Muscle is the key word here because the amount of muscle that you have directly affects your metabolism, not to mention your looks. The lean and hard phsique that men desire as well as the tight and toned look that women want can both ne achieved by adding more muscle. The way to add more muscle is to workout with weights, consume adequate amounts of protein throughout the day and get enough rest.
Contrary to popular belief you do not build musce when you workout. When you lift weights, you are actually breaking down your muscle tissue. Immediately after working out your body begins to repair and recover. It does this by taking protein and breaking it down into amino acids then reconfiguring them and placing them wherever they need to go. Instead of just rebuilding your muscle back to normal your body super compensates. This happens if you have enough protein in your system. If you don't have enough your body goes into a catabolic state. This happens to most people in this situation your body does not have enough protein to rebuild so it begins to break down your existing muscle to repair what was broken in the gym. This is not productive. That is why so many people don't see the results they want. You can avoid this by taking in an adequate amount of protein each and every day.
Figuring out what an "adequate" amount of protein is for your body is fairly simple. Women should have anywhere form 1 to 1.5 grams per pound of bodyweight. (so a 130 pound women would take he body weight and times it by 1.5. 130 x 1.5 = 195. 195 grams of protein she need each day.) Men should have anywhere from 1 to 2 grams per pound of bodyweight (a 200 pound man would have to have 300 grams per day 200 x 1.5 = 300.) If you divided that by how many meals you eat that would be the number of grams you need each meal. For ex. I eat 5 times a day. (125 x 1.5 = 187.5 / 5 =37.50.) 37.50 grams per meal for me. Now unless you have time to pre-pare your meals and take them with you, eating enough protein all the time will be next to impossible. Thats why protein supplements are essential.
Why we should eat more often if I'm trying to lose weight? Shouldn't I eat less? Not at all. The biggest mistake that people make who are trying to lose body fat is not eating enough, The body is designed to be fed every 3 to 4 hours. That is how our metabolism burns. By eating only 3 times per day you are putting your body into a fast state. That means that if you wait longer than 4 hours to eat your body is more likely to store that meal as fat, even if it is a good meal. On the other hand if you eat every 4 hours hours your metabolism will kick in all day long.
Earlier I said that having more muscle will lead to a faster metabolism. This is becuase what powers your body is muscel. Every action from walking to breathing and even blinking is powered by muscle. Muscle is the only part of your body that burns calories, so the more you have the faster your metabolism will be. Period. Even sitting on your but doinf nothing you can burn more calories by having 5 more pounds of muscle. The less muscle mass the slower your metabolism.
You will fins that once you increase your protein you will sleep better, recover faster, and have TONS of energy!
*Note to women: working out with weights will not make you "bulky". Testosterone causes men to get that way. By putting on more muscle you will increase your metabolism and look more like a fitness model than a bodybuilder.