The activating effect of dietary protein on linoleic acid desaturation

The Zone Diet phenomenon: a closer look at the science behind the claims

The purported health benefits of low-carbohydrate diets have been advocated intermittently over the last century and have enjoyed increasing popularity over the last decade. Although most revolve around the emphatic theme that carbohydrates are to blame for many chronic diseases, their specific ideologies are more variable and in some cases quite sophisticated. The Zone Diet phenomenon represents a new generation of modern low carbohydrate food fad with sales placing it among the most popular diet books in recent history. The Zone is a 40% carbohydrate, 30% protein and 30% fat eating plan that advocates only sparing use of grains and starches. The precise 0.75 protein to carbohydrate ratio required with each meal is promoted to reduce the insulin to glucagon ratio, which purportedly affects eicosanoid metabolism and ultimately produces a cascade of biological events leading to a reduction in chronic disease risk, enhanced immunity, maximal physical and mental performance, increased longevity and permanent weight loss. There is presently little scientific support for the connections made between diet, endocrinology and eicosanoid metabolism. In fact, a review of the literature suggests that there are scientific contradictions in the Zone Diet hypothesis that cast unquestionable doubt on its potential efficacy. The purpose of this review is to evaluate the scientific merit of the Zone Diet and its health claims in an effort to help delineate what is and what is not sound nutrition science.

[Chronologic studies of the effects of a hypoproteinic diet followed by an equilibrated diet on delta-6 and delta-5 desaturations of linoleic acid in liver microsomes in the rat]

A low protein diet affects amounts of linoleic and arachidonic acids in hepatic microsomal phospholipids of growing rats. Are the changes related to modifications in microsomal delta 6- and delta 5- linoleic acid desaturase activities? Two groups of Wistar rats weighing 80 +/- 5 g at the beginning of the experiment were used: Control group (T) was fed on a 16% gluten + 4% casein diet for 53 days; Experimental group (E) was fed on a 4% gluten + 1% casein diet for 26 days (MP) then Control diet for 27 days (RE). After 2, 14 and 26 days of MP and 2, 15 and 27 days of RE, rats of each group were sacrificed. Protein and water contents of liver, quantitative fatty acid, composition of total lipids in liver and hepatic microsomes were determined. delta 6- and delta 5- linoleic acid desaturase activities were estimated from incubation of liver microsomes with [1-14C] C 18: 2 n-6 or [2(14)C] C 20: 3 n-6 respectively. The low protein diet stops practically ponderal growth. The fatty-acid compositions of microsomal total lipids of E rats were affected in comparison with values of T rats. These modifications persist after 27 days of RE. The C 20: 4 n-6/C 18: 2 n-6 ratio in microsomal total lipids was slightly different between T and E rats but increased strongly during refeeding. Same modifications take place in the fatty-acid composition of hepatic total lipids. After two days of MP, delta 6- and delta 5- desaturase activities were depressed, phenomenon that not persist in the course of MP. These enzyme activities increase to higher values than those of the T after two days of RE.

Microsomal fatty acid desaturase activities in vitamin A-deficient rat liver

The activity of microsomal fatty acid delta 9-desaturase was significantly higher in liver microsomes of vitamin A-deficient rats as compared with their controls. Feeding of vitamin A-supplemented control diet to the deficient rats restored the delta 9-desaturase activity to the control values. The activity of delta 6-desaturase was not affected by vitamin A deficiency.

Activities of fatty acid desaturases and fatty acid composition of liver microsomes in rats fed beta-carotene and 13-cis-retinoic acid

The fatty acid composition of microsomal lipids and the activities of delta 9- and delta 6-desaturases in liver microsomes of rats fed diets supplemented with beta-carotene and two levels of 13-cis-retinoic acid were studied. Four groups of male, weanling rats were fed semipurified diets containing 0 or 100 mg beta-carotene per kg diet, and 20 or 100 mg 13-cis-retinoic acid per kg diet. After 11 weeks of feeding, the rats were killed, liver microsomes were prepared and assayed for delta 9-desaturase and delta 6-desaturase activities. The activity of delta 9-desaturase was lower in liver microsomes of rats fed beta-carotene-supplemented diet or the diet supplemented with the higher level of 13-cis-retinoic acid. Microsomal delta 6-desaturase activity was, however, higher in liver of rats fed 13-cis retinoic acid; there was no effect of beta-carotene on delta 6-desaturase activity. The fatty acid compositional data on total lipids of liver microsomes were consistent with the diet-induced changes in fatty acid desaturases. Phospholipid composition of liver microsomes was also altered as a result of feeding beta-carotene or 13-cis-retinoic acid-containing diets. The proportions of phosphatidylethanolamine were generally higher, whereas those of phosphatidylcholine were lower in the experimental groups as compared with the control.

Phospholipid composition of liver in rats fed high levels of 13-cis retinoic acid

The composition of liver phospholipids was studied in rats fed for 4 weeks diets containing 0, 100 or 300 mg 13-cis retinoic acid per kg diet. There was a significant decrease in phosphatidylcholine content, whereas the levels of phosphatidylethanolamine were slightly increased in liver phospholipids of rats fed 13-cis retinoic acid. The fatty acid composition of total phospholipids, PC, PE, and P1 + PS fractions revealed a general increase in the levels of 18:2 and 20:3 omega 6, whereas the levels of 20:4 omega 6 and C22 fatty acids were reduced in most of the hepatic phospholipids isolated from rats fed 13-cis retinoic acid containing diets. A decrease in the double-bond index of fatty acids was also observed in phospholipids of rats fed 13-cis retinoic acid. The data suggest that high levels of 13-cis retinoic acid may possibly be influencing the activities of microsomal desaturating and chain-elongating enzymes in the liver.

Effect of different carbon sources on delta 5 desaturation activity of HTC cells

The effect of different carbon sources on delta 5 desaturation activity in Minimal Deviation Hepatoma 7288 c (HTC) cells was studied. When HTC cells were incubated in the absence of carbon sources in the medium (fasted cells) the conversion of [1-14C]eicosa-8,11,14-trienoic acid to arachidonic acid increased significantly compared to those cells incubated in S-77 medium (fed cells). This activity was not modified if fasted cells were refed with glucose. However, the activity decreased significantly by the addition of lactalbumin hydrolysate to fasted cells. This effect can not be attributed to individual aminoacids since the addition of them to an aminoacid free medium produced no changes on delta 5 desaturation activity. The incubation of HTC cells with S-77 medium supplemented with glucose, pyruvate or glucose plus pyruvate, showed similar delta 5 desaturation activity to those cells incubated in S-77 medium alone.

Involvement of cytochrome b5 in the oxidative desaturation of linoleic acid to gamma-linolenic acid in rat liver microsomes

The effects of antibodies against microsomal electron-transport components on the in vitro activity of delta6-desaturation of linoleic acid to gamma-linolenic acid have been studied in intact microsomal membranes of rat liver. Reduced nicotinamide adenine dinucleotide (NADH) or reduced nicotinamide adenine dinucleotide phosphate (NADPH) (0.87 mM) served as electron donors, and effectively prompted the delta6-desaturase activities with yields of about 1.1 to 1.3 nmol per mg of protein in 10 min. Of the two antibodies studied under the same in vitro conditions, i.e., rabbit antisera preparations against rat liver microsomal hydrophilic parts of cytochrome b5 and NADPH-cytochrome c reductase, only the antibody against cytochrome b5 demonstrated a marked ability to inhibit the delta6-desaturase activity. This evidence supports a participation of cytochrome b5 in the delta6-desaturation of linoleic acid and suggests a pathway analogous to the delta9-desaturation of stearyl-CoA.

Effect of different carbon sources on the biosynthesis of polyunsaturated fatty acids of alpha-linolenic acid family in culture of minimal deviation hepatoma 7288 C cells

The effect of three different carbon sources on the biosynthesis of polyunsaturated fatty acids of the alpha-linolenic acid series was investigated in hepatoma tissue culture (HTC) cells. Alpha linolenic acid was converted to higher homologs by a desaturating route that synthetized mainly 18:4 (delta6, 9, 12, 15), 20:4 (delta8, 11, 14, 17) and 20:5 (delta5, 8, 11, 14, 17) and an elongating route that produced 20:3 (delta11, 14, 17) and 20:4 (delta5, 11, 14, 17) acids. "Fasting" decreased both biosynthetic routes whereas glucose reactivated only the elongating pathway. Lactabumin hydrolysate enhanced significantly only the desaturating route whereas glycerol was inactive. Glucose and aminoacids increased similarly the incorporation of labeled alpha linolenic acid in the cells. The results are independent of hormonal effects.

Biosynthesis of fatty acids by the carp, Cyprinus carpio L., in relation to environmental temperature

Incorporation in vivo of sodium 1-14C-acetate into different lipid classes and fatty acids of total lipids and phospholipids of warm adapted and cold adapted carp livers was studied at 5 C and 22 C, respectively. The fatty acid composition of total lipids and phospholipids was also determined. The level of long chain polyunsaturated fatty acids in both total lipid and phospholipid fraction was higher in cold adapted fish than in warm adapted ones. The distribution of radioactivity among different lipid classes depended only on the actual incorporation of the temperature history of the animals. Livers of fish incorporated a higher percentage of radioactivity into long chain polyunsaturated fatty acids of total lipids and phospholipids in 5 C than in 22 C. The distribution of radioactivity among different fatty acids was dependent on the experimental temperature rather than on the temperature to which the fish were adapted. The results suggest that fish are able to adjust the pattern of the biosynthesis of fatty acids very rapidly to the prevailing temperature and to assure by this way the proper physicochemical properties of their membranes. The possible mechanisms involved in this rapid response are discussed.

Dietary and hormonal effects upon activity of "soluble" protein and particulate fraction of fatty acid desaturation system of rat liver microsomes

Rat liver microsomes were extracted with a buffered 0.15 M KCl and 0.25 M sucrose solution and fractionated by centrifugation into a particulate component and a supernatant containing a protein factor necessary for fatty acid desaturation. The delta 6 fatty acid desaturation activity of the extracted microsomes was reduced significantly, and the readdition of the supernatant restored the enzymatic activity to the original value of the whole microsomes. A protein diet or a fat-free diet increased the delta 6 desaturation activity of the whole microsomes. The activating effect was evoked upon the particulate components of the enzymatic desaturation system and not upon the protein factor present in the supernatant. Fasting, refeeding, and refeeding plus glucagon and theophylline treatments of rats also modified the delta 6 desaturation activity of whole liver microsomes. The effect also was evoked on the delta 6 desaturation system tightly bound to the microsomal membrane but not on the protein factor of the supernatant. Accordingly, the protein factor of the supernatant is considered to be different from the cyanide sensitive factor and the desaturase.