I don't think there are any valid sources to support the idea of 30 g protein per meal claim is all the body can handle. There isn't any fixed limit as to calories or carbohydrates, either. [But, it's probably easier on your stomach to limit your fiber grams to something manageable, say 5-15 g per meal, depending on the size of your stomach. Most bb diets wouldn't even approach this limit, though, since they are usually so heavily based non-plant food sources.]
The idea with the protein limitation is more to keep a steady stream going through your gi tract at all times, and optimizing wanted effects by taking in fast acting proteins and slower ones at certain times. But, it isn't clear that this is necessary or will result in any visible benefits. There haven't been any clinical trials to prove or disprove it.
However, there does seem to be a very good reason for limiting fat grams to 30 per meal:
Curr Atheroscler Rep. 2003 Nov;5(6):445-51.
Dietary fat and postprandial lipids.
Sanders TA.
Nutrition Food and Health Research Centre, King's College London, Franklin- Wilkins Building, 150 Stamford Street, London SE1 9NN, United Kingdom.
[email protected]
Impaired clearance of chylomicron remnants is associated with increased risk of atherosclerosis and cardiovascular disease. An intake of 40 to 50 g of fat in a meal results in significant lipemia in healthy adults, with consecutive fat-containing meals enhancing the lipemia. This would suggest that limiting fat intake to approximately 30 g on each eating occasion would minimize postprandial lipemia. Sedentary behavior and obesity independently impair the postprandial metabolism of lipids. Postprandial lipemia causes endothelial dysfunction and results in a transient increase in factor VII activated (FVIIa) concentration. Plasminogen activator inhibitor type-1 activity is associated with fasting plasma triacylglycerol concentration, but is not influenced by postprandial lipemia. Trans-18:1 acid appears to increase cholesterol ester transfer activity acutely compared with oleate. Randomized stearic acid-rich fats result in less postprandial lipemia and a lower postprandial increase in FVIIa, whereas unrandomized cocoa butter results in similar postprandial lipemia and increases in FVIIa compared with oleate. A background diet containing in excess of 3 g/d of long-chain omega-3 fatty acids decreases postprandial lipemia by stimulating lipoprotein lipase expression and decreasing very low-density lipoprotein synthesis, but a diet enriched in alpha-linolenic acid (up to 9.5 g/d) does not show these effects. Future research on diet and postprandial lipids needs to exploit newly gained knowledge on the regulation of adipocyte metabolism by adipokines and nuclear hormone receptors, particularly with regard to fat patterning and reverse cholesterol transport.
PMID: 14525677 [PubMed - in process]