Magnesium intake and risk of type 2 diabetes in men and women

Caffeine ingestion is associated with reductions in glucose uptake independent of obesity and type 2 diabetes before and after exercise training

OBJECTIVE

We investigated the effect of caffeine ingestion on insulin sensitivity in sedentary lean men (n = 8) and obese men with (n = 7) and without (n = 8) type 2 diabetes. We also examined whether chronic exercise influences the relationship between caffeine and insulin sensitivity in these individuals.

RESEARCH DESIGN AND METHODS

Subjects underwent two hyperinsulinemic-euglycemic clamp procedures, caffeine (5 mg/kg body wt) and placebo, in a double-blind, randomized manner before and after a 3-month aerobic exercise program. Body composition was measured by magnetic resonance imaging.

RESULTS

At baseline, caffeine ingestion was associated with a significant reduction (P < 0.05) in insulin sensitivity by a similar magnitude in the lean (33%), obese (33%), and type 2 diabetic (37%) groups in comparison with placebo. After exercise training, caffeine ingestion was still associated with a reduction (P < 0.05) in insulin sensitivity by a similar magnitude in the lean (23%), obese (26%), and type 2 diabetic (36%) groups in comparison with placebo. Exercise was not associated with a significant increase in insulin sensitivity in either the caffeine or placebo trials, independent of group (P > 0.10).

CONCLUSIONS

Caffeine consumption is associated with a substantial reduction in insulin-mediated glucose uptake independent of obesity, type 2 diabetes, and chronic exercise.

Insulin resistance, low-fat diets, and low-carbohydrate diets: time to test new menus

PURPOSE OF REVIEW

Insulin resistance increases the risk of cardiovascular disease and diabetes, and the risk of cardiovascular disease increases further once diabetes has developed. As insulin resistance is a precursor to diabetes, it is critically important to identify cost-effective means, such as dietary changes, by which to reduce insulin resistance. The purpose of this review is to evaluate recent findings concerning dietary composition and insulin resistance, with particular focus on low-fat diets compared with the currently popular low-carbohydrate diets.

RECENT FINDINGS

Recent findings indicate little support for the value of low-carbohydrate diets as therapies for insulin resistance. In contrast, the limited data available suggest that the higher fat content of typical low-carbohydrate diets may exacerbate insulin resistance in the long term. Preliminary data indicate that proteins from different sources may have differing effects on insulin resistance. Preliminary data also suggest the potential value of whole grains, fruits and vegetables in therapeutic diets to reduce insulin resistance.

SUMMARY

Current evidence supports the inclusion of whole grains, fruits and vegetables, and lean sources of animal proteins including low-fat dairy products in dietary therapies for insulin resistance. Those who wish to follow a low-carbohydrate diet should be encouraged to follow a new menu low in fat, and with most of the protein derived from plant sources.

Mitochondria are intracellular magnesium stores: investigation by simultaneous fluorescent imagings in PC12 cells

To determine the nature of intracellular Mg2+ stores and Mg2+ release mechanisms in differentiated PC12 cells, Mg2+ and Ca2+ mobilizations were measured simultaneously in living cells with KMG-104, a fluorescent Mg2+ indicator, and fura-2, respectively. Treatment with the mitochondrial uncoupler, carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP), increased both the intracellular Mg2+ concentration ([Mg2+]i) and the [Ca2+]i in these cells. Possible candidates as intracellular Mg2+ stores under these conditions include intracellular divalent cation binding sites, endoplasmic reticulum (ER), Mg-ATP and mitochondria. Given that no change in [Mg2+]i was induced by caffeine application, intracellular IP3 or Ca2+ liberated by photolysis, it appears that no Mg2+ release mechanism thus exists that is mediated via the action of Ca2+ on membrane-bound receptors in the ER or via the offloading of Mg2+ from binding sites as a result of the increased [Ca2+]i. FCCP treatment for 2 min did not alter the intracellular ATP content, indicating that Mg2+ was not released from Mg-ATP, at least in the first 2 min following exposure to FCCP. FCCP-induced [Mg2+]i increase was observed at mitochondria localized area, and vice versa. These results suggest that the mitochondria serve as the intracellular Mg2+ store in PC12 cell. Simultaneous measurements of [Ca2+]i and mitochondrial membrane potential, and also of [Ca2+]i and [Mg2+]i, revealed that the initial rise in [Mg2+]i followed that of mitochondrial depolarization for several seconds. These findings show that the source of Mg2+ in the FCCP-induced [Mg2+]i increase in PC12 cells is mitochondria, and that mitochondrial depolarization triggers the Mg2+ release.

Glycemic index and dietary fiber and the risk of type 2 diabetes

OBJECTIVE

To examine associations between type 2 diabetes and fiber, glycemic load (GL), dietary glycemic index (GI), and fiber-rich foods.

RESEARCH DESIGN AND METHODS

This was a prospective study of 36,787 men and women aged 40-69 years without diabetes. For all self-reported cases of diabetes at 4-year follow-up, confirmation of diagnosis was sought from medical practitioners. Case subjects were those who reported diabetes at follow-up and for whom there was no evidence that they did not have type 2 diabetes. Data were analyzed with logistic regression, adjusting for country of birth, physical activity, family history of diabetes, alcohol and energy intake, education, 5-year weight change, sex, and age.

RESULTS

Follow-up was completed by 31,641 (86%) participants, and 365 cases were identified. The odds ratio (OR) for the highest quartile of white bread intake compared with the lowest was 1.37 (95% CI 1.04-1.81; P for trend = 0.001). Intakes of carbohydrate (OR per 200 g/day 0.58, 0.36-0.95), sugars (OR per 100 g/day 0.61, 0.47-0.79), and magnesium (OR per 500 mg/day 0.62, 0.43-0.90) were inversely associated with incidence of diabetes, whereas intake of starch (OR per 100 g/day 1.47, 1.06-2.05) and dietary GI (OR per 10 units 1.32, 1.05-1.66) were positively associated with diabetes. These relationships were attenuated after adjustment for BMI and waist-to-hip ratio.

CONCLUSIONS

Reducing dietary GI while maintaining a high carbohydrate intake may reduce the risk of type 2 diabetes. One way to achieve this would be to substitute white bread with low-GI breads.

Whole grain intake and insulin sensitivity: evidence from observational studies

Observational studies have found that diets rich in whole-grain foods are associated with improved insulin sensitivity. The improved insulin sensitivity may be mediated in part by magnesium and dietary fiber, two nutrients found in whole-grain foods. By incorporating whole-grain foods into the diet, therefore, insulin sensitivity might be improved.

Increased diabetes incidence in Greek and Italian migrants to Australia: how much can be explained by known risk factors?

OBJECTIVE

The aims of the study were to determine whether Greek or Italian migrants to Australia have an elevated incidence of type 2 diabetes compared with Australian-born individuals and to what extent any differences can be explained by known risk factors.

RESEARCH DESIGN AND METHODS

This was a prospective study of 34,097 men and women aged 40-69 years born in Greece, Italy, Australia, or New Zealand who were free from diabetes at baseline. For all self-reported cases of diabetes at the 4-year follow-up, a confirmation of diagnosis was sought from medical practitioners. Of these, anyone for whom there was no evidence against a diagnosis of type 2 diabetes was considered a case subject.

RESULTS

Follow-up was completed by 29,331 (86%) participants, and 334 case subjects were identified. The cumulative incidence of type 2 diabetes among Greek (2.6%) and Italian (2.4%) migrants was more than three times that in Australian-born (0.7%) individuals. After adjusting for age only, the odds ratios (ORs) for being of Greek and Italian origin compared with Australian origin were 3.8 (95% CI 2.9-5.0) and 3.3 (2.6-4.3), respectively. The only known risk factor for type 2 diabetes that materially affected these ORs was BMI. After adjusting for BMI and age, the ORs for being of Greek and Italian origin, respectively, were 2.4 (1.8-3.2) and 2.0 (1.5-2.6).

CONCLUSIONS

Greek and Italian migrants to Australia have a more than three times greater incidence of type 2 diabetes than Australian-born individuals, and this is only partly explained by BMI. Although weight control will remain important for these high-risk groups, identification of other risk factors is required.

The potential biological mechanisms of arsenic-induced diabetes mellitus

Although epidemiologic studies carried out in Taiwan, Bangladesh, and Sweden have demonstrated a diabetogenic effect of arsenic, the mechanisms remain unclear and require further investigation. This paper reviewed the potential biological mechanisms of arsenic-induced diabetes mellitus based on the current knowledge of the biochemical properties of arsenic. Arsenate can substitute phosphate in the formation of adenosine triphosphate (ATP) and other phosphate intermediates involved in glucose metabolism, which could theoretically slow down the normal metabolism of glucose, interrupt the production of energy, and interfere with the ATP-dependent insulin secretion. However, the concentration of arsenate required for such reaction is high and not physiologically relevant, and these effects may only happen in acute intoxication and may not be effective in subjects chronically exposed to low-dose arsenic. On the other hand, arsenite has high affinity for sulfhydryl groups and thus can form covalent bonds with the disulfide bridges in the molecules of insulin, insulin receptors, glucose transporters (GLUTs), and enzymes involved in glucose metabolism (e.g., pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase). As a result, the normal functions of these molecules can be hampered. However, a direct effect on these molecules caused by arsenite at physiologically relevant concentrations seems unlikely. Recent evidence has shown that treatment of arsenite at lower and physiologically relevant concentrations can stimulate glucose transport, in contrary to an inhibitory effect exerted by phenylarsine oxide (PAO) or by higher doses of arsenite. Induction of oxidative stress and interferences in signal transduction or gene expression by arsenic or by its methylated metabolites are the most possible causes to arsenic-induced diabetes mellitus through mechanisms of induction of insulin resistance and beta cell dysfunction. Recent studies have shown that, in subjects with chronic arsenic exposure, oxidative stress is increased and the expression of tumor necrosis factor alpha (TNFalpha) and interleukin-6 (IL-6) is upregulated. Both of these two cytokines have been well known for their effect on the induction of insulin resistance. Arsenite at physiologically relevant concentration also shows inhibitory effect on the expression of peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor important for activating insulin action. Oxidative stress has been suggested as a major pathogenic link to both insulin resistance and beta cell dysfunction through mechanisms involving activation of nuclear factor-kappaB (NF-kappaB), which is also activated by low levels of arsenic. Although without supportive data, superoxide production induced by arsenic exposure can theoretically impair insulin secretion by interaction with uncoupling protein 2 (UCP2), and oxidative stress can also cause amyloid formation in the pancreas, which could progressively destroy the insulin-secreting beta cells. Individual susceptibility with respect to genetics, nutritional status, health status, detoxification capability, interactions with other trace elements, and the existence of other well-recognized risk factors of diabetes mellitus can influence the toxicity of arsenic on organs involved in glucose metabolism and determine the progression of insulin resistance and impaired insulin secretion to a status of persistent hyperglycemia or diabetes mellitus. In conclusions, insulin resistance and beta cell dysfunction can be induced by chronic arsenic exposure. These defects may be responsible for arsenic-induced diabetes mellitus, but investigations are required to test this hypothesis.

Oral Magnesium supplementation improves insulin sensitivity in non-diabetic subjects with insulin resistance. A double-blind placebo-controlled randomized trial

OBJECTIVE

Although hypomagnesemia reduces insulin sensitivity, benefits of magnesium supplementation to non-diabetic insulin resistant subjects has not been established. Our purpose was to determine whether oral magnesium supplementation with magnesium chloride (MgCl2) 2.5 g daily modify insulin sensitivity in non-diabetic subjects.

MATERIAL AND METHODS

This study was a 3 months randomized double-blind placebo-controlled trial. Apparently healthy subjects were eligible to participate if they had insulin resistance (HOMA-IR index equal or greater than 3.0) and hypomagnesemia (Serum magnesium levels equal or lower than 0.74 mmol/l). Subjects were randomized to receive either, MgCl2 2.5 g daily or placebo by 3-months.

RESULTS

At baseline there were not significant anthropometric or laboratory differences between both groups. At ending of the study, magnesium-supplemented subjects significantly increased their serum magnesium levels (0.61 +/- 0.08 to 0.81 +/- 0.08 mmol/l, p<0.0001) and reduced HOMA-IR index (4.6 +/- 2.8 to 2.6 +/- 1.1, p<0.0001), whereas control subjects did not (0.62 +/- 0.08 to 0.61 +/- 0.08 mmol/l, p=0.063 and 5.2 +/- 1.9 to 5.3 +/- 2.9, p=0.087).

CONCLUSIONS

Oral magnesium supplementation improves insulin sensitivity in hypomagnesemic non-diabetic subjects. Clinical implications of this finding have to be established.