The fastest safe way for a man over 65 to lose weight is to create a moderate calorie deficit, while protecting muscle with protein and strength training rather than doing a crash diet.23512

The most effective approach is:

• Eat fewer calories than you burn; older-adult guidance still emphasizes calorie control, portion limits, and tracking intake.2312
• Prioritize protein at each meal to reduce muscle loss; sources for older adults recommend roughly 25–35 g per meal or about 1 g/kg body weight per day.131113
• Lift weights at least 2 times per week; resistance training helps preserve muscle while losing fat.31011
• Do regular aerobic activity, aiming for at least 150 minutes per week of moderate-intensity exercise.51213
• Cut liquid calories, sweets, alcohol, and highly processed foods, which makes the calorie deficit easier to maintain.1311
• Sleep consistently and hydrate well, since both are repeatedly recommended in older-adult weight-loss guidance.101113

If you want the quickest reasonable rate, a common safe target is about 1–2 pounds per week, not rapid crash weight loss.5 For many men over 65, that means combining:

• a 500–750 calorie daily deficit2
• high protein
• full-body strength training
• walking or other cardio most days

If you have diabetes, heart disease, kidney disease, take diabetes medications, or have unintentional weight loss, it is important to check with a clinician before starting a weight-loss plan, because older adults can lose muscle and run into medication or nutrition problems more easily.31112

If you want, I can turn this into a simple 7-day weight-loss plan for a 65+ man.