Thanks Jodelle and Carola.
Carola, I weigh about 134, and if I assume I need 1 g protein per kg weight, and I eat 1600 calories/day for weight loss (which has been working for me so far), that's about 15.2% protein. I can definitely do that. I see you mentioned gaining muscle on a diet with about that much protein. So, that recommendation (~1 g protein/kg weight) is for anyone, including those (like me) that lift heavy weights 3 times a week and do intense cardio as well?
Does anyone have an opinion on the soy safety question, though?
Soy is a little controversial. The concern is that the estrogen-like substances (isoflavones) in soy may stimulate the estrogen receptors of breast cancer cells and make them grow.
There are others who say that soy protects women from breast cancer based on the belief that the isoflavone's estrogen-like power is weak, much less than the body's own estradiol, therefore if the weaker isoflavones occupy the estrogen receptors instead of your body's stronger own estrogen the theory is that the receptors will be less "turned on" and therefore inhibit growth.
It is often referenced that women in Asia have much less occurance of breast cancer. Personally I believe it has nothing to do with soy but everything to do with the fact that they keep their animal protein intake low, rarely eat dairy and eat much higher amounts of fruits and vegetables than we do. People in Asia don't eat soy breakfast sausages, smoothies made from soy protein, etc.
I had breast cancer, therefore, unless they find conclusive evidence that soy is actually beneficial for me I'll stay away from it and stick with other protein sources.
If you like soy, I'd probably stick with fermented soy because it is easier digestible, limit it to a few times per week and stay away from soy protein powders, processed soy, etc. because it really is unknown if the soy isoflavones are safe in such high concentrations.
As to your other question and this is only my personal opinion and experience, I don't buy that high amounts of protein would really make a difference in performance or muscle building, I think it's a myth, created by some clever marketing.
From my own experience on higher protein diets my performance actually suffered greatly. Now on my vegetarian diet of course I have some days better than others but in general I can see a lot improvement. I lift heavy 2 to 3 times per week (chest up to 20 lbs, biceps 15 lbs, back 25 lbs, shoulders .... ummh let's not talk about that one
) and I do cardio 5 to 6 times per week, mostly running. I have at least one 10 mile run per week, the others are usually between 3 to 6 miles. 3 to 4 of them are HIIT.
I would consider that pretty high intensity, I don't notice any negative effects, I am visibly building muscle and considering that I had major surgery end of November I have made enormous progress over the past few months. In fact, I was never able to run 10 miles before. It seems like my "low-protein" diet is fueling the workouts pretty well.
That being said, everyone is different and this is what works for me.
HTH
