Cholesterol And Insects

The diet of humans living in industrialized nations is frequently high in lipids, such as fat and cholesterol. Unfortunately, an increase in serum cholesterol levels seems to increase the risk of a person developing coronary heart disease(1). More than a million Americans have heart attacks each year and more than one-half die as a result. Therefore the National Institutes of Health has recommended that one way individuals can reduce their risk of illness is to make changes in their diet (2).

With the amount of publicity that this recommendation has received, almost everyone knows of ways to reduce their dietary intake of cholesterol. Surveys indicate that health-conscious Americans are, indeed, trimming fat from meat, replacing red meat with poultry and fish, eating fewer eggs and using skim milk in their diet (2). The sales of natural products, such as oat bran, which apparently enhances the excretion of cholesterol, are booming. With all of this current interest in nutrition, it is certainly worthwhile to explore the sterol biochemistry of other lesser known food items, such as insects, and to evaluate their possibilities as low-cholesterol food additives.

Interestingly, most eucaryotic organisms require sterol, or sterol-like molecules, as structural components of their membranes (3). These molecules apparently interdigitate with the tails of the phospholipid molecules in the lipid bilayer and so help regulate the fluidity of the membrane. For example, the major sterol in many animal membranes is cholesterol (Figure 1, page 5) which has a double bond at carbon 5 and no alkyl group at carbon 24 (i.e., it is a Δ5-24-desalkylsterol). Plants and fungi also contain sterols but they tend to synthesize and utilize ones that have a methyl or ethyl group on the side chain at carbon 24, and have a double bond at carbon 5, or 7, or at both 5 and 7 (i.e., they are Δ5-Δ7-, or Δ5,7-24-alkylsterols).

The differences in the structure of the sterols in these different organisms helps to explain why a diet high in plant products is healthier than a diet high in animal products. Fortunately, humans can neither absorb nor metabolize 24-alkylsterols to cholesterol (3). Therefore, the plant sterols in corn oil pass harmlessly through the human digestive tract whereas the cholesterol in butter is absorbed and can contribute to the endogenous pools of cholesterol. (The human liver synthesizes cholesterol de novo in order to ensure that there are adequate

amounts for cell membrane biosynthesis as well as to serve as precursors for bile salts and hormones, such as estrogen and testosterone.)

What about insects as a part of the human diet? Since they are animals, do their tissues also contain cholesterol? Insects are very interesting, in that they too need sterols for the biosynethesis of membranes and as precursors to hormones (.e.g., the ecdysteroids), but they are unable to synthesize them de novo. (4). Therefore, they must obtain these molecules exogenously, from their diet or from symbionts. Those insects that feed on animal products (e.g., the hide beetle, Demestes vulpinus [5]) can easily obtain cholesterol from their diet and so

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usually utilize this sterol directly in their tissues. In contrast, those that feed on plants or fungi consume 24-alkylsterols and so must absorb and either use these sterols directly in their tissues or metabolize them to more utilizable ones. It appears that many insects (e.g., the tobacco hornworm, Manduca sexta [4] and the house cricket, Acheta domesticus [6] ) contain the enzymes for the dealkylation of delta 5-24-alkylsterols and so convert such sterols to cholesterol. Therefore, like other animals, insects are approximately 0. 1% sterol (i.e., 1 mg sterol/g tissue).

Does this mean that the consumption of insects must entail the ingestion of unwanted dietary cholesterol as well? Not necessarily. Since there are probably well over one million different species of insects, it is not surprising to learn that there are some that are unable (or do not expend the energy) to convert delta 5-alkylsterols to cholesterol. Such species include the honey bee, Apis mellifera (7). The latter insect has avoided the necessity of producing 24-desalkylsterols in order to biosynthesize ecdysteroids. It simply uses makisterone A (an alkylated ecdysteroid) as its molting hormone!

In addition, studies in my laboratory have shown that replacing the Δ5-sterols in the diet of the corn earworm, Heliothis zea with delta 7-, delta 5,7-, or delta O-sterols results in an insect that contains little, if any, cholesterol (8,9). This lepidopteran dealkylates the new dietary sterols but does not hydrogenate the double bonds or introduce new ones. The structures of the resulting tissue sterols render them unabsorbable by the normal human digestive tract. Therefore, if one was interested in producing insects, such as A. domesticus, with a low cholesterol level, one might try feeding them, for example, a diet rich in alfalfa sterols (i.e., Δ7-sterols). Perhaps they, like H. zea (10), would utilize the Δ7-sterols predominately in their tissues. The alfalfa weevil, Hypera postica, which is a pest of alfalfa, uses Δ7-sterols and routinely lacks cholesterol in its tissues (10). Other insects, which naturally feed on diets that contain sterols other than Δ5-sterols, and do not contain cholesterol in their tissues, include the fly, Drosophila pachea (11), and the leaf-cutting ant, Atta cephalotes isthmicola (12).

Figure 1:

In conclusion, the inability of insects to synthesize their own sterols forces them to be dependent upon exogenous sterols in order to complete their growth and development. This unusual sterol requirement of insects means that the sterol composition of their tissues may change when they feed on different diets. Thus, insect species whose tissues are low in cholesterol, either naturally or due to special feeding, may be an especially useful addition to the human diet.

by Dr. Karla S. Ritter (University of Wisconsin-Madison)

1. Blum, C.B.; Levy, R.I., 1989. Current therapy for hypercholesteremia. JAMA 261:3582-3587.
2. Schucker, B.; Bailey, K.; Heimbach, J.T.; Mattson, MM.; Wittes, J.T.; Haines, C.M.; Gordon, DJ.; Cutler, J.A.; Keating, V.S.; Goor, R.S.; Rifkind, B.M. 1987. Change in public perspective on cholesterol and heart disease: Results from two national surveys. JAMA 258:3527-3531.
3. Nes, W.R.; McKean, M.L., 1978. Biochemistry of Steroids and Other Isopentenoids. University Park Press, Baltimore, 690 pp.
4. Svoboda, J.A.; Thompson, Mg., 1985. Steroids. In: Comprehensive lnsect Physiology, Biochemistry and Pharmacology. Vol. 10. G.A. Kerkut; Gilbert, L.I., eds. Pergamon Press, New York. pp. 137-175.
5. Clayton, R.B.; Bloch, K., 1963. Sterol utilization in the hide beetle, Dermestes vulpinus. J. Biol. Chem. 238:586-591.
6. Martin, M.M., and Carls, G.A. 1968. The lipids of the common house cricket, Acheta domesticus L. III. Sterols. Lipids 3:256-259.
7. Feldlaufer, M.F.; Herbert Jr., E.W.; Svoboda, J.A.; Thompson, MJ.; Lusby, W.R. 1985. Makisterone A. The major ecdysteroid from the pupa of the honey bee, Apis mellifera. Insect Biochem. 15:597-600. Biochem. 15:597-600. Biochem. 15:597-600.
8. Ritter, K.S. 1994. Metabolism of ΔO-,Δ5-, and Δ7-sterols by larvae of Heliothis zea. Arch. Insect Biochem. Physiol 1:218-296.

9.Ritter. K.S. 1986. Utilization of Δ5,7- and Δ8-sterols by larvae of Heliothis zea. Arch. Insect Biochem. Physiol. 3:349 -362

10. MacDonald, D.L.; Nham, D.N.; Cochran, W.K.; Ritter, K.S. Differences in the sterol composition of Heliothis zea fed Zea mays versus Medicago sativa. Submitted for publication.

11. .Goodnight K.C.; Kircher, H.W.1971. Metabolism of lathosterol by Drosophila pachea. Lipids 6:166-169.

12. Ritter, K.S.; Weiss, B.A.; Norrbom, A.L.; Nes, W.R. 1982. Identification of Δ5,7-24-methylene- and methylsterols in the brain and whole body of Atta cephalotes isthmicola.. Comp. Biochem. Physiol. 71 B:345-349.

(Ed. This is the second in a series of invited articles on the biochemistry of insects as related to their use and nutritional value as food. Dr. Ritter has conducted research on insect sterol biochemistry since 1978, first at Drexel University, Philadelphia, and subsequently at the University of Wisconsin-Madison.)

CORRECTION

In the article, “The Identity of Grasshoppers Used as Food by Native American Tribes,” in the November 1989 Newsletter, a reference was made to “huge wingless Boopedon females.” It should have read “huge wingless Brachystola females.”

“Good” versus “Bad” Cholesterol
Sometimes the terms “good” versus “bad” cholesterol appear in the lay literature and can be confusing to persons attempting to regulate the cholesterol level of their diet. Just what is the difference between “good” and “bad” cholesterol? Interestingly, these adjectives don’t refer to different structural forms of cholesterol. Instead, they are simple terms used to describe the ways in which this molecule can be transported in the human circulatory system.

Cholesterol is a lipid and, just as oil and water don’t mix, neither does cholesterol and blood. Therefore, in order for cholesterol to move from one part of the body to another (e.g., from the digestive tract to fat tissue), without precipitating out of solution and causing blockage of the blood vessels, it has to bind to an amphipathic (i.e., detergent-like) molecule that can shield it from the aqueous medium and so transport it through the blood
stream. Lipoproteins, such as HDL and LDL, are the molecules that transport cholesterol (and other lipids) through the circulatory system of humans.

Although hypercholesterolemia can occur with the elevation of any lipoprotein class, elevation of LDL levels increases the risk of coronary heart disease whereas elevation of HDL levels reduces the risk. Therefore, persons with a high level of “bad” cholesterol (i.e., LDL) should attempt to reduce the concentration of these lipoproteins in their blood.

Insects also have lipoproteins that transport their sterols through their body (see accompanying article). However, since they have an open circulatory system, they don’t have to worry about heart attacks! Humans don’t have to avoid consuming “bad’ vertebrate, or invertebrate, lipoproteins in their diet since such molecules are digested in the intestinal tract. K.S.R.