When someone gains weight by eating clean, the composition of weight gain can vary depending on several factors, including their overall calorie intake, macronutrient distribution, exercise routine, and individual genetics. 

Eating clean typically refers to consuming whole, minimally processed foods that are rich in nutrients. If someone is in a calorie surplus while eating clean, the weight gained can be a combination of muscle and fat, although the exact ratio will depend on various factors.

To maximize muscle gain and minimize fat gain during weight gain, it’s important to focus on a few key factors:

1. Calorie surplus: To gain weight, you need to consume more calories than your body burns. However, the surplus should be moderate and controlled to minimize excess fat gain.

1. Adequate protein intake: Protein is essential for muscle synthesis. Including sufficient protein in the diet, ideally spread throughout the day, can support muscle growth and repair.

1. Resistance training: Engaging in regular strength training exercises is crucial for stimulating muscle growth and promoting an anabolic environment in the body.

1. Overall nutrient balance: While focusing on clean eating, it’s important to ensure that you’re consuming a balanced diet that includes an appropriate amount of carbohydrates, proteins, and fats. Each macronutrient plays a role in supporting muscle growth and overall health.

It’s worth noting that the rate and extent of muscle gain can vary between individuals, and there may be genetic factors that influence the proportion of muscle and fat gained during weight gain.

Monitoring body composition through methods such as body fat measurements or visual assessments can provide a better understanding of the changes occurring during weight gain.

Working with a qualified fitness professional or registered dietitian can help develop an appropriate nutrition and exercise plan tailored to your specific goals, helping you optimize muscle gain while minimizing fat gain.