Twenty years ago I thought: "What's the difference between eating a bloody steak and killing my dog, slitting him open and roasting him?" I've always loved animals but it was around the late 80s that I realised I had to go vegetarian. A lot of things converged in my life then – musically, emotionally – but mainly it was my love of animals and spending so much time touring that made me decide I had to change my diet. In the Runaways we used to eat a ton of junk food. So something had to give.

My attitude to food has been a slow burner. I grew up in Philadelphia. My parents never aspired to eat well; they didn't cook too great or experiment. I was a major carnivore. Burgers, steaks, the rarer the better; I got barbaric pleasure from eating meat that was bleeding.

When the Runaways was drawing to a close, I ate nothing but breakfast food for a year. Just French toast, pancakes and muffins, and I started to lose the taste for meat. Then I read John Robbins's Diet for a New America which got me thinking about farm to plate and soon I was thinking about slaughterhouses all day long. That was pretty disturbing. I'm not a preacher, I'm not dogmatic, I let people come to their own conclusions, but I promote it at a healthy level through Peta [People for the Ethical Treatment of Animals] or through small talk. I've handed out leaflets but I feel better mentally and physically so I like to mention that.

A lot of vegetarian food is repulsive. Take quiche and soufflé – why would you eat that? I like pasta with good olive oil and garlic. I also love tomatoes and make a great passata to go on top. In the summer I love to wood grill vegetables like people grill meat. Portobello mushrooms are great for that.

I like my vegetables plain. I don't stuff veg with veg, I'd rather eat boiled potatoes with a little salt. I'm a side dish person. When I go out, I order three side dishes. On the tour bus, I like Indian food: chana masala, naan, boiled rice. The Indians get vegetarianism like no other nation. They love their beans. That's my main source of protein, but it's crazy that people always ask me about my protein intake. I'm like, chill out; we need a quarter of the protein that we actually eat. It's OK to use your status to influence people about this kind of thing. At the end of the day, billions of animals are still going to die – I just hope that I can help reduce the number.

Dr John Briffa assesses Joan's diet:

Olive oil

This versatile and tasty oil is very rich in so-called 'monounsaturated' fat that is believed to have benefits for the heart and cardiovascular system generally. Olive oil also contains 'phenolic' compounds, that protect the body against freed radical damage, and have been linked with a reduced risk of some forms of cancer including breast cancer.

Pasta

Vegetarians often rely quite heavily on pasta as a staple food. What a shame, I think, that it's got generally low nutritional value and is generally disruptive to blood sugar levels in a way that can predispose to a range of ills.

Tomatoes

One of the nutritional highlights of this fruit is the 'carotenoid' nutrient lycopene, which research links with protection from conditions such as heart disease and cancer. Stewing tomatoes in oil enhances the absorption of this important nutrient, which is good news for passata-loving Joan.

Grilled portobello mushrooms

Mushrooms are generally a good source of the trace mineral selenium, with some evidence suggesting that this important nutrient has cancer-protective potential.

Naan bread

Naan bread, like pasta, offers little from a nutritional perspective and will have, particularly when eaten in quantity, generally deleterious effects on the body's biochemistry. Emphasis should be on more nutritious and less disruptive legume and vegetable-based curries , with perhaps a little naan on the side.

Chana masala

The chickpeas that form the base of this dish are a reasonably nutritious, relatively slow-sugar releasing food. The presence of tomatoes onions (rich in disease-protective compounds known as 'flavonoids') in this dish add to its nutritional value. All-in-all, a decent veggie offering.

Garlic

Garlic offers a range of compounds which, collectively, seem to offer some potential in the prevention of chronic diseases including heart disease and cancer.