morning class, please take a seat and class will begin

its good to know i've built a rep here now with training/supps and working out

the question "protein and weight change" in a nut shell

High protein/low carb diets are used for weight loss, its attatched to the unhealthy atkinds diet fad sadly.
Carbs are a high source or energy and calories especially in heavy foods such as pototoes, rice, pasta and bread.
By removing this energy source and mainly fuelling the fire by taking in high amounts of protein and fats, the body primarily burns fat, including body fat... like switching a log fire for a coal fire, it all burns but at different rates and effeciency.

However:

Potential diet danger exists for diabetics in that recommended carbohydrate intake is as low as 1/5 of the recommended minimum for diabetics.

Diabetics or those with sugar/glucose sensitivities should consult their doctor before following a dietary plan that so inadequately provides the minimum carbohydrate recommended.

One important danger relates to the excess acidity from a high protein/high fat diet (as evidenced particularly by the ketone acids produced in ketosis).

Excess acidity can lead to loss of calcium from the bones. Calcium is an alkaline mineral, and when the body experiences excess acidity, it uses what it can to restore ph balance. Consequently, there is more calcium excreted in the urine and more calcium leached out of bone tissue.

This atkins diet problem has broad, long term implications. The Cornell University site discusses this aspect of atkins diet danger which suggests that over time, calcium loss could lead to weaker bones which would increase risk of fracture.

Our teeth are made of calcium also, and aside from the fact that excess dietary acidity can actually erode teeth from acidity in the mouth, a reduction of overall calcium levels would affect the strength of teeth. These atkins diet dangers would not immediately manifest in our body, but would over time.

Metabolic acidosis, as this diet danger of excess acidity is called, also leads to a depression of the central nervous system.

Those with kidney or liver problems face potential atkins diet problems relating to the excessive load the atkins diet places on those organs. Due to the excess acidity caused by ketosis, there is extra work for the kidneys as the urinary system is the primary mechanism the body uses to deal with the ketone bodies produced.
i do not recommend low carb diets, i prefer the route of slow and steady weight loss that the heart can adapt to easily without placing the whole body under strain.
Removing carbs from veg/grains and other sources also limits the important nutritional benefits we take in as well as potential health benefits.

More on subject:

Protein is used for muscle fibre growth and reconstruction, protein itself is made up of amino acids.

For a laymans approach:

*if you want to gain muscle mass (size): you need to increase your protein intake to a minimum of 1gram per 1lb of bodyweight. To ease the strain on the kidneys you increase the water intake to a minimum of 3 litres a day.
the carbs need to be around 2-3.5g per lb of bodyweight to adequetly supply your energy needs and ensure your not working in a deficit (promoting weight loss instead of gain).
This is a common diet approach

*crash weight gain: more commonly called a bulking diet in bodybuilding, i've been on this before... jacking up the calories to 5000 a day, 400g protein a day, not carb counting and just gorging on heavy carb meals. 7 meals a day to ease the strain on the digestive system.
I gained the most amount of muscle in my life using this diet for 8 months due to my diet allowing a 100% growth environment but i also gained a lot of fat Its all part and parcel of bodybuilding...
long term diet for 6-12+ months depending on goals

* crash weight loss: after my bulking phase and 230lbs later i wanted to shed the fat to reveal what was under the whale blubber.
dropping the carbs by certain levels, raising them to trick the metabolism, lowering them and then finding a maintenance phase...
http://www.teenbodybuilding.com/duncan2.htm good example
You need to keep the protein intake high to stop the body going into a catabolic state and devouring muscle fibre for a energy source... muscle is easier to assimiltae for energy than bodyfat :P

these diets usually last for 8-12 weeks

*slow steady weight loss whilst gaining muscle This is called clean bulking, its hard to achieve as your in effect telling your body to reduce its bodyfat stores to shed the weight whilst tyring to use the energy stores to gain weight muscle wise...
YOU CANT TURN FAT CELLS INTO MUSCLE FIBRE AND VISA VERSA... ignore mother dearest when she chimes up with this

The calories are around 3000-3500 but from cleaner sources than just grabbing food of the supermarket shelf. The carbs are kept steady and the protein high, aiming for a slow and steady fat loss whilst slowly building up muscle mass.

To help out what food are good sources for muscle mass and keeping the gut, in check here is a list to help you out....

Proteins:

Boneless, Skinless Chicken Breast
Tuna
Fish (salmon, seabass, halibut)
Shrimp
Extra Lean Ground Beef or Ground Round (92-96%)
Egg Whites or Eggs
Ribeye Steaks or Roast
Top Round Steaks or Roast (aka Stew Meat, London Broil, Stir Fry)
Top Sirloin (aka Sirloin Top Butt)
Beef Tenderloin (aka Filet, Filet Mignon)
Top Loin (NY Strip Steak)
Flank Steak (Sir Fry, Fajita)
Eye of Round (Cube Meat, Stew Meat, Bottom Round , 96% LeandGround Round)
Ground turkey, Turkey Breast Slices or cutlets (fresh meat, not deli cuts)
Low-Fat or Fat-Free Cottage Cheese
Low-Fat or Fat-Free Milk
Protein Powder

Complex Carbs:

Oatmeal (Old Fashioned or Quick Oats)
Sweet Potatoes, Yams
Beans (pinto, black, kidney)
Brown Rice
Potatoes (red, new)

Fibrous Carbs:

Green Leafy Lettuce (Green Leaf, Red Leaf, Romaine)
Broccoli
Asparagus
String Beans
Spinach
Bell Peppers
Brussels Sprouts
Cauliflower
Celery
fiber one cereal

Other Produce & Fruits:

Cucumber
Green or Red Pepper
Onions
Garlic
Tomatoes
Zucchini
Fruit (if acceptable on diet): bananas, apples, grapefruit, peaches, strawberries, blueberries, raspberries
Lemons or Limes

Healthy Fats:

Olive Oil or Safflower Oil
Nuts (peanuts, almonds)
Flaxseed Oil
Fish Oil Caps