Q: how many grams of protein is recommended (per day) for someone training seriously and regularly? Why is it important to keep fat intake relatively high when weight training?

A: People will give varying advice on this, but I think a good lower bound is 1 gram of protein per pound of bodyweight per day. If you want to set your aim high, try for 1.5 grams of protein per pound of bodyweight per day. High protein intake does NOT impact your kidneys negatively and does NOT suppress calcium uptake into your system (two common myths about the supposed dangers of protein). It is highly recommended that you drink a lot of plain water (one gallon is a good target) throughout the day to help your kidneys flush out any excess wastes. It's also always a good idea to eat lots of natural foods, raw fruits and veggies, etc. instead of heavily processed or synthetic foods. This will make your kidneys' jobs easier and will keep you much healthier overall.

As to fat intake, it's not really that high -- I recommend around just 30% of total calories. An average person in US who is not on any kind of diet typically gets 50% or more of calories from fats! So 30% shouldn't appear high by any means. Some motivation for why fat is important in your diet comes from the fact that fatty acids are some of the most common building blocks of all the trillions of cells in your body. So fat is not just the fuel for your body (which is the way we typically think of it), but also a major building block. The same is true of protein. Carbs, on the other hand, are used almost exclusively as fuel only (though, of course, carb-containing foods also contain many vitamins and minerals).

How the 30% figure was arrived at, nobody can really tell you. Millions of people weight training on their own and many conducting studies over time came to roughly the same conclusions about what appears to be optimal balance in proteins, carbs, and fats in one's diet. That advice is to get 40% from protein, 30% from carbs, 30% from fats when trying to lose fat and to get 30% from protein, 40% from carbs, 30% from fats when trying to gain muscle. These are of course rough guides and it doesn't mean you won't reach your goals if you don't follow them exactly. But if you want a dietary target, they are there. The other big asterisk is, of course, the kinds of foods you eat -- getting your 30% carbs and 30% fats all from ice cream won't get you very far.