A 5-month-old Russian Blue cat's loss of appetite can stem from several factors, including a monotonous diet, digestive issues, or hairballs. Owners can address this by diversifying their cat's food, introducing digestive enzymes, and using hairball remedies like cat grass or hairball paste.

1. Change the Diet
Sometimes, cat owners might opt for a simple, repetitive diet, which can lead to boredom and a lack of appetite in cats. To tackle this, consider adding variety to your cat's meals. Besides their main food, you can supplement with cooked chicken breast, beef, or fish 3 to 4 times per week.
2. Supplement with Digestive Enzymes
Appetite loss might not always be diet-related. A deficiency in digestive enzymes can hinder a cat's ability to digest food, directly impacting their willingness to eat. In such cases, adding digestive enzymes can be helpful. You can try offering honey water, plain yogurt, or a specialized feline digestive supplement like "Miao Xiao Li" (or a similar product).
Lastly, sometimes it could just be a case of a fussy cat. You can try withholding food for a meal or two to see if hunger encourages them to eat. It's also important to consider if hairballs are causing the issue. Ensure you're regularly providing cat grass or hairball paste. Additionally, always have fresh, lukewarm water available for your cat to drink.