs always switching up your meals and sources which might leave you feeling constantly bloated and then you have no consistently to be able to pin point what may or may not be causing the issue.