4. Tissue mobilisation in the obese

A closer examination of the pattern of tissue mobilization during starvation in 'obese' and 'normal' subjects reveals some interesting features. Figure 3 shows the pattern of urinary nitrogen loss during the 31-day fast in a normal weight subject (60.6 kg) studied by BENEDICT (1915) and compares it with an obese subject (154 kg) studied by GILDER et al. (1967) and starved for a comparable period. After the first week, the obese subject lost less fasting urinary nitrogen than the normal weight subject.

This difference in nitrogen loss is further highlighted if we compare the P ratio of normal subjects (Table 1) with that of obese subjects (Table 2). The obese have a lower P ratio and appear to mobilise about 5% of their energy from protein breakdown, in contrast to normal weight subjects, who mobilised approximately 20% of their energy from protein breakdown. It therefore appears that normal weight and obese subjects respond differently during starvation, and that the adipose tissue exerts a profound influence on protein mobilisation.

Although there are no data on P ratio and direct body composition measures in humans, weight and height measured in subjects cited in Tables 1 and 2 were supplemented with single point measurements of BMR and nitrogen loss during fasting on seven more subjects. These values are shown in Figure 4. It can be seen that a close agreement exists between the P ratio and the BMI at the start of the fast. The Figure also shows GARROW's (1983) proposed relationship between BMI and body fat. It is clear from the graph that the P ratio declines in a curvilinear fashion with increasing adiposity. It therefore appears that the tendency to use protein as a fuel in starvation is much lower in the obese than the non-obese human, as has already been suggested by experiments on laboratory animals (HENRY, 1984).

Table 2. P ratio in obese human subjects during fasting

Subject No.	Initial body weight [kg]	Duration of fast [d]	Fasting urinary nitrogen [g/d]	BMR [kcal/d]	P ratio
1	123	38	3.90	1985	0.049
2	132	38	4.00	1707	0.058
3	147	38	3.27	1773	0.046
4	154	21	4.40	1740	0.063
5	190	21	4.56	2480	0.046
6	166	14	3.41	2100	0.040
7	73	14	2.83	1200	0.059
8	124	21	5.70	2328	0.061
9	120	18	3.20	1984	0.040
10	103	4	4.50	1509	0.074
				Mean	0.0536
					± 0.011

Figure 3. Mean fasting urinary nitrogen loss in normal weight and obese subjects.

Figure 4. Relationship between P ratio, W/H2 and percentage fat in humans.