I have a friend from my hometown who, in my opinion, is really ruining his life. When we were in high school, we were both athletes. Neither of us is now, but I still love to get a lot of exercise and take part in physical activities. I still work out, and I still eat right. My friend… well, not so much.

He eats terribly, never works out, and drinks more than I’ve ever seen anyone else drink. And it’s making him fat. I don’t want to sound shallow – I really don’t care what my friend looks like. I’m worried about his health! He says he’s just getting older, and that “my metabolism will slow down, too.” What can I do to show him that what he puts in his body matters?

There can be no doubt that what we eat and drink have everything to do with our fitness and our overall health. Nearly everything we need to keep our insides in good shape is taken in through food and drink, and failing to eat and drink the right things – or consistently indulging in the wrong ones – is a good way to wreck your health and to gain weight.

The evidence is clear. Let’s start with diet. Your friend’s poor diet is almost certainly the primary driver of his weight gain, and while metabolisms can slow down, it’s unlikely that his has done so in such a dramatic way at such a young age. Your friend needs to be fueling his body with whole foods, particularly healthy plant-based foods. He needs to eat them in the proper amount, of course, and should steer clear of excessive snacks and highly processed foods, which overwhelmingly tend to be unhealthier than whole foods. It’s not just the volume and calories that matter, here – it’s the nutrients and balanced nature of a good diet, which can make you healthier as well as skinnier. Some experts believe a great diet can even reverse diabetes.

Exercise is key, too – experts say we should be getting 30 minutes a day at the minimum. Exercise is unlikely to help your body conquer a bad diet (and even if you do manage to stay skinny, you won’t be truly healthy), but it’s a key part of a healthy life that should work in combination with a good diet.

Finally, a word needs to be said about your friend’s unhealthy alcohol habit. It’s not just the things we should put in our bodies that matter to our health – it’s the things we shouldn’t, too. Having a couple of drinks a day can be safe and even healthy, but binge drinking and excessive alcohol consumption are terrible for your health. Alcoholics tend to have a lot of health problems, say psychologists at one of the top rehabs in Florida. Alcohol can kill through overdoses and accidents, but it can also cause long-term damage to everything from your liver to your waistline.

The facts are squarely on your side. Your friend needs to listen. You should approach the conversation as tactfully as possible, and you should emphasize your concern for his health (not his appearance). You should probably avoid drawing direct comparisons between his current physical condition and yours, and you should express your support, not indulge in excessive criticism. Ultimately, however, it is up to your friend to decide whether or not he’s going to listen to reason.

“Good habits are worth being fanatical about.” – John Irving