The effect of high-protein, low-carbohydrate diets in the treatment of type 2 diabetes: a 12 month randomised controlled trial.

AIMS/HYPOTHESIS: Short-term dietary studies suggest that high-protein diets can enhance weight loss and improve glycaemic control in people with type 2 diabetes. However, the long-term effects of such diets are unknown. The aim of this study was to determine whether high-protein diets are superior to high-carbohydrate diets for improving glycaemic control in individuals with type 2 diabetes. METHODS: Overweight/obese individuals (BMI 27-40 kg/m(2)) with type 2 diabetes (HbA(1c) 6.5-10%) were recruited for a 12 month, parallel design, dietary intervention trial conducted at a diabetes specialist clinic (Melbourne, VIC, Australia). Of the 108 initially randomised, 99 received advice to follow low-fat (30% total energy) diets that were either high in protein (30% total energy, n = 53) or high in carbohydrate (55% total energy, n = 46). Dietary assignment was done by a third party using computer-generated random numbers. The primary endpoint was change in HbA(1c). Secondary endpoints included changes in weight, lipids, blood pressure, renal function and calcium loss. Study endpoints were assessed blinded to the diet group, but the statistical analysis was performed unblinded. This study used an intention-to-treat model for all participants who received dietary advice. Follow-up visits were encouraged regardless of dietary adherence and last measurements were carried forward for study non-completers. RESULTS: Ninety-nine individuals were included in the analysis (53 in high protein group, 46 in high carbohydrate group). HbA(1c) decreased in both groups over time, with no significant difference between groups (mean difference of the change at 12 months; 0.04 [95% CI -0.37, 0.46]; p = 0.44). Both groups also demonstrated decreases over time in weight, serum triacylglycerol and total cholesterol, and increases in HDL-cholesterol. No differences in blood pressure, renal function or calcium loss were seen. CONCLUSIONS/INTERPRETATION: These results suggest that there is no superior long-term metabolic benefit of a high-protein diet over a high-carbohydrate in the management of type 2 diabetes. TRIAL REGISTRATION: ACTRN12605000063617 ( www.anzctr.org.au ). FUNDING: This study was funded by a nutritional research grant from Meat and Livestock Australia (MLA). J.E. Shaw is supported by NHMRC Fellowship 586623.

Comparison of seal oil to tuna oil on plasma lipid levels and blood pressure in hypertriglyceridaemic subjects.

As meat is a rich source of the omega-3 fatty acid docosapentaenoic acid (DPA) and Australians consume six times more meat than fish, investigation of the potential health benefit of DPA is warranted. The aims were to compare the effects of seal oil supplementation with fish oil, on measures of plasma lipids and blood pressure in hypertriglyceridaemic subjects. Forty-eight volunteers were recruited from the Wollongong community and were randomly allocated to one of three groups either receiving 1 g/day of long-chain omega-3 polyunsaturated fatty acids (LC n-3 PUFA) using one of three oils: seal oil capsules (340 mg eicosapentaenoic acid (EPA), 230 mg DPA, 450 mg DHA), fish oil capsules (210 mg EPA, 30 mg DPA, 810 mg DHA) or placebo capsules (containing sunola oil) for 6 weeks. Plasma triglycerides remained unchanged in the placebo group, whilst reductions of 7 and 14% (P < 0.05) were seen in the fish oil and seal oil groups respectively. Systolic blood pressure improved by 8 and 5 mmHg with seal oil and fish oil respectively (P < 0.05). The mean arterial pressure was significantly lower after seal oil supplementation (P < 0.005) compared with the placebo group. These results indicate that seal oil is as effective as fish oil in lowering plasma triglycerides and blood pressure.

Short-term diets enriched in stearic or palmitic acids do not alter plasma lipids, platelet aggregation or platelet activation status.

OBJECTIVE: To determine whether healthy males who consumed increased amounts of dietary stearic acid compared with increased dietary palmitic acid through the consumption of commercially available foods, exhibited any changes in plasma lipids, platelet aggregation or platelet activation status. DESIGN: A randomised cross-over dietary intervention. SUBJECTS AND INTERVENTIONS: Nine free-living healthy males consumed two experimental diets (stearic acid enriched, diet S, and palmitic acid enriched, diet P) for 3 weeks in a randomised cross-over design separated by a 3 week washout phase. The diets consisted of approximately 30% of energy as fat (30% of which was derived from the treatment diets) providing approximately 13 g/day as stearic acid and 17 g/day as palmitic acid on diet S and approximately 7 g/day as stearic acid and 22 g/day as palmitic acid on diet P. The dietary ratio of stearic to palmitic acids was 0.76 on diet S compared with 0.31 on diet P. Blood samples were collected on days 0 and 21 of each dietary period. RESULTS: LDL cholesterol levels and platelet aggregation response to the agonist ADP were significantly decreased (P<0.025) in subjects on diet S compared with day 0. Apart from that, there were no significant changes in plasma lipids, platelet aggregation, mean platelet volume and platelet reactivity between diets. There were no significant changes in stearic or palmitic acid levels in plasma phospholipid or triacylglycerol. There was a significant difference in palmitic acid levels in platelet phospholipids between the two diets. CONCLUSIONS: Use of commonly available foods led to a 27% increase in stearic acid (diet S) and a 19% increase in palmitic acid (diet P), on diets S and P respectively, and no significant differences between the two diets in plasma lipoprotein concentrations, platelet aggregation or platelet activation status.

A stearic acid-rich diet improves thrombogenic and atherogenic risk factor profiles in healthy males.

OBJECTIVE: To determine whether healthy males who consumed increased amounts of dietary stearic acid compared with increased dietary palmitic acid exhibited any changes in their platelet aggregability, platelet fatty acid profiles, platelet morphology, or haemostatic factors. DESIGN: A randomized cross-over dietary intervention. SUBJECTS AND INTERVENTIONS: Thirteen free-living healthy males consumed two experimental diets for 4 weeks with a 7 week washout between the two dietary periods. The diets consisted of approximately 30% of energy as fat (66% of which was the treatment fat) providing approximately 6.6% of energy as stearic acid (diet S) or approximately 7.8% of energy as palmitic acid (diet P). On days 0 and 28 of each dietary period, blood samples were collected and anthropometric and physiological measurements were recorded. RESULTS: Stearic acid was increased significantly in platelet phospholipids on diet S (by 22%), while on diet P palmitic acid levels in platelet phospholipids also increased significantly (8%). Mean platelet volume, coagulation factor FVII activity and plasma lipid concentrations were significantly decreased on diet S, while platelet aggregation was significantly increased on diet P. CONCLUSION: Results from this study indicate that stearic acid (19g/day) in the diet has beneficial effects on thrombogenic and atherogenic risk factors in males. The food industry might wish to consider the enrichment of foods with stearic acid in place of palmitic acid and trans fatty acids.

Selected micronutrient intake and status in men with differing meat intakes, vegetarians and vegans.

Dietary factors play a critical role in human health. The aim of this cross-sectional study was to examine micronutrient intake and status of subjects who were habitual meat eaters eating different quantities of meat with those who were habitual vegetarians or vegans. One hundred and thirty-nine healthy male subjects (vegan, n = 18; ovolacto-vegetarian, n = 46; moderate meat-eater, n = 65; and high meat-eater, n = 18) aged 20-55 years were recruited in metropolitan Melbourne. Each volunteer completed a semiquantitative Food Frequency Questionnaire (FFQ) and gave a fasting venous blood sample. Dietary sodium/potassium ratio was significantly lower and vitamin C, fibre and iron intakes were higher in vegetarians than in meat-eaters. High meat-eaters had a significantly higher calcium, retinol and zinc intake than did the other three dietary groups; moderate meateaters had the lowest mean intake of fibre, vitamin C and ß-carotene. Vegans had a significantly higher ß-carotene intake than did the other groups. Serum ferritin and vitamin B12 levels, and haemoglobin concentration were significantly lower in vegetarians than in meat-eaters. Vegans had a significantly higher serum folate concentration than did ovolacto-vegetarian and moderate meat-eater groups. There was no significant difference in serum α-tocopherol concentration. There are differences between the four diet groups that have potential to affect the subjects' health and susceptibility to chronic diseases including cardiovascular disease and cancer. Based on the present data, high meat-eaters may particularly benefit from altering their dietary pattern to reduce their sodium and saturated fat intake, and moderate meat-eaters from increasing their fibre and antioxidant consumption. Vegetarians, especially vegans, may need to increase their vitamin B12 and zinc intakes.

The effect of diet on plasma homocysteine concentrations in healthy male subjects.

OBJECTIVE: To determine the effect of habitual omnivorous and vegetarian diets on folate and vitamin B12 status and the subsequent effect on homocysteine concentration. DESIGN: Cross-sectional comparison of free-living habitual meat-eaters and habitual vegetarians. SETTING: The study was conducted at RMIT University, Melbourne. SUBJECTS: One hundred and thirty-nine healthy male subjects (vegans n=18, ovolacto vegetarians n=43, moderate meat-eaters n=60 and high meat-eaters n=18) aged 20-55 y who were recruited in Melbourne. OUTCOME MEASURES: Fasting plasma or serum from each subject was analysed for folate, vitamin B12 and homocysteine concentration. A semi-quantitative Food Frequency Questionnaire was completed by a subset of subjects from each group to determine methionine intake. RESULTS: The two meat eating groups consumed significantly greater levels of methionine (P<0.001). There was no clear trend in plasma folate status between groups, however the plasma vitamin B12 concentration decreased progressively from the high-meat-eating group to vegans (P<0.05). An inverse trend was observed with plasma homocysteine concentration, with vegans showing the highest levels and high meat eaters the lowest (P<0.05). CONCLUSIONS: Dietary methionine intake has no observable effect on plasma homocysteine concentration. In habitual diets, where folate intake is adequate, lowered vitamin B12 intake from animal foods leads to depleted plasma vitamin B12 concentration with a concomitant increase in homocysteine concentration. The suggested mechanism is the failure to transfer a methyl group from methyl tetrahydrofolate by vitamin B12 in the remethylation of homocysteine to methionine.

Contribution of meat fat to dietary arachidonic acid.

Arachidonic acid (AA) in the diet can be efficiently absorbed and incorporated into tissue membranes, resulting in an increased production of thromboxane A2 by platelets and increased ex vivo platelet aggregability. Results from previous studies have shown that AA is concentrated in the membrane phospholipids of lean meats. However, the concentration of AA in the visible fat portion of meats also may be significant despite being ignored in most studies. The aim of this study was to accurately quantitate the AA content of visible fat and the lean portion of beef, lamb, pork, chicken, duck, and turkey. The visible fat of meat contained a significant quantity of AA, ranging from 20 to 180 mg/100 g fat, whereas the AA content of the lean portion of meat was lower, ranging from 30 to 99 mg/100 g lean meat. Beef and lamb meats contained lower levels of AA in both the visible fat and lean portion than that from the other species. The highest level of AA in lean meat was in duck (99 mg/100 g), whereas pork fat had the highest concentration for the visible fats (180 mg/100 g). The lean portions of beef and lamb contained the higher levels of n-3 polyunsaturated fatty acids (PUFA) compared with white meats which were high in AA and low in n-3 PUFA. The present data indicate that the visible meat fat can make a contribution to dietary intake of AA, particularly for consumers with high intakes of fat from pork or poultry meat.

Short-term diets rich in arachidonic acid influence plasma phospholipid polyunsaturated fatty acid levels and prostacyclin and thromboxane production in humans.

Two small-scale dietary intervention studies were conducted to examine the effect of diets rich in arachidonic acid (AA) and n-3 long-chain polyunsaturated fatty acids (n-3 LCP), on the in vivo production of prostacyclin (PGI2) and thromboxane (TXA2). The first was a pilot study and contained insufficient numbers for statistical analyses. It involved a 7-d intervention with 10 subjects divided into three groups, consuming diets rich in AA (500 mg/d), rich in AA and docosahexaenoic acid (DHA) (500 mg/d of each), or rich in DHA and eicosapentaenoic acid (EPA) (approximately 1500 mg/d of n-3 LCP). Plasma phospholipid (PL) levels of AA increased in all subjects in groups 1 (n = 4) and 2 (n = 3). DHA levels increased in all subjects in Groups 2 and 3 (n = 3), and EPA levels increased in all subjects from Group 3 but fell in all subjects from Group 1. The in vivo production of PGI2, measured as its urinary metabolite, was increased in two subjects in Group 1 and one subject in Group 2, with all other subjects showing little change. Urinary TXA2 metabolite increased in all subjects from Group 1. The second study was conducted in seven subjects, who consumed a low fat diet for 2 wk: the 1st wk was a vegetarian diet (no LCP) followed by a 2nd wk where the subjects were required to consume 500 g (raw weight) of kangaroo meat daily (305 mg/d AA, 325 mg/d n-3 LCP). The meat diet was associated with a marked rise in the serum PL levels of AA, EPA and docosapentaenoic acid 22:5(n-3) and with a significant increase in the urinary output of the prostacyclin metabolite, but no effect on TXA2 production, as measured by its urinary metabolite level. The results of these studies have shown that diets that contain both AA and n-3 LCP are associated with an increase in PGI2 production, without affecting TXA2 production. Further studies with purified LCP are warranted.

The arachidonic acid content of the Australian diet is lower than previously estimated.

Linoleic acid [18:2(n-6)] is the most abundant polyunsaturated fatty acid in the Western diet and is considered to be the primary source of tissue arachidonic acid [20:4(n-6)]. Dietary 20:4(n-6) may also contribute to tissue 20:4(n-6) levels in humans, but the extent of this contribution is unclear. We believe that literature estimates of 20:4(n-6) intake of 200-1000 mg/d are too high, possibly because of incorrect values in food composition tables where high amounts of 20:4(n-6) are recorded in margarines, some vegetable products and animal fat. We assessed the 20:4(n-6) content of common Australian foods and found that the 20:4(n-6) levels (on a 100-g edible basis), were 891 mg and 390 mg, respectively, for duck and chicken egg yolks, 294 mg for liver, 153 mg for kidney, 75 mg for skinless turkey, 56 mg for lean pork, 49 mg for lean lamb, 31 mg for chicken breast, 56 mg for chicken legs and 35 mg for lean beef. Eicosapentaenoic acid [20:5(n-3)] levels were < 10 mg/100 g in chicken meat, turkey meat, emu meat and chicken eggs, whereas the values for 20:5(n-3) for beef, lamb, liver, kidney and duck egg yolk ranged from 11 to 138 mg/100 g food. Applying our current 20:4(n-6) measurements to previously determined food intakes of Australian adults determined in an Australiawide survey in 1983, we estimated the mean 20:4(n-6) intake for Australian adult males to be 130 mg/d and females 96 mg/d. Whether such intakes of dietary 20:4(n-6) make an important contribution to tissue 20:4(n-6) levels is uncertain.

The effect of linoleic, arachidonic and eicosapentaenoic acid supplementation on prostacyclin production in rats.

We examined the effect of dietary supplementation of linoleic acid (LA), arachidonic acid (AA) or eicosapentaenoic acid (EPA) to rats fed a diet low in linoleic acid on in vitro and in vivo production of prostacyclin. Male Sprague Dawley rats were fed a high-fat diet (50% energy as fat, 1.5% linoleic acid) for two weeks. Three of the groups were then supplemented orally with either 90 mg/d of LA, AA or EPA, all as the ethyl esters, for a further two weeks while remaining on the high-fat diet. Forty-eight hour urine samples were collected at the end of the second and fourth weeks. In vivo prostacyclin production was determined by a stable isotope dilution, gas chromatography/mass spectrometry assay for the major urinary metabolite of prostacyclins (2,3-dinor-6-keto-PGF1 alpha or PGI2-M and delta 17-2,3-dinor-6-keto-PGF1 alpha or PGI3-M). In vitro prostacyclin production was determined by radioimmunoassay of the stable metabolite (6-keto-PGF 1 alpha) following incubation of arterial tissue. Oral supplementation with AA resulted in a rise in plasma and aorta 20:4n-6, and increased in vitro prostacyclin and urinary PGI2-M production. EPA supplementation resulted in a rise in plasma and aorta 20:5n-3 and 22:5n-3, and a decline in plasma 20:4n-6, but not in the aorta. In the EPA-supplemented group, the in vitro prostacyclin and the urinary PGI3-M increased, but urinary PGI2-M decreased. The increase in in vitro prostacyclin production in the EPA-supplemented rats was unexpected and without obvious explanation.(ABSTRACT TRUNCATED AT 250 WORDS)