Trafficking of dietary fat and resistance to obesity

Abstract

The task of maintaining energy balance involves not only making sure that the number of calories ingested equals the number of calories burned but also involves ensuring nutrient balance. This means that over time, the quantity of carbohydrate, fat and protein consumed equals the amount of each oxidized. While the body has the ability to convert protein to carbohydrate and carbohydrate to fat, over long periods of time the body establishes nutrient balance with a high degree of accuracy storing excess nutrients as fat. To make decisions about food intake, the brain must assimilate information about the quantity of nutrients ingested and their disposition through the body over time. This is a very complex time ordered process as different tissues may be in different states of energy balance at different intervals following food ingestion. The fundamental task for the brain is to assess the influx of nutrients relative to stored pools of those nutrients and the rate at which they are being oxidized. It has been suggested that this task is particularly difficult for dietary fat because the stored pool of lipid is quite large compared to either the stored pools of carbohydrate and protein or the quantity of fat ingested per day. It is clear that some organisms resist weight gain even in the face of highly palatable diets. In fact most individuals eat less on any given day than they could given their maximal capacity for consumption. A central question then is: what restrains food intake in the setting of widely available highly palatable food? In this paper we will discuss the evidence that the movement of dietary fat between tissues may play an important role in the fidelity of nutrient sensing and as a result, resistance or susceptibility to obesity. In particular, the relative metabolism of dietary fat favoring oxidation over storage may be associated with more robust signaling of positive energy balance and resistance to dietary induced obesity in both humans and rats.

Energy balance, body composition, sedentariness and appetite regulation: pathways to obesity

“…However, efforts to suppress energy intake by the stimulation of fatty acid oxidation have failed to consistently show an effect on food intake (82,83). This may be because changes in whole body fat oxidation provide a weak regulatory signal for food intake, with the amount of fat ingested or oxidised on a daily basis very small relative to the total amount of energy stored as adipose tissue (84). Indeed, while it has been suggested that that nutrient metabolism may exert negative feedback on food intake e.g.…”

Dietary fat sensing via fatty acid oxidation in enterocytes: possible role in the control of eating

“…Therefore, only minimal mealrelated fluctuations of hepatic FAO can be expected. In fact, labeling studies demonstrated that ingested fat reaches the liver in substantial amounts not before 2-3 h after the onset of ingestion, i.e., long after meal termination (13). With ad libitum access to food, fatty acids derived from ingested TAG may in fact become available to hepatocytes as an energy source at just about the time when a new meal begins, at least in the rat.…”

“…Given the replacement of other nutrients by fats as fuels during the postprandial period, meals may actually be initiated during a gradual increase rather than a decrease in whole body FAO, indicating that the latter does not provide a physiological hunger signal. On the other hand, meal-related changes in the oxidation of dietary triacylglycerol (TAG)-derived fatty acids may occur at critical sensor sites in particular tissues, and ample evidence indicates that differences in FAO or, more specifically, differences in the organism's capacity to increase the oxidation of dietary TAG-derived fatty acids in response to increases in dietary fat intake are one of the factors determining the susceptibility to dietary fat-induced overeating and weight gain (13,65,76,149). A critical question, thus, is where in the body oxidation of dietary TAG-derived fatty acids is monitored to influence energy intake and expenditure.…”

Dietary fat oxidation as a function of body fat

“…A recent review [23] concluded that the partitioning of dietary fat between tissues might play an important role in the resistance or susceptibility to obesity. The relative metabolism of dietary fat, favoring oxidation over storage, may be associated with more robust signaling of positive energy balance and resistance to dietary-induced obesity.…”