THERMOGENESIS - what it means for those on a diet!
The thermic (thermogenic) effects of certain dietary components have been known for many years, but were generally dismissed as of more academic than practical significance. It is now known that, far from being only a research curiosity, these effects can be utilized to significantly increase rates of weight loss, and that their exploitation in this way may also be associated with other benefits relating to behavioural and psychological aspects of the weight loss process. Furthermore, application of the thermogenic approach to weight loss plans is closer to a causal therapy of obesity than any dietary treatment hitherto tried or postulated.
Overweight patients are put on diets to create energy deficits, so that, to obtain the necessary energy to cover their basal metabolic requirements (keeping the body running; Resting Metabolic Rate; RMR) and their daily activities, they have to burn off their surplus fat. Unfortunately, the body adapts to this "time of famine"; the metabolic rate gradually falls and the body tries to preserve its not-so-secret stores of energy reserves as long as possible.
Frustrating for the patient and for those involved in treatment. So how do we overcome this problem and make sure that fat is eliminated at the greatest possible rate?
The first step is to keep the metabolic rate up by stimulating thermogenesis. This is an awkward word meaning "creation of heat", and in the context of dieting it means creating energy, thus increasing energy requirements.
There are various types of thermogenesis, of which dietary thermogenesis is the foremost. In simple terms, the body responds to intake of major nutrients by imposing a sort of handling charge, which is the energy the metabolism must generate to cover the cost of digesting, absorbing, breaking down and rebuilding the nutrients. It can be quite significant; for example, about 30% of the caloric value of protein is required to cover the processing in the body. It is rather less for other nutrients, and most experts now agree that people who tend to become fat easily have poor thermogenic response to nutrients, particularly fat and carbohydrate (Heleniak & Aston, 1989; Jéquier, 1987, 1989).
A minimal exercise program can also increase thermogenesis (a few minutes per day is enough; strenuous exercise can increase food intake). Physical exercise by itself is often a failure (to quote Garrow, 1978; "The suggestion that weight can be effectively lost by exercise . . . is not supported by published results"), but various investigators (cited by Miller, 1969) have reported that exercise-induced thermogenesis can increase the RMR by 10% - 20% for periods of days after the exercise. This property of exercise has been put to good use in combatting the fall in RMR that occurs in dieting overweight patients. For example, Van Dale et al. (1989) have shown that the RMR fell significantly less in women on a diet who were exercising than in a comparable group of dieting women who did not exercise.
Thermogenic responses are not restricted to food intake and exercise; natural constituents of some foods can also induce thermogenesis, either directly (by an effect on the metabolism) or indirectly, by stimulating heat loss (which has to be made up by the metabolism, so that body temperature remains constant).
The most common is caffeine, in tea and coffee, as well as similar compounds in cocoa and chocolate; these substances (collectively known as methylxanthines) work directly on the metabolism (James, 1983). This should not be taken to imply that good dietary programs can be based exclusively on chocolate bars and coffee!
There are herbs and spices which increase thermogenesis, again directly or indirectly. For example, Ephedra (also known as Ma huang) has been used as food or food adjunct since antiquity (Gerard, 1597; Mahdihassan and Mehdi, 1989; Katiyar et al., 1990). This herb has been characterized as "possessing a harmonious combination of trace elements, vitamins and other biologically active substances in ratios optimal for a human organism" (Gerasimova and Barelko, 1980). The activity of Ephedra is due to its content of ephedrine and related alkaloids, which have a direct stimulant effect on thermogenesis. Research has shown that these substances can significantly improve rates of weight loss on diets (Malchow-Moller et al., 1981; Pasquali et al., 1987), and also show significant protein-sparing effects (Pasquali et al., 1992); they work synergistically with caffeine and are effective at very low dosage. Unfortunately, side effects of an often uncomfortable nature are quite common, though rarely dangerous.
The alkaloids present in many Citrus species have much the same effect as those in the Ephedra herb, but are usually present in most Citrus fruits in such small amounts that consumption of oranges can play no role in weight loss programs! However, a recent entry to the market is the immature Bitter Orange(Citrus aurantium), which appears to be even more effective than Ephedra but without side effects (Jones, 1998; Colker et al., 1999). Most recently, it was shown that two of the alkaloids in Bitter Orange are specific beta-3 agonists, which means they target fat cells and initiate fat breakdown (Carpene et al., 1999; Fontana et al., 2000).
Other foods or substances in our environment can either stimulate thermogenesis or create conditions favourable for thermogenesis. Salicylates, such as aspirin, are thermogenic, as is nicotine, and one of the reasons why smokers "put on weight" when they quit is the sudden loss of this thermogenic effect! For obvious reasons, neither aspirin nor smoking can be advocated as part of a diet program.
However, the presence of fat in the diet, and in particular essential fatty acids (EFAs), has also been related to increases in rates of weight loss. The fact that patients on low calorie diets are at greater risk of EFA deficiency than the general population is well known, and in itself sufficient justification for assuring adequate EFA intake, but EFAs do more than prevent skin problems, gall bladder problems or a rise in cholesterol; they actually contribute to fitness and weight loss. EFAs of the omega-6 and omega-3 families have been shown to increase thermogenesis. It is not known whether this is an intrinsic consequence of their mechanisms of action (as precursors for eicosanoids and as membrane constituents), or whether it merely represents the rectification of a pre-existent but unsuspected EFA deficiency.
Goubern et al. (1990), for example, showed that brown adipose tissue cells recovered from EFA-deficient rats responded poorly to noradrenaline (norepinephrine; the hormone which stimulates thermogenesis), but that addition of linoleic acid (omega-6 EFA) and the saturated fatty acid, palmitic acid, to the medium normalized the response. Alam et al. (1995) also presented evidence that cyclic adenosine monophosphate (cAMP) production can be impaired in EFA deficiency, which would manifest as decreased sensitivity to catecholamines, with subsequent reduced thermogenesis.
Clandinin et al. (1992) similarly showed that linoleic acid increased the binding of insulin to adipose tissue cells (and thus improved their metabolic responses). These researchers also reported that omega-3 EFAs increased the responsiveness of muscles to insulin, and significantly increased the rate of glucose uptake by the muscle. Takada et al. (1994) showed that a dietary intake of gammalinolenic acid increased the ability of the liver to oxidize fats.
At an empirical level, Bucci (1994) cites studies which have shown that supplementation with long chain omega-3 EFAs (from fish oil) improves aerobic metabolism, while some research groups (Cunnane et al., 1986; Jones and Schoeller, 1988) have shown that increases in EFA intake improve rates of weight loss by a presumed thermogenic mechanism and also improve the efficiency of energy-generating metabolic processes in the body.
Finally, what about stimulating thermogenesis by increasing heat loss directly? The naked human body is in equilibrium with the environment at about 26°C (79°F), and below this temperature the metabolism has to work harder to generate the heat needed to maintain body temperature.
Thus in both theory and in practice, patients seeking to lose weight during the winter can do so more readily if they set back their heating thermostat by a couple of degrees and wear fewer clothes! If taken to extremes, then another type of thermogenesis would also occur, the so-called shivering thermogenesis (which is not a pure metabolic event but also includes involuntary muscular actions). However, the discomfort for the patient, and the risk of hypothermia, precludes induction of shivering as a practical way of increasing weight loss.
So far, the methods noted favor loss of fat by simply maintaining a large difference between "energy out" and "energy in", so that the body has little choice but to burn fat. It is possible, however, to stimulate the biological process, lipolysis, responsible for releasing fat from the fat stores. To become available for use, stored fat must be converted into free fatty acids, and if lipolysis is stimulated to release more fat, a greater amount will be used for energy generation (some tissues, such as muscle, prefer free fatty acids as a source of energy), thus sparing some protein and carbohydrate.
In practice, stress
("fight or flight" response) increases lipolysis, but is hardly
to be promoted as a way of losing weight. Most appetite suppressant
drugs also increase lipolysis, but lose their efficacy rapidly, and
research is gradually turning the circle again, back to the natural
approach. In fact, the most recent research (Carpene et al., 1999; Fontana
et al., 2000) shows that the Bitter
Orange alkaloids are superior stimulants of lipolysis with no side-effects
when correctly used, and persistent action which may even increase with
time (compare Astrup et al., 1986). At a correct dosage, they may also
help suppress hunger!