Specific appetite
Encyclopedia
Specific appetite, also known as specific hunger is a drive to eat foods with specific flavors or other characteristics.
Regulation of homeostasis
is essential to the survival of animals. Because the nutritional content of a diet will vary with environmental and other conditions, it is useful for animals to have a mechanism to ensure that their nutritional needs are within the appropriate range. Specific appetite is one such mechanism. Specific appetite has been demonstrated in various species for a number of vitamins and minerals, as well as calories, protein, and water. Unfortunately, specific appetite is very difficult to study experimentally, as there are a number of factors that influence food choice. Very little is known about the specific mechanisms inducing specific appetite, and the genes encoding for specific appetites are mostly speculative.
Very few specific appetites for particular nutrients have been identified in humans. The most robustly identified are salt appetite/sodium appetite. The problem with many other nutrients is that they do not have distinctly identifiable tastes, and only two other specific appetites, for iron and calcium, have been identified with experimental rigour so far. Other appetites are thus currently classified as learned appetites, which are not innate appetites that are triggered automatically in the absence of certain nutrients, but learned behaviours, aversions to or preferences for certain foods as they become associated with experiences of malnutrition and illness.
, which alleviates the consequences of heat stress Learned specific appetites are not necessarily a result of an animal's ability to detect the presence of a nutrient. Because nutrient deficiencies of various types can have stressful effects which vary depending on the missing nutrient, subsequent ingestion of that nutrient is associated with relief of certain signs. An animal may therefore associate the flavor of a food that is high in a certain nutrient with relief of the signs of that nutrient deficiency, while not seeking out other foods rich in the same nutrient.
via infusion directly into the brain developed a strong sodium appetite which persisted for months. However, the conclusions of this experiment have been contested. Nicotine implants in rats have been shown to induce a specific appetite for sucrose, even after removal of the implants.
who develop hypocalcemia prefer cheese with greater amounts of calcium added . Exercise also increases the preference for salt . Addison’s disease
is known to induce a specific craving for sodium, although other diseases causing hyponatremia may not induce the same response. . Extreme sodium depletion in human volunteers has been demonstrated to improve the palatability of high-salt foods .
is investigating candidate genes for calcium specific appetites using two strains of mice: the B6 strain, which shows no specific appetite, and the PWK mouse which has a demonstrated specific appetite for calcium.
Regulation of homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
is essential to the survival of animals. Because the nutritional content of a diet will vary with environmental and other conditions, it is useful for animals to have a mechanism to ensure that their nutritional needs are within the appropriate range. Specific appetite is one such mechanism. Specific appetite has been demonstrated in various species for a number of vitamins and minerals, as well as calories, protein, and water. Unfortunately, specific appetite is very difficult to study experimentally, as there are a number of factors that influence food choice. Very little is known about the specific mechanisms inducing specific appetite, and the genes encoding for specific appetites are mostly speculative.
Very few specific appetites for particular nutrients have been identified in humans. The most robustly identified are salt appetite/sodium appetite. The problem with many other nutrients is that they do not have distinctly identifiable tastes, and only two other specific appetites, for iron and calcium, have been identified with experimental rigour so far. Other appetites are thus currently classified as learned appetites, which are not innate appetites that are triggered automatically in the absence of certain nutrients, but learned behaviours, aversions to or preferences for certain foods as they become associated with experiences of malnutrition and illness.
Learned appetite
If a food source has an identifiable flavor, an animal can learn to associate the positive effects of alleviation of a certain nutrient deficiency with consumption of that food. This has been demonstrated in a variety of species: lambs offered free choice of various foods will compensate for phosphorus, sodium, and calcium deficiencies. Domestic fowl have demonstrated specific appetites for calcium, zinc, and phosphorus, thiamine, protein in general, and methionine and lysine. Heat-stressed fowls seek out vitamin CVitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...
, which alleviates the consequences of heat stress Learned specific appetites are not necessarily a result of an animal's ability to detect the presence of a nutrient. Because nutrient deficiencies of various types can have stressful effects which vary depending on the missing nutrient, subsequent ingestion of that nutrient is associated with relief of certain signs. An animal may therefore associate the flavor of a food that is high in a certain nutrient with relief of the signs of that nutrient deficiency, while not seeking out other foods rich in the same nutrient.
Unlearned appetite
An unlearned appetite is one that an animal possess at birth, without conditioning through prior stress followed by alleviation with specific foods. An unlearned appetite suggests a physiological mechanism for detecting the absence of a nutrient as well as a signalling component that directs the animal to seek out the missing nutrient. An example of an unlearned appetite might be caloric appetite, as seen in all domestic animals. Other unlearned appetites are more difficult to demonstrate. In one study, protein-deficient rats that had not previously experienced protein deficiency demonstrated strong preferences for high-protein foods such as soybean, gluten, and ovalbumin within thirty minutes of food presentation. This preference was not seen in controls, and was also exhibited by pregnant females with higher protein needs who were not experimentally protein-deficient. Rats also seem to have an unlearned appetite for calcium and sodium. In addition, zinc-depleted chicks show preferences for zinc-rich feeds.Indirect manipulation of specific appetite
Specific appetite can be indirectly induced under experimental circumstances. In one study, normal (sodium-replete) rats exposed to angiotensin IIAngiotensin
Angiotensin, a peptide hormone, causes blood vessels to constrict, and drives blood pressure up. It is part of the renin-angiotensin system, which is a major target for drugs that lower blood pressure. Angiotensin also stimulates the release of aldosterone, another hormone, from the adrenal cortex...
via infusion directly into the brain developed a strong sodium appetite which persisted for months. However, the conclusions of this experiment have been contested. Nicotine implants in rats have been shown to induce a specific appetite for sucrose, even after removal of the implants.
Specific appetite in humans
There is very little strong evidence for specific appetite in humans. However, it has been demonstrated that humans have the ability to taste calcium , and indirect evidence supports the idea that patients on kidney dialysisDialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...
who develop hypocalcemia prefer cheese with greater amounts of calcium added . Exercise also increases the preference for salt . Addison’s disease
Addison's disease
Addison’s disease is a rare, chronic endocrine disorder in which the adrenal glands do not produce sufficient steroid hormones...
is known to induce a specific craving for sodium, although other diseases causing hyponatremia may not induce the same response. . Extreme sodium depletion in human volunteers has been demonstrated to improve the palatability of high-salt foods .
Ongoing research
While the most common nutritional disorders in humans concern excessive intake of calories, malnutrition remains a problem. For example, the link between insufficient dietary calcium and bone disorders is well established Michael Tordoff’s lab at the Monell Chemical Senses CenterMonell Chemical Senses Center
The Monell Chemical Senses Center is a non-profit independent scientific institute located at the University City Science Center research campus in Philadelphia, Pennsylvania...
is investigating candidate genes for calcium specific appetites using two strains of mice: the B6 strain, which shows no specific appetite, and the PWK mouse which has a demonstrated specific appetite for calcium.