You may have heard about research that shows exercise helps you perform better under stress. That shows exercising at moderate intensity for 20 minutes will elevate your mood for up to 12 hours. That shows exercise increases the production of a protein that supports the function, growth, and survival of brain cells.

Maybe you've even heard about a 30-year Harvard study that shows exercise is one of the five daily habits that can not only increase your lifespan by 12 to 14 yearsbut may also cut your risk of Alzheimer's in half.

Problem is, you hear about a lot of other stuff. Paleo diets. Keto diets. Mediterranean diets. Low-carb diets, high-protein diets, low-fat diets. Intermittent fasting. HIIT workouts. Isometrics. Plyometrics. Weight training. Cardio training. Tabata.

Since the options seem endless, figuring out what you should do seems impossible.

As does finding the time to eat healthier and get fitter.

As with most things, getting and staying healthier and fitter, over the long term, comes down to following a handful of basic principles.

Follow these, and then you can layer in the fancy stuff:

Drink a glass of water before every meal. 

We all need to drink more water. Even mild cases of dehydration make you feel more gloomy and pessimistic, possibly because certain neurons may detect dehydration and alert areas of the brain that impact your mood.

But instead of trying to drink 10 glasses of water a day, just drink a glass before every meal. That's an easy way to up your intake.

Plus, there's a side benefit if you're trying to control your weight: If you drink about 20 minutes before you eat, you'll feel a little more full when you sit down to eat -- and won't be as tempted to eat past the point of hunger.

To lose weight, consume fewer calories than you burn.

Some calories are better for you than others. Everyone has a different metabolic rate. Some people do have medical conditions that make losing weight really, really hard.

But for the vast majority, losing weight comes down to taking in less calories than you burn. You can go on a diet consisting solely of ice cream ... and as long as you consume fewer calories than you burn, you may not be particularly healthy but you will lose weight.

Still, that weight loss may not occur as quickly as you want. Even if you go on an extremely strict calorie-reduction plan, you may not lose much weight for days.

That's because severely reducing calories triggers the release of more cortisol, which typically increases the amount of fluid you retain: While you do lose fat, you also retain more water.

But it all shakes itself out after a period of time, which is why some people suddenly lose several pounds over the course of a couple days. 

So with all that said: If you want to lose four pounds in a month, you'll need to burn 500 more calories per day than you consume. (Generally speaking, 3,500 equals a pound.) 

You can do that by eating 500 calories less than you normally do, or burning 500 calories more than you normally do, or a combination of the two. Either way, do that for a month and you will lose four pounds.

If you don't lose four pounds, that means you either undercounted the amount of calories you took in or overcounted the amount of extra calories you burned.

If you find that -- no matter what diet you're following -- you aren't losing weight, then you need to eat a little less and move a little more. It really is that simple.

You won't find a single scientific study that proves otherwise.

To eat healthy, shoot for 80/20.

It may be possible to construct, from a nutrition point of view, the perfect diet.

But who wants to spend the rest of their life eating that way?

An occasional slice of pizza won't kill you. Or an occasional slice of cake. Or an occasional cheat meal.

Unless your doctor -- and not a trendy new diet's marketing -- says otherwise, if 80 percent of what you eat is healthy -- vegetables, fruits, lower-fat proteins, whole grains, etc. -- then you're doing great. 

And, since you won't feel like a slave to your diet plan, you'll be much more likely to eat healthier over the long term.

Which is what really matters.

Do some cardio ...

Cardio training can improve your aerobic capacity, lower your blood pressure, lower your body fat ...

Plus, as little as 20 minutes of low to moderate intensity aerobic exercise will make you feel less fatigued and more energized. (And if you do it first thing in the morning, that exercise will improve your mood and reduce your levels of stress for the next 12 hours.)

Problem is: How much cardio training is enough?

Good question. Some studies recommend up to 300 minutes a week of moderate cardio. On the other end of the spectrum, a study published in Medicine & Science in Sports & Exercise shows that one 23-minute HIIT session per week is nearly as effective as doing three 23-minute sessions per week. 

So it all depends on where you're starting. And on your eventual goals.

If you're starting from zero and want to get fitter, doing 15 minutes of moderate to somewhat vigorous cardio -- meaning you can't carry on a conversation while you're exercising -- three times a week is awesome. (In my experience, people who can exercise and talk are either incredibly fit or aren't working hard enough.)

The type of exercise you choose is up to you. While some forms of cardio may be "better" than others, the best kind of cardio is the one you like well enough -- either because you "enjoy" it or because it works so well -- that you can stick with it.

Because "optimal" is irrelevant if you never do it. 

And do some resistance training.

Better muscle strength and tone helps protect your joints from injury. Helps you maintain flexibility and balance. Boosts your metabolism. May help reduce or prevent cognitive decline. 

Plus it can make you look better, which usually means you'll feel better -- especially about yourself.

You can use weights. You can do bodyweight exercises. Shoot, you can just do the big four: pushups, pull-ups, squats, and dead lifts.  

If you're just starting out, three times a week is plenty. And if you don't rest a lot between sets -- which will add a little cardio oomph to the workout -- then 20 minutes or so might be plenty, especially at first. That's enough to do 10 or 12 sets.

But whatever workout you choose, whether cardio or resistance, and whatever diet you decide to follow ...

Stick to it for at least two weeks.

Yep: Two weeks. No matter what.

Why? One, you can do anything for two weeks. (If you can't, then you've clearly chosen a goal that doesn't mean enough to you.) 

More important, at the end of two weeks you will have enjoyed some level of success. Of improvement. Of return on effort.

In short, you'll have given whatever you decide to do a chance to actually work -- at least a little.

So keep your head down, don't focus on results -- because for the first few days there won't be any results -- and stick to the plan. Don't think about the next week. Or next month. Just focus on today, each day for two weeks.

By the end of the second week, you'll have worked out the kinks. You'll have a better feel for the exercises. You'll have a better feel for meal prep. You'll be better, not just in terms of results but at the process. 

That will give you the motivation to keep going -- instead of starting over by switching to another shiny new diet or fitness routine that catches your eye.

Because, as with most things, process is important.

But consistency matters most.