Nutrients for Blood Glucose Control: A Review and Clinical Guide
Illnesses related to poor blood glucose control and insulin resistance are amongst the most commonly encountered in clinical practice. Although type-2 diabetes, which affects more than one in 20 people (1), is the most apparent example there are a host of other common insulin resistance related illnesses including obesity, hyperlipidaemia, hypertension, dermatological disorders (e.g. acne, acrochordons, and acanthosis nigricans) and polycystic ovary syndrome (PCOS) (2).
Dietary change, exercise, behavioural changes and medication are typically the first-line treatment for improving blood glucose control however there are concerns about the real-world benefits and potential side effects of conventional medications (3,4). Nutritional supplements are receiving increasing interest as a viable treatment option and in some cases there is rigorous evidence that certain supplements could be included in standard care for improving blood glucose control (5).
Because the effectiveness of nutritional interventions is influenced by factors such as dietary intake and variations in requirements and metabolism it is unlikely that a “one size fits all” prescription is appropriate. An individualized prescription based on the best available evidence would therefore be a logical approach to improving blood glucose control with nutritional supplements. This article summarizes a selection of nutritional supplements with compelling evidence for clinical efficacy and concludes with a summary and guide for developing a personalized prescription (table 1).
The culinary spice cinnamon was found in a meta-analysis of 6 clinical studies to improve blood glucose control with statistically significant improvements in glycosylated haemoglobin A1c (HbA1c) and fasting blood glucose (FBG) in studies of up to 4 months duration (6). A more recent meta-analysis of 10 clinical studies however found inconsistent evidence for cinnamon (7). Compared to medications the reduction in fasting glucose from cinnamon (–24.59 mg/dL) was lower than metformin (–58 mg/dL) and superior to sitagliptin (–16 to –21 mg/dL). Cinnamon is very promising but it appears better quality research is needed to clarify its clinical use (8).
Chromium has been subject to several clinical trials but these have generally produced inconsistent findings with some showing benefit while others do not (9). A review of clinical studies suggested that individuals with more severe insulin resistance might have a better response to chromium supplementation; specifically individuals with type-2 diabetes and marked elevations in fasting glucose and HbA1c (10).
Pycnogenol is a standardized extract of French maritime pine bark composed of procyanidins and phenolic acids. One open label preliminary study and two randomized controlled trials (RCTs) have demonstrated the ability of pycnogenol to improve blood glucose control in diabetics (11-13). The RCTs also found benefit for cardiovascular risk factors suggesting pycnogenol may be particularly useful for reducing associated cardiovascular risk (14).
Both intravenous and oral administration lipoic acid has been studied in type-2 diabetics and it appears the effects or oral supplementation are modest, with improvements insulin sensitivity following supplementation typically less than 20% (15). However lipoic acid is a potent anti-oxidant and improves nerve function making it a potentially useful supplement for managing complications of elevated blood glucose and oxidative stress including vascular disease and neuropathy (16,17).
A particular sweet potato (Ipomoea batatas) extract has been used as a dietary supplement for type-2 diabetes in a number of studies. A systematic review of 3 clinical trials found a statistically significant improvement in HbA1c at three to five months with 4 grams a day of the sweet potato dietary supplement compared to placebo (18).
Several studies have found myo-inositol to be very effective for the treatment of PCOS, primarily due to its ability to improve insulin sensitivity (19). Emerging evidence also suggests that myo-inositol may be effective the treatment of other disorders related to insulin resistance including gestational diabetes mellitus, metabolic syndrome as well as the management of diabetic complications (20).
There is very strong evidence to show that low dietary magnesium intake is associated with risk of type-2 diabetes, in a dose-response manner (21). Furthermore a meta-analysis of 9 clinical studies of magnesium supplementation in type-2 diabetes found consistent benefits for reducing plasma fasting glucose levels and raising HDL cholesterol at a median dose of 360 mg per day for 4-16 weeks (22).
Vitamin D deficiency is an emerging risk factor for insulin resistance, type-2 diabetes, metabolic syndrome and PCOS (23,24). Various lines of evidence suggest that the association between vitamin D deficiency and insulin resistance may be causal (25). However clinical studies examining the effect of vitamin D supplementation on blood glucose, insulin resistance and HbA1c have produced mixed results (26). Despite this it should be noted that studies so far have been poorly designed and not accounted for important factors such as baseline vitamin D status or provision of an adequate dose to correct deficiency, so further research is needed before firm conclusions can be made (27).
Table 1: Summary and guide for developing a personalized prescription
Nutritional supplements in practice
The use of safe, relatively low-cost nutritional supplements should be considered in the management of insulin resistance, type-2 diabetes, metabolic syndrome and PCOS due to their ability to improve blood glucose regulation and insulin signalling (see image 1).
Image1: Nutritional supplements and the insulin signaling cascade.
Supplements such as cinnamon and sweet potato could be used as general insulin-sensitizing agents in addition to dietary and lifestyle changes. Lipoic acid appears to have wider reaching benefits for preventing complications of elevated blood glucose while myo-inositol is effective for women with PCOS. The identification and correction of deficiencies in chromium, magnesium and vitamin D is likely to improve blood glucose and have a number of general health benefits.
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