The issue of protein toxicity is mainly a concern when a diet is not balanced, i.e. without sufficient fibre, carbs and fats.
The body is remarkably capable of dealing with excesses of all types, including prolonged excesses. The amount of protein that becomes toxic is not precisely known: the general rule of thumb is - as FreeFaller suggests - is around 200g/day.
Variables in this include:
- Size (lean mass);
- Exercise levels;
- Exercise type;
- Dietary balance (i.e. what else balances out the protein);
- Age (advancing age means the impact of protein supplementation over a regular diet are greater);
From what I've read, it would seem that bodybuilders/lifters will need somewhere around 100-125g/day (on average): I'll throw an additional 20% variability in there just because... excesses just disappear through your digestive system.
However, because an individual is USING more protein as part of their body's daily maintenance, the level of protein toxicity will be increased. This is why getting precise ANYTHING for the human body is very difficult without proper measurement.
In other words, keep your protein consumption below 200g/day and you probably won't have a problem. I'd personally aim for about 150g/day, simply for cost/benefit reasons, but that's just me.
