Last Friday, I was crossing 5th Avenue, heading Eastbound on 23rd Street, in the Flatiron district of Manhattan. I was en route to a personal training appointment, when a stranger approached me.

"Danny!" he exclaimed, "It's great to see you. You and your brother Al are a huge inspiration to me. I've been doing calisthenics for five years, and I'm in the best shape of my life thanks to you!"

Wow! Stuff like this always makes my day. How could it not? I'll stop and chat with everybody and anybody when it comes to push-ups, pull-ups, and of course, the human flag. Anything workout related, really. I'm happy to hear their stories and I consider myself lucky to have these opportunities.

After talking shop for a few minutes, though, the man surprised me with a rare demonstration of transparency. "Danny, I drink," he said solemnly.

Perhaps the fact that we were perfect strangers enabled him to open up. Or maybe he knew that I'm the only guy in the game who dedicated a whole chapter to drinking—along with the ones about things like protein, veggies, and exercise—in a book supposedly about abs. Who knows? But then he followed up with the real question: "It's bad for gains, right?"

Good question. And the answer is easy enough to explain, but not easy for some people to hear and put into action. So, let's unpack it.

We Like Some Things That Are Bad For Us

First off: I'm not going to give a loud, Nancy Reagan-style "just say no" when it comes to mixing booze and brawn. Plenty of people have already done that. But, like many issues, this subject is too complicated to be dismissed with a simple, "It's bad. Don't do it." The fact is that an enormous number of people all over the world—even people who work out, look amazing, and can do amazing stuff—like to drink.

And to be perfectly candid, the pull-up bar is not the only bar I've known. Prior to becoming a personal trainer in 2006, one of my many previous—and strange—careers was as a promotional marketer for a well-known liquor brand. (Think "Danny as Duffman"… not too far of a stretch, right?)

Long story short, I'd go on the road for months on end, traveling to and from "party towns" to promote the hard stuff. Believe it or not, despite my erratic, late-night schedule and sometimes self-destructive lifestyle, I still worked out four days a week and continued to get in better shape. Call it an inconvenient truth.

Obviously I'm well acquainted with the unfavorable effects that drinking can have on your fitness, but in my experience, the occasional use of alcohol does not have to be a complete deal breaker. At least it hasn't been for me. You just need to go in with your eyes wide open and not lie to yourself about the downsides—and there are definitely downsides. So, let's get them out in the open.

Downside 1: It Dehydrates You

One of the main reasons for your throbbing skull and dry mouth the day after a big night on the town is dehydration. You lose much more water than you gain when you drink—even if you drink a lot. Ever notice how often you run to the bathroom? That's part of it. And because there's not enough to go around, water that should go to the brain is redirected to other organs, hence that headache.

Downside 2: It Can Contribute to Weight Gain

Alcohol isn't a carbohydrate, fat, or protein. It's just alcohol, or basically, a fourth macronutrient. That means that it has calories, even if you're drinking the hard stuff straight out of the bottle with no mixers. And the way alcohol consumption affects your body is definitely different than, say, what you get from a solid dose of protein or some vegetables.

At 7 calories per gram, alcohol is also the emptiest of calories. An average alcoholic beverage is anywhere from 100-200 calories, but we all know that in drinking, one man's average is another man's warm-up, so the numbers can go way higher.

And to be clear, all these extra calories, when not metabolized, will be stored as fat. Not the look you were going for when you started training, huh?

Downside 3: It Can Disrupt Muscle Growth

Not only does alcohol have zero nutritional value, some studies say it may even disrupt muscle protein synthesis, and muscle growth. (Google it if you have a day or two to kill.)

Of course, many studies are questionable, reporting only on "chronic" drinkers or even non-human subjects. That said, anyone who has taken ninth grade biology knows that alcohol is metabolized before other nutrients, blocking them from being absorbed. This can hinder protein synthesis.

Downside 4: It Interrupts Your Sleep

Plenty of people—but definitely not all of them—find they fall asleep better after a drink or two. But the sleep that comes afterward can be poor quality, to say the least. And sleep quality matters big-time in fitness.

You need sleep so your body can grow, repair, and get strong. Sleep also seriously affects your levels of testosterone, and by extension, muscle building. Remember, muscle is built when you recover. You do not want to stand in the way of that.

Knowing All This, Why Do I Drink?

I'm in my mid-forties and I perform better, lift more, and have more muscle than I did in my early twenties. And I occasionally drink. Why? Because I enjoy it, and I'm confident I can do it right.

Clearly, there are certain adjustments you can implement to turn something that's bad for you into something that's slightly less bad for you. Follow these guidelines and you'll be helping yourself more than you can imagine, should you choose to both train and drink. (It goes without saying that you should never work out while under the influence.)

You already know my strength rules. Here are my booze rules.

Rule 1: Consume Lots of Water

It is extremely important to drink plenty of water before, during, and after cocktails. In addition to several glasses throughout the day for general hydration, I suggest also having one glass for every alcoholic beverage you consume.

Then, at night, sleep with a bottle of water close by so you have easy access. If your urine is dark, you need more water.

Rule 2: Drink the Good Stuff

I don't understand how anyone can work their ass off in the gym, spend their hard-earned money on quality food and high-end supplements, and then binge out once a week on low-end well liquor or cheap beer.

Go for the good stuff! Like master chefs, craftsmen of finer wines and spirits pay careful attention to their ingredients—and even the soil from which they came—not to mention the proper production process. The essence of everything we put in our bodies matters, even when it comes to libations.

Another advantage of drinking good stuff is that it costs more. You can consider this to be automatic portion control.

Rule 3: Not All Alcohol Is Created Equal

This one is going to rub some people the wrong way, but my personal rule is "No mixed drinks, no beer, no exceptions." Give me the good stuff, straight, or give me nothing at all.

Most mixed drinks are loaded with sugar, sweeteners, and in many cases, chemical and other uncategorizable crap. If you need a mixer, use club soda. Popular "hard" lemonades and ciders are usually some of the worst alcoholic beverages you can drink, as they combine the lowest quality alcohol with the lowest quality sugar. Yes, there are also high-quality craft ciders out there, but the amount of sugar in them is still a deal-breaker for me. These beverages will put you on the road to weight gain faster than a glass of bourbon, or even vino.

Beer, too, is packed with calories, and your body absorbs it like a big bowl of sugar. If you choose to have beer, do it for the flavor and cut yourself off after one rather than five or "I lost count," as too many beers can definitely turn your six-pack into a keg. Remember, anything your body does not use for fuel will be stored as fat.

Rule 4: Never Substitute Alcohol for Sleep

Don't laugh. We've all seen somebody do this and we know what it looks like. Or maybe we've even done it ourselves.

All I'm saying is, if you're going to tear it up, don't do it when you have to be at work at 7 a.m., or when you have an important training session going down the next day. You need a full night's rest, probably even more to compensate. Enjoy your drinks, but not in place of sleep.

An even better approach? Don't "tear it up" at all. Have a quality drink or two, enjoy it a lot, and go to bed at a normal, reasonable time.

Rule 5: Don't Do It Every Day

This is the most important rule on the list. Just like I say regarding dietary decadence, if you have a rule and you "cheat" every day, it's not cheating—it's a lifestyle.

We are a product of our day-to-day habits. We have birthday cake at parties, and it's OK because it's not the norm—it's the exception. I am fond of a good wine or whiskey, but like birthday cake—which I'm also fond of—I only have it enough to make it special. If I consumed it every day, my life could become unmanageable.

Cheers to Results

Adults make their own decisions, and I'm not here to tell you what to do. I simply want to promote what I've observed to be true regarding training and drinking, as objectively as possible. In the world of wellness, fear-mongering tactics and absolutism often get more attention than life experience, observation, and common sense. Not for me, though.

Obviously, too much alcohol consumption is toxic and can ruin your life, your testosterone, and your gains. But I don't see why a grown-up can't enjoy the ol' firewater from time to time, hold onto their muscle gains, and still be in great shape. If you're eating right 90 percent of the time, training hard, and following my rules, then you've probably earned it.

As for the stranger who stopped me on 23rd Street, that's what I told him, albeit a shorter version. Cheers!