Americans spend a ton of money on diet and exercise--health clubs alone take in over $75 billion a year in revenue--in spite of the fact that most of us have no clue as to which nutrition and exercise plans are actually best for us.

Instead, we guess. Or we do what feels right. Or we do what's trendy. Or we get generic advice from a book or an article or a personal trainer at our local gym who gives everyone pretty much the same tips.

And then we don't get the results we hope for.

But not anymore.

Professional athletes have long followed highly personalized diet and training plans because they have access to results from genetic analysis and sophisticated diagnostics. Now, some companies are offering DNA testing for the rest of us, providing data that lets you optimize your diet and fitness training, based not on feel or hunch or trends...but on what is truly best for you.

In short, you can follow plans based on real data--which means those plans will actually produce results.

I never recommend products or services without trying them first, so to see how genetic testing works I chose FitnessGenes, mostly because its co-founders, Dr. Dan Reardon and Dr. Sam Decombel, are entrepreneurs. (Who doesn't love entrepreneurs?)

Here's how it works.

Place your order and FitnessGenes sends a kit you use to provide a saliva sample. Mail back the kit and a few weeks later you get a comprehensive online report showing your results across 41 different genetic variations. Some of these genes affect how you should train, some affect nutrition, and some affect both. And you also get diet and training recommendations based on your individual results.

Fortunately, you don't have to be a scientist to understand the report; it's extremely detailed and provides through explanations for each marker.

Some of your results won't come as a surprise. I have two copies of the slow metabolism allele, which means I have an efficient metabolism: My body does a good job of storing food as energy so, unlike some, I don't need to eat just before I work out in order to have enough energy to work out. I also clear lactic acid relatively quickly; I can ride up a mountain and, while my legs will burn the whole way, after a quick downhill burst I'm ready to go again. And I have two copies of the allele associated with sleep cycles, which means I'm a morning person and the best time for me to work out is around noon.

But other results surprised me. A lot.

Where weight lifting is concerned, I've always thought I got the best results by using relatively light weights and doing more reps. It turns out I had that all wrong--and so I've been training all wrong.

From a muscle-building perspective, I need to train with higher weights and less sets and reps; based on how I'm made, that's the best way to optimize muscle growth and protein synthesis. High volume training simply can't give me the same level of improvement.

And that brings up a key point about genetic tests. Many genetic tendencies don't exist in isolation; the key is to understand how multiple genes interact. Since I clear lactic acid quickly, I never felt like I had a good workout unless I did plenty of sets with short rest periods in between. I wanted to feel like a wrung-out dishrag when I walked out of the gym so I did lots of drop sets, lots of supersets, etc.: high volume, high intensity.

That way, I walked out of the gym feeling like I had accomplished a lot--but for my body, I really hadn't.

I've also assumed I'm more disposed to endurance sports than activities requiring power and/or speed. Wrong again: I have two copies of the "power/strength" allele and two copies of the "sprinter" allele. In cycling, I always thought I was better suited to climbing than riding hard on the flats or sprinting, but that's just because I liked to train on hills and mountains.

Developing more power seemed a lot harder, and I definitely didn't enjoy I rationalized that I must not be suited to it.

That's the best part of genetic testing for health and fitness purposes. We tend to make decisions about our bodies based not on science or data but on what feels right or what feels easier. As with most things, doing what we want to do usually doesn't provide the same results as doing what we need to do.

I know that's true for me. A few years ago, I spent a few months focused on lifting heavy. Looking back, I realize I got stronger and bigger relatively quickly. But when I went to lighter weights and more reps to give my body a change of pace, I never cycled back to lifting heavy.

Why? I don't like lifting heavy. It hurts in a different way than the burn I get from doing more reps.

Simply put, lifting heavy isn't fun...but neither is spending an hour or so in the gym every day and not feeling like I'm making as much headway as I should.

When you want to accomplish something--when you're putting in the effort to accomplish something--the only real fun is in seeing results from that effort.

That's why, when you actually know how you're made, it's a lot easier to follow a diet and exercise plan that will actually work for you.

Update: A number of readers have emailed asking if you also get nutrition advice. Yep. I focused on training guidance because I have never been too far off in terms of what I eat... although I did learn that I need to add a higher percentage of (healthy) fat to my diet. You can get recommendations for different scenarios, like fat loss or adding muscle or maintenance. 

And if you're interested: Here's the plan I once used to lose 10 pounds in 30 days. (And no, I'm not selling anything.)