Everyone who trains with weights should know the benefits of protein for lifting. Increased protein intake is an absolute requirement for maximal gains. Unfortunately, many gym goers take this to extremes and eat far more protein than they could ever use. The protein, while not causing health problems--there's no indication that high protein intake is harmful to HEALTHY kidneys--simply becomes an expensive source of calories since the excess gets converted to glucose.
A protein intake of 2.2 - 3.3 grams/kilogram of bodyweight is plenty as long as you're getting enough calories and the sources of protein are high quality. There are a lot of good food sources of protein: meat, fish, poultry, beans and dairy; even vegetables have a small amount.
If there's one place weight trainers go wrong, it's the misguided quest of trying to eliminate fat from the diet. We need certain types of fats, but not others. Fat quality is far more important than fat quantity. In general, a fat intake of 20-25%, coming from healthy sources such as vegetable oils, seeds, nuts, etc., should be consumed by all who train.
This helps to provide sufficient calories without being excessive. Saturated fats from animal products should be minimized since they are associated with health problems. But there are no absolutes.
Carbohydrates are both under and over-rated for strength athletes. On the one hand, many athletes tend to over-consume carbohydrates (especially highly refined ones), usually in lieu of healthy fats. On the other hand, there are some diet gurus telling people that they can grow without carbohydrates in their diet.
Such diets are not ideal for maximal growth. Carbohydrates are required for high-intensity activity, and a low-carbohydrate diet will eventually sap training intensity--not a recipe for good gains.
In general, carbohydrates may make up 50-55% of a persons diet. Of that percentage, some should be starchy foods such as breads and grains, with the other portion from high-fiber vegetables and fruits. That ensures adequate glycogen levels for training, along with adequate fiber and nutrient intake.
You should ensure a large amount of carbohydrates right after training (preferably with protein) as this has been shown to improve protein synthesis and recovery.
Fruit and Vegetables
If there's another place many weight trainer make mistakes, it's in not consuming sufficient amounts of fruit and vegetables. Fruit and vegetables are crucial for proper nutrition. Five to seven servings of fruits and vegetables per day should not be hard to achieve: that's 2-4 pieces of fruit and 3-5 servings of vegetables. A serving of vegetables is not as large as you think (a cup of vegetables equals two servings, for example).
The high fiber intake will keep your colon healthy, and improve nutrient assimilation. Of course, too much fiber (50+ grams/day) can be detrimental. Balance, again, is key.
Drink lots of water. Five clear urinations per day should be the minimum. Many people survive on soda and coffee, which don't count as water intake. The water content of milk, fruits and juices does count. Most people are a little dehydrated chronically, which is not healthy. Get a water bottle and fill and empty it 2-3 times per day; then you'll be on the right track. Eventually it will become a habit.
There's a lot of confusion over how to set calories for different goals. Part of this confusion stems from the fact that any equation can only be an estimate. It should not be taken as holy. Like training, caloric intake will have to be adjusted based on goals. As a general rule, a caloric intake of about 34 calories per kilogram will maintain weight. To lose weight (fat), reduce this by 10-15%; to gain weight, increase by 10-15%. After a few weeks of the change, check results and make further adjustments until you find the numbers that are ideal for you.
Eating more smaller meals tends to help with appetite, avoids overloading digestion, and makes it easier to consume sufficient calories. Four meals/day is bare minimum and five-six meals (this includes snacks) would be better.
Gaining weight is not difficult. To gain mass you must eat more than you require. Either your body will burn off the excess or it will be deposited as tissue in your body. What type of tissue (muscle or fat) depends on how intensively you're training, and how much you're eating.
If you're not gaining weight, the solution is simple: eat more. There's nothing magical, and no special nutrient combination; just keep adding calories until you start moving up in weight.
Losing weight is fundamentally no more difficult than gaining it. To force your body to mobilize stored fat tissue, you have to eat less than you need. All diet plans, no matter what they claim, simply get you to eat less (or exercise more) so that stored tissue is mobilized for fuel. No magic, but just simple physiology.
If you want to lose weight, reduce your calories little by little until you start seeing changes. Then stay there until they stop, and reduce a little bit more. As long as your strength in the gym isn't dropping, you're not losing muscle. If your strength is dropping, you need to eat a little more until your strength stops dropping.
Increasing cardio a little bit--and you should be doing some anyway--can help with fat loss.