Leonard H Storlien

Skeletal Muscle Triglyceride Levels Are Inversely Related to Insulin Action

In animal studies, increased amounts of triglyceride associated with skeletal muscle (mTG) correlate with reduced skeletal muscle and whole body insulin action. The aim of this study was to test this relationship in humans. Subjects were 38 nondiabetic male Pima Indians (mean age 28 ± 1 years). Insulin sensitivity at physiological (M) and supraphysiological (MZ) insulin levels was assessed by the euglycemic clamp. Lipid and carbohydrate oxidation were determined by indirect calorimetry before and during insulin administration. mTG was determined in vastus lateralis muscles obtained by percutaneous biopsy. Percentage of body fat (mean 29 ± 1%, range 14–44%) was measured by underwater weighing. In simple regressions, negative relationships were found between mTG (mean 5.4 ± 0.3 μmol/g, range 1.3–1.9 μmol/g) and log10M (r = −0.53, P ≤ 0.001), MZ (r = −0.44, P = 0.006), and nonoxidative glucose disposal (r = −0.48 and −0.47 at physiological and supraphysiological insulin levels, respectively, both P = 0.005) but not glucose or lipid oxidation. mTG was not related to any measure of adiposity. In multiple regressions, measures of insulin resistance (log10M, MZ, log10[fasting insulin]) were significantly related to mTG independent of all measures of obesity (percentage of body fat, BMI, waist-to-thigh ratio). In turn, all measures of obesity were related to the insulin resistance measures independent of mTG. The obesity measures and mTG accounted for similar proportions of the variance in insulin resistance in these relationships. The results suggest that in this human population, as in animal models, skeletal muscle insulin sensitivity is strongly influenced by local supplies of triglycerides, as well as by remote depots and circulating lipids. The mechanism(s) underlying the relationship between mTG and insulin action on skeletal muscle glycogen synthesis may be central to an understanding of insulin resistance.

Substituting dietary saturated for monounsaturated fat impairs insulin sensitivity in healthy men and women: The KANWU study

Aims/hypothesis. The amount and quality of fat in the diet could be of importance for development of insulin resistance and related metabolic disorders. Our aim was to determine whether a change in dietary fat quality alone could alter insulin action in humans. Methods. The KANWU study included 162 healthy subjects chosen at random to receive a controlled, isoenergetic diet for 3 months containing either a high proportion of saturated (SAFA diet) or monounsaturated (MUFA diet) fatty acids. Within each group there was a second assignment at random to supplements with fish oil (3.6 g n-3 fatty acids/d) or placebo. Results. Insulin sensitivity was significantly impaired on the saturated fatty acid diet (-10 %, p = 0.03) but did not change on the monounsaturated fatty acid diet ( + 2 %, NS) (p = 0.05 for difference between diets). Insulin secretion was not affected. The addition of n-3 fatty acids influenced neither insulin sensitivity nor insulin secretion. The favourable effects of substituting a monounsaturated fatty acid diet for a saturated fatty acid diet on insulin sensitivity were only seen at a total fat intake below median (37E %). Here, insulin sensitivity was 12.5 % lower and 8.8 % higher on the saturated fatty acid diet and monounsaturated fatty acid diet respectively (p = 0.03). Low density lipoprotein cholesterol (LDL) increased on the saturated fatty acid diet ( + 4.1 %, p < 0.01) but decreased on the monounsaturated fatty acid diet (MUFA) (–5.2, p < 0.001), whereas lipoprotein (a) [Lp(a)] increased on a monounsaturated fatty acid diet by 12 % (p < 0.001). Conclusions/interpretation. A change of the proportions of dietary fatty acids, decreasing saturated fatty acid and increasing monounsaturated fatty acid, improves insulin sensitivity but has no effect on insulin secretion. A beneficial impact of the fat quality on insulin sensitivity is not seen in individuals with a high fat intake ( > 37E %). [Diabetologia (2001) 44: 312–319]

Influence of dietary fat composition on development of insulin resistance in rats. Relationship to muscle triglyceride and omega-3 fatty acids in muscle phospholipid.

High levels of some but not all dietary fats lead to insulin resistance in rats. The aim of this study was to investigate the important determinants underlying this observation. Insulin action was assessed with the euglycemic clamp. Diets high in saturated, monounsaturated (ω-9), or polyunsaturated (ω-6) fatty acids led to severe insulin resistance; glucose infusion rates [GIR] to maintain euglycemia at ∼1000 pM insulin were 6.2 ± 0.9, 8.9 ± 0.9, and 9.7 ± 0.4 mg · kg−1 · min−1, respectively, versus 16.1 ± 1.0 mg · kg−1 · min−1 in chow-fed controls. Substituting 11% of fatty acids in the polyunsaturated fat diet with long-chain ω-3 fatty acids from fish oils normalized insulin action (GIR 15.0 ± 1.3 mg · kg−1 · min−1). Similar replacement with short-chain ω-3 (α-linolenic acid, 18:3ω3) was ineffective in the polyunsaturated diet (GIR 9.9 ± 0.5 mg · kg−1 · min−1) but completely prevented the insulin resistance induced by a saturated-fat diet (GIR 16.0 ± 1.5 mg · kg−1 · min−1) and did so in both the liver and peripheral tissues. Insulin sensitivity in skeletal muscle was inversely correlated with mean muscle triglyceride accumulation (r = 0.95 and 0.86 for soleus and red quadriceps, respectively; both P = 0.01). Furthermore, percentage of long-chain ω-3 fatty acid in phospholipid measured in red quadriceps correlated highly with insulin action in that muscle (r = 0.97). We conclude that 1) the particular fatty acids and the lipid environment in which they are presented in high-fat diets determine insulin sensitivity in rats; 2) impaired insulin action in skeletal muscle relates to triglyceride accumulation, suggesting intracellular glucose–fatty acid cycle involvement; and 3) long-chain ω-3 fatty acids in phospholipid of skeletal muscle may be important for efficient insulin action.

The Relation between Insulin Sensitivity and the Fatty-Acid Composition of Skeletal-Muscle Phospholipids

BACKGROUND Insulin resistance and hyperinsulinemia are features of obesity, non-insulin-dependent diabetes mellitus, and other disorders. Skeletal muscle is a major site of insulin action, and insulin sensitivity may be related to the fatty-acid composition of the phospholipids within the muscle membranes involved in the action of insulin. METHODS We determined the relation between the fatty-acid composition of skeletal-muscle phospholipids and insulin sensitivity in two groups of subjects. In one study, we obtained samples of the rectus abdominis muscle from 27 patients undergoing coronary artery surgery; fasting serum insulin levels provided an index of insulin sensitivity. In the second study, a biopsy of the vastus lateralis muscle was performed in 13 normal men, and insulin sensitivity was assessed by euglycemic-clamp studies. RESULTS In the patients undergoing surgery, the fasting serum insulin concentration (a measure of insulin resistance) was negatively correlated with the percentage of individual long-chain polyunsaturated fatty acids in the phospholipid fraction of muscle, particularly arachidonic acid (r = -0.63, P < 0.001); the total percentage of C20-22 polyunsaturated fatty acids (r = -0.68, P < 0.001); the average degree of fatty-acid unsaturation (r = -0.61, P < 0.001); and the ratio of the percentage of C20:4 n-6 fatty acids to the percentage of C20:3 n-6 fatty acids (r = -0.55, P < 0.01), an index of fatty-acid desaturase activity. In the normal men, insulin sensitivity was positively correlated with the percentage of arachidonic acid in muscle (r = 0.76, P < 0.01), the total percentage of C20-22 polyunsaturated fatty acids (r = 0.76, P < 0.01), the average degree of fatty-acid unsaturation (r = 0.62, P < 0.05), and the ratio of C20:4 n-6 to C20:3 n-6 (rho = 0.76, P = 0.007). CONCLUSIONS Decreased insulin sensitivity is associated with decreased concentrations of polyunsaturated fatty acids in skeletal-muscle phospholipids, raising the possibility that changes in the fatty-acid composition of muscles modulate the action of insulin.

Development of Muscle Insulin Resistance After Liver Insulin Resistance in High-Fat–Fed Rats

Muscle and hepatic insulin resistance are two major defects of non-insulin-dependent diabetes mellitus. Dietary factors may be important in the etiology of insulin resistance. We studied progressive changes in the development of high-fat–diet–induced insulin resistance in tissues of the adult male Wistar rat. In vivo insulin action was compared 3 days and 3 wk after isocaloric synthetic high-fat or high-starch feeding (59 and 10% cal as fat, respectively). Basal and insulin-stimulated glucose metabolism were assessed in the conscious 5- to 7-h fasted state with the euglycemic clamp (600 pM insulin) with a [3-3H]-glucose infusion. Fat feeding significantly reduced suppressibility of hepatic glucose output by insulin after both 3 days and 3 wk of diet (P < 0.01). However, a significant impairment of insulin-mediated peripheral glucose disposal was only present after 3 wk of diet. Further in vivo [3H]-2-deoxyglucose uptake studies supported this finding and demonstrated adipose but not muscle insulin resistance after 3 days of high-fat feeding. Muscle triglyceride accumulation due to fat feeding was not significant at 3 days but had doubled by 3 wk in red muscle (P < 0.001) compared with starch-fed controls. By 3 wk, high-fat—fed animals had developed significant glucose intolerance. We concludethat fat feeding induces insulin resistance in liver and adipose tissue before skeletal muscle with early metabolic changes favoring an oversupply of energy substrate to skeletal muscle relative to metabolic needs. This may generate later muscle insulin resistance.

Physiological Importance of Deficiency in Early Prandial Insulin Secretion in Non-Insulin-Dependent Diabetes

Patients with non-insulin-dependent diabetes mellitus (NIDDM) have a deficiency in early prandial insulin secretion. To determine the contribution of this early deficiency to prandial hyperglycemia, exogenous intravenous insulin (1.8 U over 30 min) was delivered to eight NIDDM subjects in a profile designed to simulate the normal initial rise in insulin levels. The same dose of insulin was also administered 1) in the same profile but delayed by 30 min and 2) as a constant infusion over 180 min. Augmentation of the early insulin response caused a 33 ± 4% reduction in the glycemic response to a mixed meal (P < .005); the peak blood glucose increment above baseline was reduced by 1.4 mM (P < .005) to an increment identical to nondiabetic subjects (3.3 ± 0.3 vs. 3.2 ± 0.2 mM), and blood glucose levels were still 0.9 mM lower after 180 min (P < .05). In contrast, the delayed profile or constant infusion did not significantly alter the glycemic response to the meal. Early insulin augmentation resulted in elevated peripheral insulin levels initially (peak level 81 ± 11 mU/L), but subsequent insulin and C-peptide levels were lower than in the control study (at 180 min after the meal, 22 ± 5 vs. 33 ± 8 mU/L, P < .05, and 4.0 ± 0.5 vs. 5.3 ± 0.6 μg/L, P < .02, respectively). Early insulin delivery caused free-fatty acid (FFA) levels to fall at a faster rate after the meal and also attenuated the initial rise in glucagon levels typical of NIDDM. We conclude that the deficiency in early prandial insulin secretion contributes to prandial hyperglycemia and late hyperinsulinemia and may be partially responsible for the abnormal FFA and glucagon responses in NIDDM.

In vivo insulin resistance in individual peripheral tissues of the high fat fed rat: assessment by euglycaemic clamp plus deoxyglucose administration

SummaryWe have examined peripheral insulin action in conscious rats chronically fed high fat (60% calories as fat) or high carbohydrate (lab chow) diets using the euglycaemic clamp plus 3 H-2-deoxyglucose technique. A response parameter of individual tissue glucose metabolic rate (the glucose metabolic index, based on tissue deoxyglucose phosphorylation) was used to assess diet effects in eight skeletal muscle types, heart, lung and white and brown adipose tissue. Comparing high fat with high carbohydrate fed rats, basal glucose metabolism was only mildly reduced in skeletal muscle (only diaphragm was significant,p<0.05), but was more substantially reduced in other tissues (e.g. white adipose tissue 61% and heart 33%). No evidence of basal hyperinsulinaemia was found. In contrast, widespread insulin resistance was found during the hyperinsulinaemic clamp (150 mU/l) in high fat fed animals; mean whole body net glucose utilization was 34% lower (p<0.01), and the glucose metabolic index was lower in skeletal muscle (14 to 56%,p< 0.05 in 6 out of 8 muscles), white adipose (27%,p<0.05) and brown adipose tissue (76%,p<0.01). The glucose metabolic index was also lower at maximal insulin levels in muscle and fat, suggesting the major effect of a high fat diet was a loss of insulin responsiveness. White adipose tissue differed from muscle in that incremental responses (maximal insulin minus basal) were not reduced by high fat feeding. The heart showed an effect opposite to other tissues, with an increase in insulin-stimulated glucose metabolism in high fat versus chow fed rats. We conclude that high fat feeding, without a major increase in body weight or basal hyperinsulinaemia, causes widespread but varying degrees of in vivo insulin resistance in peripheral tissues, with major effects in principally oxidative skeletal muscle.

SKELETAL MUSCLE MEMBRANE LIPIDS AND INSULIN RESISTANCE

Skeletal muscle plays a major role in insulin-stimulated glucose disposal. This paper reviews the range of evidence in humans and experimental animals demonstrating close associations between insulin action and two major aspects of muscle morphology: fatty acid composition of the major structural lipid (phospholipid) in muscle cell membranes and relative proportions of major muscle fiber types. Workin vitro andin vivo in both rats and humans has shown that incorporation of more unsaturated fatty acids into muscle membrane phospholipid is associated with improved insulin action. As the corollary, a higher proportion of saturated fats is linked to impairment of insulin action (insulin resistance). Studiesin vitro suggest a causal relationship. Among polyunsaturated fatty acids (PUFA) there is some, but not conclusive, evidence that ω-3 (n−3) PUFA may play a particular role in improving insulin action; certainly a high n−6/n−3 ratio appears deleterious. In relation to fiber type, the more highly oxidative, insulin-sensitive type 1 and type 2a fibers have a higher percentage of unsaturated fatty acids, particularly n−3, in their membrane phospholipid, compared to the insulin-resistant, glycolytic, type 2b fibers. These variables, however, can be separated and may act in synergy to modulate insulin action. It remains to establish whether lifestyle (e.g., dietary fatty acid profile and physical activity), genetic predisposition, or a combination are the prime determinants of muscle morphology (particularly membrane lipid profile) and hence insulin action.

Effects of fish oil supplementation on glucose and lipid metabolism in NIDDM.

Fish oils, containing omega-3 fatty acids (ω3FAs), favorably influence plasma lipoproteins in nondiabetic humans and prevent the development of insulin resistance induced by fat feeding in rats. We studied the effects of fish oils in 10 subjects (aged 42–65 yr) with mild non-insulin-dependent diabetes mellitus (NIDDM). Subjects were fed a standard diabetic diet plus 1) no supplementation (baseline), 2) 10 g fish oil concentrate (30% ω3FAs) daily, and 3) 10 g safflower oil daily over separate 3-wk periods, the latter two supplements being given in radom order by use of a double-blind crossover design. At the end of each diet period, fasting blood glucose (FBG), insulin, and lipids were measured, and insulin sensitivity was assessed with a hyperinsulinemic-euglycemic clamp performed with [3-3H]glucose. FBG increased 14% during fish oil and 11% during safflower oil supplementation compared with baseline (P < .05), whereas body weight, fasting serum insulin levels, and insulin sensitivity were unchanged. The absolute increase in FBG during each supplementation period correlated with the baseline FBG (fish oil, r = .83, P < .005; safflower oil, r = .75, P = .012). Fasting plasma triglyceride levels decreased during fish oil supplementation in the 4 subjects with baseline hypertriglyceridemia (>2 mM) but were not significantly reduced overall. There was no significant change in fasting plasma total, high-density lipoprotein, and low-density lipoprotein cholesterol levels. In summary, dietary fish oil supplementation adversely affected glycemic control in NIDDM subjects without producing significant beneficial effects on plasma lipids. The effect of safflower oil supplementation was not significantly different from fish oil, suggesting that the negative effects on glucose metabolism may be related to the extra energy or fat intake. These data indicate that fish oil supplementation should be used with caution in subjects with NIDDM.

Dietary n-3 and n-6 fatty acids alter avian metabolism: metabolism and abdominal fat deposition

The effects of dietary saturated fatty acids and polyunsaturated fatty acids (PUFA) of the n-3 and n-6 series on weight gain, body composition and substrate oxidation were investigated in broiler chickens. At 3 weeks of age three groups of chickens (n 30; ten birds per group) were fed the fat-enriched experimental diets for 5 weeks. These diets were isonitrogenous, isoenergetic and contained 208 g protein/kg and 80 g edible tallow, fish oil or sunflower oil/kg; the dietary fatty acid profiles were thus dominated by saturated fatty acids, n-3 PUFA or n-6 PUFA respectively. Resting RQ was measured in five birds from each treatment group during weeks 4 and 5 of the experiment. There were no significant differences between treatments in total feed intake or final body mass. Birds fed the PUFA diets had lower RQ and significantly reduced abdominal fat pad weights (P<0.01) compared with those fed tallow. The dietary lipid profile changes resulted in significantly greater partitioning of energy into lean tissue than into fat tissue (calculated as breast lean tissue weight:abdominal fat mass) in the PUFA groups compared with the saturated fat group (P<0.01; with no difference between the n-3 and n-6 PUFA groups). In addition, the PUFA-rich diets lowered plasma concentrations of serum triacylglycerols and cholesterol. The findings indicate that dietary fatty acid profile influences nutrient partitioning in broiler chickens.