Using just urinalysis alone would not show you the whole picture (fecal, sweat). Research has established that a minimum protein intake of 1.5 grams protein/kg of BW is necessary to achieve nitrogen balance (1, 2, 3), although carbohydrates can certainly be protein sparing of course. There is also a high variability, and if you feel comfortable that you are safe with 120g/day then that is perfectly fine - I just tend to stick with recommendations that err on the high side.
The following is based on some information found in Lyle's ketogenic diet book:
In a study looking at the metabolic effects of various dietary approaches, subjects were studied under a total of 6 different dietary conditions (4). The first was a 400 calorie diet consisting of 100 grams of protein. In this group the average negative nitrogen balance was -2 grams, the equivalent of 12 grams of body protein broken down to make glucose. An additional 20 grams of dietary protein (for a total
of 120 grams/day) would have provided this amount of glucose and prevented any nitrogen losses. The second group was given 800 calories as 200 grams of protein. There was a positive nitrogen balance of almost 8 grams/day, the
equivalent of 48 grams of protein. This suggests that a protein intake of 152 grams would have been sufficient to achieve nitrogen balance. The difference between the two values for protein intake determined in this study (120g vs 152g) can not be explained from the data presented.
In another study, eight men were placed on diets of 1800 calories, containing 115g protein for 9 weeks (5). Carbohydrate varied from 104 grams to 60 grams to 30 grams. Nitrogen balance was slightly negative during the first week of the diet. Approximately 2g of nitrogen were lost (equal to 13g of protein converted to glucose), meaning that
22 additional grams of protein would have been required to attain nitrogen balance. Added to the 115 grams of protein given, this yields a total of 137 grams of protein to prevent all nitrogen losses. Nitrogen balance was achieved during week 2 and became slightly positive during the third week.
So, it appears that a protein intake of at least 120-150g/day should be sufficient to maintain nitrogen balance, regardless of calorie levels. However, this value does not include any additional protein needed to cover exercise, and none of these studies have discussed weight training individuals. Already muscular and lean individuals also tend to lose more protein during hypocaloric conditions.
1. Phinney S. Exercise during and after very-low-calorie dieting. Am J Clin Nutr (1992) 56: 190S-194S
2. Davis PG and Phinney SD. Differential effects of two very low calorie diets on aerobic and anaerobic performance. Int J Obes (1990) 14: 779-787.
3. Bistrian BR et. al. Effect of a protein-sparing diet and brief fast on nitrogen metabolism in mildly obese subjects. J Lab Med (1977) 89:1030-1035
4. Bell J. et. al. Ketosis, weight loss, uric acid, and nitrogen balance in obese women fed single nutrients at low caloric levels. Metab Clin Exp (1969) 18:193-208. (Abstract not available)
5. Young CM et. al. Effect on body composition and other parameters in young men of carbohydrate reduction in diet. Am J Clin Nutr (1971) 24: 290-296. (Abstract not available)