Middle Age: Why are fit people like me keeling over?

I'm 46, a lifelong non-smoker, moderate meat and alcohol intake (less than 20 units a week), cyclist/walker/runner/swimmer, BMI 21-23, and no family history of heart trouble. I should have no worries, but I'm mindful of the ostensibly healthy men who have keeled over in middle age (Alan Ball, Geoff Hamilton, Ken Barrington, Leonard Rossiter, Robin Cook, Billy Bremner, Douglas Adams...). I've always pushed myself hard when I run and swim. Should I ease up now I'm over 45? What medical tests should I request? I do get brief spells of apparently fluttering heart-beats, but I've always had this. Should I take these as a warning?

Dr Fred Kavalier answers your health question:

You are leading an exemplary life, and doing everything possible to reduce risks of cardiovascular disease, and of cancer and other conditions. There's no need to slow down just because of your age. Your body will tell you when it's time to slow down. Check your blood pressure; high blood pressure would counteract many of the positive things you are doing. A cholesterol test would be useful if it turned out that your cholesterol was very high, but with your history and lifestyle, it is far from essential. The occasional fluttering heartbeat is not a sign of heart disease. If you are looking for a guarantee of eternal health, no tests will do that. Keep up the good work.

Please mail your questions for Dr Fred to health@independent.co.uk. He regrets that he is unable to respond personally to questions.