Insect Fatty Acids Higher In The Polyunsaturates

Most of the attention on insects as a food source has focused on their high protein content. The malnutrition problem in much of the developing world is the result, however, not principally of a deficiency in protein or protein quality, but rather of a deficiency in total calories. While most North Americans are not looking for more calories, we are looking for animal sources with a lower proportion of saturated fatty acids and a higher proportion of monounsaturated and polyunsaturated fatty acids. It is of interest, then, that insects range from low to high in fat, from less than 10% to more than 30% on a fresh weight basis, and are relatively high in the C18 fatty acids, oleic acid (18: 1), linoleic acid (18:2) and linolenic acid (18:3).

Calvert et al (1969), while conducting studies on the protein quality of house fly pupae (Musca domestica L.) fed to broiler chicks, also analyzed the fatty acids of the pupae and noted that the fatty acid pattern resembled those of some fish oils. The fatty acid composition of house fly pupae and of the edible stages of several species of insects used as human food are shown in Table 1 (page 2). Although diet and development exert strong influences on fatty acid profiles (Stanley-Samuelson et al 1988), the many analyses that have been conducted show a relationship between fatty acid composition and the taxonomic grouping of insects (Thompson 1973). Fast (1970) summarized the results of fatty acid analyses on insects up to that time, and it is apparent from his tabulation, for example, that the Coleoptera (beetles and weevils) in general are particularly high in C 18:2 while the Lepidoptera (butterflies and moths) are particularly high in C 18:3. Linoleic acid comprised 25 % or more of the total fatty acid composition in nearly 40% of the Coleoptera species analyzed, while linolenic acid comprised 25% or more in nearly 50% of the Lepidoptera species.

The essential fatty acids, which include linoleic acid (18:2w6) and a-linolenic acid (18:3w3). serve several physiological functions in vertebrates. As components of specific phospholipids, they are important to the integrity of cellular lipid membranes and their

associated enzyme activities. They provide the C20 fatty acid precursors for the hormone-like eicosanoid compounds needed for localized metabolic regulation in many tissues (Dadd 1983). The essential fatty acids also regulate cellular lipid metabolism and are required for growth. Biochemically, fatty acid deficiency in warmblooded vertebrates is characterized by reduced levels of the tetraene, arachidonic acid (C20:4w6) and increased levels of the triene, eicosatrienoic acid (C20:3w9) (Dadd 1983). Linoleic acid, the root member of the w6 (or n6) family, can be metabolized by carbon-chain elongation and further desaturation to arachidonic acid through the following steps (w6 family): 18:2w6 (linoleic) to 18:3w6 to 20:3w6 to 20:4w6 (arachidonic). As with linoleic acid, a-linolenic acid (Cl 8:3 or 18:3w3), which is the root member of the w3 (or n3) family can be elongated and further desaturated through a series of intermediates up to docosahexaenoic acid (22:6w3): 18:3w3 to 18:4w3 to 20:4w3 to 20:5w3 to 22:6w3 (docosahexaenoic). It was thought until recently that all EFA deficiency symptoms in mammals and birds were remitted by dietary fatty acids of the w6 family, but Dr. Ney notes that this is not true in infants and rapidly growing vertebrates. Recent research indicates that both 18:2w-6 and 18:3w3 have essential functions in warm-blooded animals. The w9 family of which oleic acid (18:1w9). is the root member is not essential.

Alpha-linolenic acid or higher members of the w3 family are the primary essential fatty acids for many fish (Dadd 1983). Fish acquire them from ingestion of plankton and other plants. The evidence from vertebrate metabolic studies is that conversions between members of the w3, w6, and w9 families do not occur. Cats are an exception to the general rule that vertebrates can convert ‘the C18 polyunsaturates of food into the physiologically necessary C20 and C22 polyunsaturates. Also, some w3-requiring fish are unable to metabolize linolentic acid to longer chain polyunsaturates, thus these animals must acquire them preformed in their food.

It has only recently become recognized that the C20 and C22 fatty acids occur generally in insects and are probably physiologically important to most, or all, species. Although a few pre-chromatographic analyses indicated substantial proportions of arachidonic acid or a similar tetraene, subsequent gas chromatographic analyses have rarely recorded polyunsaturates of carbon chain length greaterthan 18 (Fast 1970; Dadd 1983). In crude lipid extracts of whole insects the long-chain polyunsaturates generally constitute no more than 2% of the fatty acid total, although they may be present in 5-10 times this proportion in extracts Erom certain tissues (Dadd 1983, Stanley-Samuelson and Dadd 1983).

The functions of fatty acids that are essential to vertebrates apply also to insects. The major sources of fatty acids in edible insects are the phospholipids in the cell membranes and the glycerides (especially the triglycerides in the fat body). Dr. Ritter notes that, since insects are poikilotherins the degree of unsaturation of the fatty acids associated with the phospholipids is very important for helping to regulate the fluidity of the membranes. In contrast, the structure of triglycerides is less important, physiologically, and so may be more variable and influenced by diet. Fatty acids also have several other functions that are more-or-less unique to insects such as precursors in biosynthesis of waxes, pheromones, and eicosanoids, and as components of defensive secretions (Stanley-Samuelson et al 1988).

Table 2: The proportions of saturated/unsaturated fatty acids in beef, pork, poultry and fish (adapted from National Research Council 1988)

Percent of total fatty acids

Animala SFA b MFAc PFAd

Beef 52.0 (28.1) 44.2 (3.8)
Pork 44.1 (24.3) 44.3 (11.6)
Chicken 35.5 (20.2) 40.8 (23.7)
Fish 29.6 (22.6) 39.6 (30.8)

a For beef, the averages of 27 combinations of cut and grade; for pork, the averages of 16 cuts; for chicken, the averages of 8 parts; for fish, the averages of two products each of haddock, halibut and tuna.
b Saturated fatty acids; stearic acid, C 1 8:0, is shown in parentheses as a percentage of total saturated fatty acids (see text).
c Monounsaturated fatty acids.
d Polyunsaturated fatty acids.
It appears that the elongation/desaturation pathways described above for vertebrates apply to most, although not all, insects. Of particular interest is the presence of long-chain polyunsaturates in

Legend: Table 1 (Fatty Acids of Edible Insects)

1 Saturated acids: 16:0 = palmitic acid; 17:0 = margaric; 18:0-stearic; unsaturated acids:

2 A = adult; L = Larva; P = pupa; N = nymph; m = male; f = female.
3 References: 1 = Albrecht 1961; 2 = Bachstez & Aragon 1942; 3= Barlow 1964; 4 = Calvert et al. 1969; 5 = Demainovsky & Zubova 1956; 6 =Fast 1966; 7 = Fauzi et al. 1961; 8 = Giral et al 1946; 9= Grapes et al. 1989; 10 = Herodek & Farkas 1960; 11 = Hutchins & Martin 1968; 12 = McFarlane et al. 1984; 13 = Nakosone & Ito 1967; 14 = Oliveira et al. 1976; 15 = Saha et al 1966; 16 = Shaeffer 1968; 17= Scoggin & Tauber 1950; 18 = Thompson & Barlow 1972; 19 = Young 1967. References listed here but not in the References Cited can be found in Fast (1970).
4 Most insects probably contain long-chain polyunsaturates, but they generally constitute no more than 2% of the total fatty acids.
5 In Oliveira et all (1976), the same values are tabularized for palmitic (C16:0) and palmitoleic (C16: 1) acids; we have assumed that the values listed by Oliveira are correct for palmitic acid and incorrect for palmitoleic acid. The values listed above for the latter represent the difference between the totals of fatty acid percentages listed by Oliveira and 100%.
6 The data are for 2-day-old pupae.
7 M. atlantis is considered a synonym of M. sanguinipes.

phytophagous insects such as lepidopterous larvae (Dadd 1983), many species of which are used as food. Such insects presumably biosynthesize the longer-chain polyunsaturates from the C18 polyunsaturates in their food. Among insects used as food, arachidonic acid has been detected in the locust, Locusta migratoria and the cricket Acheta domesticus (Stanley-Sainuelson and Dadd 1983). Various prostaglandins have been detected in insects and Stanley-Samuelson et al (1988) review what is known about their biosynthesis. In addition to functions probably similar to those in vertebrates they are known to have a role in the reproductive biology of some species (Stanley-Samuelson and Dadd 1983; Stanley-Samuelson et al 1988),e.g., among specific food insects the crickets Acheta domesticus and Teleogryllus commodus, the silkmoth Bombyx mori, and the African termite Macrotermes subhyalinus.

Unlike vertebrates, some insects can synthesize linoleic acid de novo (there is good evidence for at least 15 species in four orders) (Stanley-Samuelson et al 1988). At least one food insect, the”‘ mealworm Tenebrio molitor, can biosynthesize linoleic and linolenic acids (Dadd 1983).

Table 2 above shows the proportions of saturated/unsaturated fatty acids in various vertebrate animal species, as averaged from market available cuts and grades (see Table 2 footnotes). The averages are derived from data tabulated by the National Research Council (1988) and are intended as fairly representative values although there is considerable variation in individual cuts, grades and parts. As shown in Table 1, the saturated/unsaturated ratio of most edible insects is less than 40% saturated, grouping them with poultry and fish. The other most notable feature of insect fatty acids is the very high ratio of the polyunsaturates, linoleic and linolenic acids, higher in general than found in poultry and fish. Less favorable for insects, but of relatively minor importance, is that they are generally lower than the vertebrates in stearic acid, C18:0, as a proportion of the saturated fatty acids. Stearic acid, unlike other saturated fatty acids, does not raise the plasma cholesterol level (NRC 1988).

The main objective here has been to bring together the available data on fatty acid composition of the edible insect groups. Their dietary value compared to other animal sources in specific situations concerning human or aminal nutrition remains to be determined. Dr. Ney notes that essential fatty acid deficiency is rare in human populations in the developed countries, and the current trend in human nutrition is not to more polyunsaturated fat intake but to less saturated fat. According to Dr. Csete, the degree of saturation of fatty acids is probably not much of an issue in developing countries, at least not in places where undernutrition is most severe. Meeting essential fatty acid requirements in the diet and also attaining enough calorie density in general, particularly in certain seasons and for children whose stomachs can take only so much bulk, are much more important. Because so much of the insect body would be indigestible but would fill the infants stomach, she raises a question as to whether insect fatty acids can meet the need for calorie-dense foods that young children, in particular, can ingest in adequate quantities to make a difference.

Research is needed. Relative to marginally nourished children’s diets, for example, the more “thin-skinned” insects among those traditionally used may play a valuable role. And processed insects that have undergone dechitinization may have a place (the chemical methodology is simple and cheap). Some robust young children have been observed in indigenous populations that rely heavily on insects as part of the diet, suggesting an ample protein/calorie intake for both mothers and children. Mountford (1946:98), who studied the Pitjendadjara aborigines in central Australia, provided an interesting photograph of a native baby, age unstated but sitting upright, ‘fat and saucy,” who “thrives on a diet of mother’s milk, white grubs, and honey ants.” Mountford drove home the point that the child’s home was in the Mann Range “where previous travelers’ reports indicated that the country was too bad to support even aborigines.” Tindale (1953) similarly states that, “Aborigines with access to witjuti grubs [leopard moth larvae, genus Xyleutes] usually are healthy and properly nourished… Women and children spend much time digging for them and a healthy baby seems often to have one dangling from its mouth in much the same way that one of our children would be satisfied with a baby comforter. “And the older literature from Africa is full of accounts of how indigenous populations grew noticeably fatter and in better condition when termites or grasshoppers were available. Relative to animal nutrition, Dr. Lindsay notes that there is currently a particularly active search for new sources of the w3 fatty acid precursors of long chain w3 polyunsaturated fatty acids and for the latter preformed for use in pond fish production. Yes, research is needed.

Gene R. DeFoliart, Editor.

References Cited

Bachstez, M.; Aragon, A. 1942. Notes on Mexican drugs. 11. Characteristics and composition of the fatty oil from ‘Gusanos de Maguey’ (caterpillars of Acentrocneme hesperiaris). J. Am. Pharm. Assoc. 3:145-46.
Barlow, J.S. 1964. Fatty acids in some insect and spider fats. Canad. J. Biochem. 42:1365-74.
Calvert, C.C.; Martin. R.D.; Morgan, N.O. 1969. House fly pupae as food for poultry. J. Econ. Entomol. 62:938-39.
Dadd, R.H. 1983. Essential fatty acids: insects and vertebrates compared. In Metabolic Aspects of Lipid Nutrition in Insects (T.E. Mittler, R.H. Dadd, eds.), pp. 107-47. Boulder, Colo.: Westview Press.
Fast, P.G. 1970. Insect lipids. In Progress in the Chemistry of Fats and other Lipids (R.T. Holman, ed.), Vol. 11, Pt. 2: 181-242. Oxford, etc.: Pergamon Press.
Grapes, M.; Whiting, P.; Dinan, L. 1989. Fatty acid and lipid analysis of the house cricket, Acheta domesticus. Insect. Biochem. 19:767-74.
Hutchins, R.F.N.; Martin, M.N. 1968. Lipids of the common house cricket, Acheta domesticus, L. I. Lipid classes and fatty acid distribution. Lipids 3:247-49.
McFarlane, J.E.; Alli, I.; Steeves, E. 1984. Studies on the group effect in Acheta domesticus (L.) using artificial diets. J. Insect Physiol. 30:103-7.
Mountford, C.P. 1946. Earth’s most primitive people. A journey with the aborigines of Central Australia. Natl. Geograph. Mag. 89:89-104.
Nakasone, S.; Ito, T. 1967. Fatty acid composition of the silkworm, Bombyx mori L. J. Insect Physiol. 13:1237-46.
National Research Council. 1988. Designing Foods. Animal Product Options in the Marketplace. Washington, D.C.: National Academy Press.
Oliveira,J.S. Santos; Carvalho,J. Passos de; Bruno de Sous,R.F.X.; Simao, M. Magdalena. 1976. The nutritional value of four species of insects consumed in Angola. Ecol. Food Nutr. 5:91-97.
Stanley-Samuelson, D.W.; Dadd, R.H. 1983. Long-chain polyunsaturated fatty acids: patterns of occurrence in insects. Insect Biochem. 13:549-58.
Stanley-Samuelson, D.W.; Jurenka, R.A.; Cripps, C.; Blomquist, G.J.; Renobales, M. de. 1988. Fatty acids in insects: composition, metabolism, and biological significance. Arch. Insect Biochem. Physiol. 9:1-33.
Thompson, S.N. 1973. A review and comparative characterization of the fatty acid compositions of seven insect orders. Copw. Biochem. Physiol. 45B:467-82.
Thompson, S.N.; Barlow, J.S. 1972. The consistency of the fatty acid pattern of Galleria mellonella, reared on fatty acid supplemented diets. Canad. J. Zool. 50: 1033-34.
Tindale, N.B. 1953. On some Australian Cossidae including the moth of the witjuti (witchety) grub. Trans. Roy. Soc. S. Austral. 76:56-65.

I wish to thank the following University of Wisconsin colleagues for reading the original draft and suggesting improvements: Dr. Denise Ney, lipids specialist in the Department of Nutritional Sciences; Dr. Joanne Csete, specialist in Third World nutrition, Dept. of Nutritional Sciences; Dr. Robert Lindsay, lipids specialist in the Department of Food Science; and Dr. Karla Ritter, insect lipids specialist presently located in the Clinical Science Center.)