3. Respiratory quotients and substrate oxidation rates in chronically energy deficient subjects

Several reports on BMRs of Indians have indicated that their RQs vary over a wide range and that they generally tend to be higher than 0.82, a value which is frequently quoted in the Western literature as a normal average (Table 1). SHIV KUMAR et al. (1961), who studied 339 apparently normal, healthy subjects aged between 18 to 80 years, noted that the RQs of these subjects varied from 0.70 to 0.97, with a mean value of 0.83 (SEM ± 0.003) and a coefficient of variation (CV) of 7.1%. These authors stated in their paper that, since the publication of the large study on BMRs of Indians with concurrent measurements of RQ by SEN and BANERJEE (1958), evidence has been accumulating that the average RQ of Indians is somewhat higher than 0.82. SHIV KUMAR and others (1961) also made the suggestion that the wide normal range of basal RQs in their study (ranging from 0.70 to 0.97) deserved greater attention and emphasis than the slight differences seen in the means as compared to the mean RQs reported in Western subjects. It is assumed that all these observations were made in apparently healthy individuals who maintained stable weight.

The large range of RQ values seen in these studies from India may be attributed to the varying nutritional status of these apparently normal adults who were weight stable. It is not unlikely that a fair proportion of these subjects had low body weights and low body mass indices (BMIs), and that a proportion of the apparently healthy individuals in this reasonably large sample were weight stable but chronically energy deficient. Table 2 summarises the RQs of undernourished or CED subjects from the same geographical region. It clearly shows that individuals of poor nutritional status have a higher basal fasting RQ than well-nourished controls from the same region. The wide range of RQ values reported in Table 1 may thus represent contributions from undernourished subjects, present in the large samples of individuals whose BMRs were reported.

Table 1. Respiratory quotients of apparently healthy Indians reported in the literature

Number of subjects	Respiratory quotient		Reference
	Mean	Range
18	0.84	0.71-0.99	MUKERJEE and GUPTA, 1931
9	0.81	0.72-0.87	AHMED and Roy, 1938
138	0.84	0.71-0.98	SEN and BANERJEE, 1958
99*	0.84	0.73-0.98	SEN and BANERJEE, 1958
339	0.83	0.70-0.97	SHIV KUMAR et al., 1961

* Female subjects.

Table 2. Respiratory quotients of chronically energy deficient or undernourished Indians

Undernourished	Well-nourished	Reference
0.94	0.84	RAMANAMURTHI et al., 1962
0.94 (0.02)	0.77 (0-03)	SHETTY, 1984
0.94 (0.02)	0.79 (0-05)	SOARES et al., 1991
0.90 (0.07)	0.87 (0.05)1	SOARES and SHETTY, 1991
0.93 (0.08)	0.84 (0.06)2	SOARES and SHETTY, 1991
0.93 (0.03)	0.81 (0.01)	PIERS et al., 1992b

1 Rural population.
² Urban population.

Figures in parenthesis are standard errors.

Table 3. Comparisons of respiratory quotients (RQ) with food quotients (FQ) in chronically energy deficient (CED) subjects

	Controls		CED Subjects
	RQ	FQ	RQ	FQ
Fasting RQ vs FQ (24-h recall)	0.86	0.84	0.95	0.94
Fasting RQ vs FQ (weighed)	0.85	0.81*	0.93	0.88*
Fasting RQ vs FQ (weighed)+	0.83	0.82	0.93	0.90*

* Statistically significant.
⁺ During 36 hours of calorimetry.

Table 4. Respiratory quotients and substrate oxidation rates of chronically energy deficient (CED) subjects in the post-absorptive, fasted state

	Respiratory quotient	Substrate oxidation rates
		g/h			kJ/kg/h
		C	F	P	C	F	P
Well-nourished	0.81	5.0	3.8	2.9	1.3	2.1	0.7
CED subjects	0.93*	8.1*	1.1*	2.4	3.1*	0.9*	0.9
C = carbohydrate		F = fat			P = protein

* Statistically significant differences by analysis of variance.

From PIERS et al., 1992b.

The higher RQs of Indians have generally been attributed to their diets containing a high proportion of carbohydrates. This seems a reasonable assumption. Since mean RQs of free-living subjects are not readily and continuously available, BLACK and her colleagues (1986) suggested a different approach to predict the RQ from the macronutrient content of the diet. This was termed the food quotient (FQ). Under conditions of energy balance and weight maintenance, the FQ must equal RQ, and the RQ should reflect the macronutrient composition of the diet.

Comparisons of FQs, made in a number of well-nourished and CED subjects, and calculated according to the methods suggested by BLACK et al., (1986), were compared with the 12-14 hour postabsorptive, early morning RQs obtained during the measurement of a BMR. No differences were seen between the FQs based on dietary recall and post-absorptive, fasting RQs in either group of subjects studied (Table 3). As noted above, the fasting RQs of the CED subjects were significantly higher than those of the well-nourished. Their FQs corroborated this, since the antecedent habitual diets of these individuals were high in carbohydrate content. The lower FQs in the well-nourished controls were accounted for by a higher fat intake than in the CED subjects, despite the carbohydrate intakes in both groups being higher than the usual Western diets. However, comparisons of FQs from weighed intakes over several days, with the RQs of the same subjects obtained during a standard BMR measurement, showed that both had FQs that were significantly lower than RQs.

Fasting RQs obtained during a 36-hour cycle in an indirect calorimeter were also computed in both well-nourished controls and undernourished CED subjects while in the whole-body calorimeter. Differences between RQs and FQs were seen only in the CED subjects, with RQs being higher than FQs. The FQs were obtained from the composition of the actual food provided to the subjects during 36-hour calorimetry runs. That food was different from their habitual diets in terms of the macronutrient composition. It seems, however, that the macronutrient composition of the diet ingested, as indicated by the mean FQ of the day, is not truly reflected in the fasting RQ of the same day. Since the individuals were maintaining stable body weights and therefore were unlikely to be in a state of energy imbalance, RQs should equal FQs. The higher fasting RQ may be compensated for by FQ > RQ at other times of the day. Records of the 24-hour RQs in these CED subjects do show that the post-exercise RQs are lower than the FQs.

When substrate oxidation rates were calculated during the postabsorptive, fasted state in these CED subjects, using indirect calorimetry and urinary N excretion (without correcting for changes in the blood urea pool), the CED subjects had significantly higher rates of carbohydrate oxidation and lower rates of fat oxidation in the fasted state than the well-nourished controls (Table 4). No differences were seen in the rates of protein oxidation; an observation that is in keeping with the evidence of similar rates of protein turnover in CED and well-nourished subjects (SOARES et al., 1991). It would appear that the CED subjects have a higher RQ largely due to their selective use of carbohydrate as fuel even in the post-absorptive, fasted state.

It is important to recognise that, contrary to general belief, fat is not necessarily the predominant substrate in the post-absorptive, fasted state and is certainly not the preferred substrate in the chronically undernourished. However, the selective utilisation of carbohydrate illustrates how closely carbohydrate oxidation is adjusted not only to its immediate availability, as demonstrated in well-nourished individuals (FLATT et al., 1985), but probably also relates to the antecedent habitual intakes of carbohydrate in the diets of the undernourished.

The selective use of carbohydrate as fuel has obvious metabolic advantages to the CED individual, since carbohydrate (glycogen) oxidation generates more ATP than iso-energetic amounts of fat or protein (LIVESEY, 1984; WATERLOW, 1988). Also the metabolisable energy equivalent, i.e., the energy equivalent of ATP gained (in kJ per mole ATP) is almost identical to that of fat (75.3 for glycogen oxidation via glycolysis and the citric acid cycle; ELIA and LIVESEY, 1992). It is hence not unlikely that the high fasting RQs of the CED reflect the metabolic efficiency of active tissues of these subjects.

FLATT (1987), in an elegant hypothesis, has implicated adequacy of carbohydrate stores as being central to ingestive behaviour in humans. The need to meet adequate carbohydrate intakes may result in the ingestion of more lipid in a habitually high-fat diet, which may result in positive energy balance and excess fat storage, thus predisposing to obesity. A corollary to the main hypothesis is that, for the purposes of long-term equilibration and body weight stability, an increase in body fat may in turn lead to an increase in fat oxidation (SCHUTZ et al., 1992). A direct extrapolation of Flatt's hypothesis must surely imply that substrate oxidation is related to the fat mass of an individual. In broader, general terms, this must mean that the fasting RQ is related to the body composition of an individual.

Meta-analysis of several recent studies done by our group in well-nourished and CED subjects who were weight stable (means of 15 separate sets of data points), was used to look at relationships between RQ and body composition parameters (Figure 1). Body weight and FFM were negatively and linearly correlated to RQ (Body weight: r = 0.92; p < 0.001 and FFM: r = -0.92; p < 0.001) as well as fat mass expressed as a ratio of active tissue mass, i.e., Fat/FFM. The latter relationships were also negative and linear (Fat mass: r = -0.87; p<0.001 and Fat/FFM: r = -0.83; p<0.001). The RQ of an individual seems to reflect his body composition and more specifically the available fat stores. The high RQs of the CED subjects may thus reflect lower than normal fat stores in the individual. FLATT (1972) has suggested that the antilipolytic effect of insulin is less effective in the presence of an increased fat mass, and an increased level of insulin is thus associated with high free fatty acid levels in obesity. Since fat oxidation has been shown to be directly related to the levels of free fatty acids (ISSEKUTZ et al., 1968; GROOP et al., 1991), it is apparent that low fasting levels of free fatty acids are likely to be associated both with the small fat mass seen in CED subjects and consequently associated with lower rates of fat oxidation. The low rates of fat oxidation in the CED will contribute to the high fasting RQs. The high RQs of the CED subjects may thus reflect both a high dietary intake of carbohydrate as well as a predominant dependence on carbohydrate as fuel and a reduced rate of fat oxidation in the presence of low levels of circulating free fatty acids associated with the low fat stores observed in these individuals.

Figure 1. Shows the linear relationship and the negative correlations between RQs and (a) Body weight, (b) FFM, (c) Fat mass, and (d) Fat mass/FFM based on a meta-analysis of several studies.