Building and maintaining muscle is critical for us males for several important reasons:
1) Muscle burns fat. It is much easier to gain weight with a low muscle to fat ratio. Have you ever felt like you just canï¿½t lose weight even when you cut back on calories? Well, if youï¿½ve lost a lot of your muscle mass, what do you expect? Muscle is your bodyï¿½s fat burner.
2) The evidence strongly indicates that Weight Lifting is a long term testosterone booster.
3) Weight lifting builds strength and strength has now been associated with decreased mortality. Read this important link on Weight Lifting for more details.
4) Muscle is a confidence builder. Even a little muscle building will make you look ten to twenty (or more) years younger. Let's face it: flat and flabby make you look worse to most women out there and you know it and she knows it. Just a little work with the iron and you can transform your appearance remarkably.
5) Muscle can also keep you from getting hurt. Building up the muscles around the knee keeps your from knee injuries. Building the lower back muscles will keep you from injuring yourselves on the weekend.
6) Strength is important during sex. Core strength and endurance is very important for many positions and a flabby, weak musculature isn't going to do the trick many times.
So how does one put on muscle easily and safely? It's no secret that you have to do strength or resistance training in order to stress the muscles and get them to grow. And the classic form of strength training is lifting weights of course.
Of course, you don't have to lift weights to put on muscle: just look at the Olympic male gymnasts or the legs of speed skaters. For most guys, though, weight training is the easiest way to do the exercising portion of gaining muscle. Unfortunately, many guys are intimidated by weight, but it does not have to be complicated nor overly time consuming. In fact, let's start out with weight lifting basics for those of you new at this game:
a) Resistance exercise, i.e. weight/machine/cable training, for 3+ sets and about 8-10 reps (covering, of course, all the basic muscle groups). There are many variations of this including Volume, but this is the starting point for most guys.
b) Increasing weight for #1 from week to week
c) Ample protein of at least 1 g per pound of body weight. The standard rule of thumb is that if you weigh 180 pounds, you should eat 180 g of protein/day. My experience is that you do not have to eat this much. (NOTE: I recommend egg whites, grains, legumes and beans for your protein sources with occasional low fat dairy as well.) CAUTION: Consumption of a Pre-Workout Protein Drink or Meal is common but potentially dangerous.
d) Consumption of about 300-400 additional calories per day than normal in order to build the muscle tissue
d) Variation of your routine each month or two (including reps and exercises)
CAUTION: Avoid low carb diets if you want to put on muscle. Carbohydrates have been found to be just as critical to muscle growth and recovery as protein.  Those with adequate carbohydrates had improved protein synthesis, which is critical for rebuilding muscle tissue. (See my page on other Potential Dangers of Low Carb or Atkins Diets as well.)
That's it. Almost anyone of any age can put on some muscle mass using this simple formula. It's definitely more difficult as the decades roll by, but certainly possible. Jack LaLanne is a prime example of this. (Of course, you must make sure you have adequate testosterone and growth hormone levels!)
By the way, you do not need to train with weights like a wild man, yelling and screaming and punishing every muscle fiber masochistically. Some body builders insist that "training to failure", as it is called, is the only system that will work. In fact, the studies show quite a different story. One 2008 study showed that training to failure was no better than a system that relied on a fixed number of reps and sets. (What really matters is muscle tension, which I cover in more detail in my section on Volume.)
Did you know that you can raise and lower your testosterone based on the foods you eat? And did you know that boosting testosterone through diet in the wrong way can lead to erectile dysfunction? Read about the High Testosterone Diet to find out how to improve both your testosterone AND your sex life.
When you're in your twenties, a basic program like the above will leave you exploding with muscle. A 25 year old male can generally put on ten pounds of muscle almost overnight. When the forties or fifties (and beyond) hit, the gains come much more slowly, but they still come.
So we can all gain mass, look twenty years younger and be happy, right? Well, the answer is "yes", but only if you do it right. there is a little more to the story. The problem can occur with c) above: consumption of all that protein.
Consider the problems with some of the proteins:
1. Beef. Range fed beef cooked with low heat is very healthy in many ways. But even under those ideal conditions, it will probably have significant saturated fat, which will cause most urban dwellering males trouble in terms of heart disease and erectile dysfunction. I cover all of this in greater detail in my Beef and Eggs link and this link on the Great Diet Smackdown.
2. Chicken, Turkey and Egg Yolks. According to David Chilton, author of Inflammation Nation, these foods are loaded with inflammatory messengers. Flooding the body with these often overloads our systems, helping to lead to the recurrent inflammatory state that is so common in western societies and has been shown recently to be one of the root causes of our current epidemic of autoimmune and heart diseases.
3. Fish. Most fish, be it farm or fresh, is loaded with Mercury, heavy metals and PCB's. It simply is not safe to eat more than once (or at most twice) per week.
4. Milk / Casein. Watch out for milk protein. Dairy has been implicated as being a strong risk factor for Prostate Cancer. And a recent animal study shows that casein, a.k.a. milk protein, actually lowered the Nitric Oxide levels of lab animals. I know of no studies that directly correlate dairy with erectile dysfunction, but you do not want to rely on anything that lowers your precious Nitric Oxide levels. 
NOTE: That leaves us with egg whites and maybe some low fat dairy - not too much because of the Protate Cancer connection - for our day-to-day use.
So how much protein do we really want? Well, first of all, several studies have shown that higher protein levels, actually higher protein/carbohydrate dietary ratios, are associated with a decrease in testosterone. One study demonstrated that a low-protein diet (10% of total energy) was associated with higher levels of testosterone compared with a diet higher in protein (44% of total energy).  And another study ten years later verified the same thing.  But it should be pointed out that a more recent study  showed that SHBG was improved - SHBG binds to testosterone which is bad - which should improve free testosterone, the kind of testosterone that is available to build mass.
So what is the answer? Well, if you want to put on muscle I would point out that the most critical time for protein is right before your workout and right after. This is the rule of thumb: eat some protein and carbs right 45 minutes before your workout and 45 minutes after. You want enough carbs to stimulate insulin, which pushes amino acids and nutrients into your muscle cells, and enough protein so that you have a ready supply of amino acids available to build those new muscle fibers.
Again, many studies have shown that it is protein (and some carbohydrates) pre and post workout that really count. One recent study actually fed weight lifters every fifteen minutes during a two hour workout and found that it reduced protein breakdown by 8 percent and increased protein synthesis - and that is what you really want - by 33 percent.  Of course, it is impractical for most of us to eat throughout our workouts, but this underscores just how critical protein availability is to build muscle mass.
So, assuming you want to put on some muscle mass, you definitely need to add this practice to your regimen if you haven't already. What proteins should you use? In my opinion, the best and most safe is egg white. 3-4 before and after will do the trick and you will notice the difference. (Do not forget some carbs along with it.)
Whey is the darling of the bodybuilding world as you may already know. However, if you use whey, I recommend undenatured or biologically active whey, which I document in my link called Yes Whey, in order to avoid excitotoxins.
So how you handle protein during your non-workout times of day? Well, you have two ways to go, which I outline below. I have tried both and put on muscle with both, but I recommend the first if you are suffering from heart disease or erectile dysfunction in particular. The latter option, though, is excellent as well:
I. Ornish (Low Fat) Diet. I tried an Ornish Diet, which is a quasi-vegan diet found that I put on several pounds of muscle while building the line down the middle of my abs. I had not seen that since my college days! Again, this kind of healthy lifestyle is what you need: building muscle, lowering fat, getting good abs and cleaning out your arteries! The Ornish Diet allows egg whites which is basically pure protein. (Although it is never stated, protein powder would probably be allowed as well as long as it does not have any fat.) So put on some muscle and get in shape and don't kill yourself while you're at it!
The Ornish Diet is primarily a vegan diet, although not in the militant sense of the word since it allows egg whites, and so one trick I did was: avoid fruit and always use whole grains, beans and legumes which are pretty high in protein. (I do have Pomegranate Juice and blueberries every day, however.) For more information, read my link on the Ornish Diet and the Great Diet Smackdown (Part II). And, by the way, the Ornish Diet also appears to stop Prostate Cancer dead in its tracks.
Remember that the Ornish Diet, because it is low fat and high fiber, will probably lower your testosterone somewhat as I cover elsewhere on the site. However, it will also clear out your arteries, increase Nitric Oxide output and improve endothelial function, all of which are critical for erections. If you are suffering erectile dysfunction or heart disease, this diet is something you should strongly consider to heal yourself of the decades of abuse you put your cardiosvascular system through.
I also want to point out that I tried an Ornish Diet, which is a quasi-vegan diet that actually reverses arteriosclerosis, and found that I put on several pounds of muscle while building a six pack on my abs! I had not seen that since my college days. Again, this kind of healthy lifestyle is what you need: building muscle, lowering fat, getting good abs and cleaning out your arteries! (I emphasized grains, legumes, beans, whole wheat bread and egg whites - all of which have a respectable amount of protein and are low fat. I weigh a little over 150 lb and get a little over 100 g of protein in a day using this technique.)
II. Mediterranean Diet. The Mediterranean Diet is a proven cancer and heart disease fighter. The Mediterranean Diet also is pro-testosterone production and will definitely improve erectile function. In other words, the Mediterranean Diet will preserve and protect both your life, your hormones AND your sex life. But the Mediterranean Diet does not emphasize protein. (Animal protein is a minor part of the Mediterranean Diet.)
So pick either one of these diets, exercise seriously and, if you feel the need, supplement some egg whites and/or whey protein. You can quietly take comfort in the fact that all the balls-to-the-wall body builders eating high fat beef, chicken and egg meals day and night will pay the price in a decade or two with massive arteriosclerosis and erectile dysfunction. (Please read this link on Muscle Madness to screw your head on straight if you are a Muscle Freak or Gym Rat. Some muscle is good, but you have to be careful or your penis and heart will pay the price.)
NOTE: It is worth noting that this is exactly what Jack LaLanne did, i.e. basically ate a vegan diet and supplemented with egg whites. And he is in his mid 90's doing very well with no cancer or heart disease. The remarkable thing about Jack LaLanne is that he performed incredible feats of strength through his early 70's and had incredible musculature even through his 80's. Imagine a guy with biceps that would make any guy at the gym proud - that was Jack LaLanne in his mid to late 80's! There's no reason that we cannot achieve a significant measure of his success, especially since he followed almost every principal advocated on this site: frequent exercise, a (slightly modified) Mediterranean or Ornish Diet and an active, upbeat pro-testosterone attitude toward life.
But, above all, remember: the goal is to build a little muscle, protect your heart AND keep youthful hormones levels....
CAUTION: New research shows that aerobics/cardio should always be done after weight training. A recent study out of Japan  has potentially uncovered the fact that weight training actually stiffens arteries - and you don't want stiff arteries if you want anything else to be stiff if you know what I mean - unless followed by cardio or aerobics. Furthermore, weight training without cardiovascular training leads to decreased Nitric Oxide production from the endothelium, which is bad - very bad. Intense weight training leads to very high, albeit brief, blood pressure levels.  These pressures may "stun" the lining of our blood vessels and arteries and possibly even do long term damage. Again, do NOT do aerobics before your weight workout - do it after. This will nullify any negative effects on the endothelium of the weight training. (Note: Some studies have not shown weight training in a negative fashion, but it is best to play it safe. ) News Flash: A recent study showed that intense weight training hardened the arteries but did not seem to damage the endothelium. To test this, they immersed the weight lifters foot in icy water and then measured artery expansion in the neck. 
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