In the short term it's in fact possible to change your body composition without eating 'clean'. But that's in the short term, in the long term it's going to be very difficult to maintain an ideal
The body, as complex as it is, breaks every macro-nutrient (protein, carbohydrate and fat) down into the same respective molecule for energy, regardless of whether it came from 'clean' food or 'junk' food; all carbohydrates are broken down to glucose, fats to fatty acids, and protein to amino acids. Therefore someone that obtains the same amount of protein, carbohydrates and fats from 'junk' food sources as someone who obtains them from 'clean' sources, is going to utilise the energy the exact same way.
However, this can't go on forever as a perfect system. Processed foods can cause different reactions in the body, for example:
- a meal with a high glycemic load will cause a large insulin spike, which can lead to fat gain at the wrong time as well as lead to health issues if allowed to occur to regularly,
- processed foods cause inflammatory responses in the gut, toxicity build up and a reduction in good bacteria,
- lack of essential nutrients such as omega-3 can cause hormonal disturbances,
- lack of fiber can lead to digestive issues, malabsorption of nutrients and irritable bowel syndrome, just to name a few.
Just as important to what you eat and how much of it, is when you eat it, also known as 'nutrient timing'. Your body utilises nutrients differently depending on the time of day and your activity (ie sitting down compared to working out). By knowing when to eat what macronutrients, you can significantly increase the potential of your results, increase your recovery rate, fat burning potential, exercise intensity, muscle building potential, sleep quality and mental performance.
So although you may get results in the short term eating 'junk' food that fits your energy requirements, you're not going to get the best potential results and thrive in the long term. This does however lead me to say that moderation is also healthy! You don't have to be one extreme or the other and you certainly don't need to eat 'clean' 100% of the time; your body can handle these foods every now and then. What it can't handle is constant nutrient void food that consistently puts it under great pressure to digest, assimilate and deal with it's adverse inflammatory and hormonal impacts. Moderation is still key!