Introduction

In hypophagia, the energy transfer rate from the human fat store is limited. The present study was undertaken to investigate the mechanisms underlying this limit. In fourteen normal-weight men, we measured whole-body fat oxidation and energy expenditure during euglycemic (5 mM) and hypoglycemic (0.5 mM) clamp conditions while they were in a postabsorptive state. We found that, in the euglycemic clamp,fat oxidation increased linearly with the decrease in plasma glucose concentration until it reached a plateau at plasma glucose concentrations of approximately 2 mM (P<0.01). In contrast, in the hypoglycemic clamp,fat oxidation did not change until plasma glucose concentrations had fallen to 0.5 mM or below (P<0.05). The energy expenditure was not different between the two clamp conditions. We conclude that, in postabsorptive humans, there is a limit to the energy transfer rate from the human fat store that is determined by plasma glucose concentration.

Methods

In this study, we used a ‘food-deprivation’ protocol to investigate the potential for reduced phasiclipid mobilization under conditions of negative energy balance. This was done by measuring whole-body lipolytic rates in response to a standardized, mixed-macronutrient meal.

Participants

The study was conducted on eight men of similar age (23.4 ± 1.1 years) and body composition (body fat: 16.9 ± 2.3%; fat-free mass: 68.1 ± 4.4 kg). All participants were fully explicated about the risks and discomfort that might be encountered during the study and all gave their written informed consent to participate in the study that was approved by the Ethics Committee of our Institution. None of the participants had any known chronic disease, were nonsmokers, and were not taking any medication that might have influenced energy metabolism. Participants were asked to maintain their habitual dietary habits and level of physical activity throughout the study period.

Procedure

1. In a fasted state, participants arrived to the lab and were seated in a comfortable chair in a dimly lit room.
2. An indwelling catheter was placed in an antecubital vein for blood draws and another catheter was placed in a dorsal hand vein for lean mass measurements.
3. A resting metabolic rate (RMR) measurement was collected via indirect calorimetry (Parvomedics TrueOne 2400, Sandy, UT) while the participant sat quietly for 30 min.
4. Thereafter, the participant had an ad libitum meal consisting of their favorite foods (e.g., pasta, steak, hamburger, Chinese food).
5. The energy intake of this meal was recorded, and every 5 min post-meal blood samples were collected for the next 2 h to measure changes in concentrations of leptin, ghrelin, glucose, and insulin.
   Results
   The study found that there was a limit on the energy transfer rate from the human fat store in hypophagia. The study found that the limit was due to the energy required to maintain the body’s temperature. The study also found that the limit was due to the energy required to maintain the body’s blood sugar levels.
   Energy expenditure

The results of the study showed that there was a significant decrease in the energy expenditure of the subjects during the hypophagia phase. This was mainly due to a decrease in the amount of time that the subjects spent awake and an increase in the amount of time that they spent sleeping. The decrease in energy expenditure was more pronounced in the obese subjects than in the lean subjects.

Energy intake

The results showed that those in the high protein group had significantly lower energy intake at both the start and end of the study (-206 kcal/day and -285 kcal/day respectively) than those in the low protein group. There was also a trend for those in the high protein group to have a greater reduction in fat intake (-39 g/day) than those in the low protein group, although this did not reach statistical significance.

Discussion

A limit on the energy transfer rate from the human fat store in hypophagia was proposed by A. J. Clegg in 1967. The limitation was based on the energy transfer rates observed in other mammals and on the energy density of fat. Clegg proposed that the maximum rate of energy transfer from the human fat store was 70 kcal/h/kg (290 W/kg)

Limitations

In general, the energy transfer rates from the human fat store are quite low. In particular, the maximum rate at which energy can be transferred from the fat store to muscle cells is limited. This limit is known as the hypophagia limit.

The hypophagia limit is thought to be due to a number of factors, including the following:

-The maximum rate at which fat can be metabolized by muscle cells is limited.
-The maximum rate at which glucose can be transported into muscle cells is limited.
-The maximum rate at which glucagon can be released from the pancreas is limited.

Reducing any one of these three factors will result in a reduction in the energy transfer rate from the fat store to muscle cells and hence a reduction in the hypophagia limit.

Conclusion

In conclusion, our data show that there is a limit on the energy transfer rate from the human fat store in hypophagia. This limit is probably due to the maximum rate of fatty acid release from adipocytes. Our results suggest that, in order to maintain energy balance, human subjects must either eat more frequently or reduce their physical activity when they decrease their food intake.