Critical Review of the Literature of the role that Dietary Factors Play in Preventing Type 2 Diabetes’
Type 2 Diabetes (T2D) is a rapidly growing international public health issue. It has been reported that 285 million 20–79 year olds had the disease in 2010 worldwide and this is estimated to grow to 439 million by 2030. (19) T2D has been associated with a variety of other health problems such as cardiovascular disease, blindness and shortened life expectancy. (4,40) The prevalence of the disease is associated with obesity and overweight as well as a ‘Western’ dietary pattern and lack of exercise. (17) It has been reported that approximately 80% of people who develop T2D are obese or overweight prior to diagnosis.(8) Diabetes is a preventable disease, (19) with weight loss being identified as one of the most powerful interventions. (4) Diet and physical activity (PA) are effective interventions in attaining this (25) and have also been linked with reduction in risk independently.
(40) The primary focus of this paper will look at diet and review the evidence on whether diet alone can help to prevent T2D. As a secondary theme it will also look at the evidence that diet can help to prevent the development of and help to control symptoms after T2D has been diagnosed. 8 papers will reviewed, (26-33) with a purpose of identifying some practical, evidence based dietary guidelines. Dietary guidelines are easier to follow when they indicate specific foods and patterns rather than nutrients and properties of foods and greater compliance can be achieved . (29,31) For this reason the studies reviewed focus on specific foods and diet styles rather than macronutrients and or micronutrients which do not translate into dietary guidelines as easily.
A literature review using the following databases was carried out; Pubmed, Web of Science, Embase, CINAHL, British Nursing Index and Medline. The following keywords were used in various combinations;
Diet, prevention, food, T2D, interventions, dietary, fruit, vegetables, nutrition. Other resources such as ‘Google scholar’ and NHS Choices ‘behind the headlines’ were also utilised. Because T2D is an international issue, papers from around the world were considered. The criteria for inclusion was less than 10 years old, the intervention had to be a specific food group or diet style, measuring new incidents of T2D, or changes is symptoms associated with T2D and adults as the sample group.
Diet as a tool to prevent T2D
6 Studies looked at whether diet can prevent T2D. Please refer to Table 1 for details of all the studies. Villegas et al (26) and Bazzano et al (27) looked at fruit, vegetable and fruit juices consumption and fruit and vegetable consumption respectively. Both of the studies used women only in their sample groups, limiting the generalizability of the findings to the wider population. (5) Dietary assessment was achieved through Food Frequency Questionnaires (FFQ) in both studies. Villegas et al (26) provided evidence of their FFQ having been validated, (21) however reported using it only 3 times within a 4.6 year timeframe, questioning the accuracy of their collected data. Bazzano et al (27) data collection was retrospective in that the FFQ was designed in 1984 and followed up at 4 year intervals. The concern with such a dated questionnaire is that it has not accounted for different trends that have occurred over time, affecting the accuracy of the results.
Villegas et al (26) confirmed incidence of T2D through subjects meeting the criteria set out by the American Diabetic Association.(2) Bazzano et al (27) used criteria set out by National Diabetes Data Group for all participants up to and including 1997.(16) The Criteria for participants after 1998 was set out by the American Diabetic Association. The reason for this was due to new criteria being published at this time. (2) The main difference being the plasma glucose reading changing from 7.8mmol/l or more to 7.0mmol/l or more. (9) The Criteria Set out by the World Health Organisation in 1985, (24) could have been used to confirm incidence of T2D in the Bazzano et al (27) study . It was published 1 year after baseline data was taken and would have resulted in a higher percentage of participants being diagnosed with the same criteria, increasing consistency
and reliability in the results.
Villegas et al (26) reported that a higher consumption of vegetables was associated with a reduced risk of T2D. Participants who had a higher vegetable intake were also less likely to smoke and have higher levels of PA, both factors that can reduce the risk of T2D questioning the causality of the reduction in risk. (4) Participants with a higher fruit intake were also less likely to smoke and have higher levels of PA. There was no association made between fruit intake and risk of T2D therefore it is possible to consider that confounders like PA and smoking may not have effected the level of risk to T2D in this study.
Bazzano et al (27) reported an inverse association between whole fruit and green leafy vegetable intake. Women who had a higher intake of fruit and vegetables were older, less likely to smoke and more likely to have higher levels of PA. Fruit Juice was positively associated with incidence of T2D which could be due to high sugar content.. (22) Also participants who had the highest fruit juice intake had the lowest levels of PA which is associated with increased risk of T2D. (20)Salas-Salvado et al (28) and Martinez-Gonzalez et al (29) both studied the risk of T2D and adherence to the Mediterranean diet (MedDiet). A MedDiet is characterised by high consumption of fruit, vegetables, whole grains, olive oil, nuts, pulses, fish and reduced consumption of red and processed meats, high fat dairy refined gains and foods high in sugar and starch. (11) Salas-Salvado et al (28) compared adherence to a MedDiet supplemented with either olive oil or nuts with a low fat diet (control group).
The supplemented items in the MedDiets were given to participants. Participants in the control group were given non dietary gifts to encourage adherence. There are concerns about the ethics of using incentives and gifts in research. (6)Within this study the use of gifts appears innocuous, the concern lies with how adherence to the diet is reliably measured. When participants are given vital ingredients, this will influence their dietary intake, making it difficult generalize the results. (13) The sample population in this study were older and had at least 3 risk factors relating to cardiovascular disease again making it more difficult to generalise results. Participants were given 7 goals, including; increasing vegetable and fruit consumption, reducing red and processed meat and increasing the consumption of either olive oils or nuts.
The control group were asked to reduce all types of fat. Results showed that participants in the MedDiet groups had a greater reduced risk of T2D. Risk was reduced by 51% in the olive oil group and 52% in the nut group. These findings are backed up by other studies. (38-39) Diabetes incidence was lower in those who attained ≥ 4 of the 7 goals. PA levels and changes in weight did not differ through all 3 groups, although the participants in the both the MedDiet groups were associated with higher levels of PA. This study was carried out on Spanish participants, who traditionally follow a MedDiet. The control group may have had a strong adherence to a MedDiet naturally, which could impact on the reliability of the results.
Martinez-Gonzalez et al (29) used participants who were nurses and university graduates. Prevalence in T2D is associated with lower socioeconomic status, (1) so by using the participants from a higher socioeconomic group may bias the findings. (13) Participants were not excluded if they had Diabetes at baseline. Data was collected via FFQ which consisted of 136 items with 9 responses ranging from never to more than 6 times a day. Points were allocated to determine a score indicating level of adherence. Questions covered areas such as cooking methods, supplements and fats and oils. This FFQ goes into a lot of detail to obtain the most information it can about participants diets, increasing the validity of the data.
However reliability is compromised as participants are asked to recall food intake from the previous year decreasing the accuracy of data collected. (5) The results indicated a significant reduction in risk of developing T2D in those who with strong adherence to the MedDiet after adjustment for age and sex. Participants with a score of ≥ 6 had an 83% reduction. Although participants in this group had the highest levels of physical activity, which is a known factor in decreasing the risk of T2D, they also had a higher baseline prevalence for increased risk factors for Diabetes such as age higher BMI and higher blood pressure. This adds weight to the findings the MedDiet can reduce the risk of developing T2D.
Fung et al (30) and VanDam et al (31) looked at more generalised dietary patterns. VanDam et al (31) used a 131 item FFQ specifying specific foods, portion sizes and frequency of intake. Over a space of 12 years data on food intake was collected 3 times. In order to make these results more reliable data collection should have occurred more frequently. The participants were all male health professionals making the sample group quite specific reducing the ability to generalise the findings to the wider population. (13) Foods were classified into groups based on nutritional profiles. Factor analysis was then applied in order to identify food patterns. Two dietary patterns were identified ‘Prudent’ and ‘Western’. Prudent was characterised by high consumption of vegetables, fish and whole grains and Western by high consumption of red and processed meat, high-fat dairy and eggs.
Men with higher Western patterns were younger, more likely to smoke and did less PA. Men with higher Prudent diet patterns were older less likely to smoke and engaged in more PA. The Prudent diet was associated with a modest reduction in risk in developing T2D with wholegrain foods having the highest inverse association. Fruit and vegetables were not considerably associated with reduced risk. The Western diet was associated with considerably higher risk of developing T2D. Processed meat, other processed foods and refined grains indicating the most significant association. This could indicate that cutting out specific foods could me more beneficial in reducing the risk of T2D than increasing intake of other foods.
Fung et al (30) used participants from the Nurses Health Study which was established in 1976. This is the same study from which Bazzano et al (27) took their participants. The same FFQ was used in this study with baseline also being the 1984 FFQ as this was the expanded 116 item version. The information obtained was then used and classified in the same way as the VanDam et al (31) study producing the same Prudent and Western dietary patterns. The results from this study focus mainly on the Western diet pattern. Similarly to the men in the VanDam et al (31) study, women who scored high in the Western diet pattern were more likely to smoke. The results also mirrored that of the VanDam et al (31) study in that it reported an increased risk of developing T2D and a Western diet pattern. This study investigated the characteristics of the Western dietary pattern further and found positive associations between red and processed meats and the development of T2D.
This could also add weight to the previous comment that cutting out specific foods, such as red and processed meats could be more beneficial than adding other food groups in preventing T2D. A replica study using the same FFQ and Prudent and Western diet pattern and using a sample group that consisted of both men and women could add strength to the finds of both of these studies.(5)
Diet as a tool in preventing the development of and giving greater control over the symptoms of T2D
Elhayany et al (32) compared a low carbohydrate Mediterranean diet (LCM) a traditional Mediterranean diet (TM) and the 2003 American Diabetic Association diet (ADA) on health parameters. Glycemic control for people with T2D diagnosis was one of the outcome measures. Participants were randomly assigned to 1 of the 3 groups, given recommendations for daily intake on nutritional elements such as calories and protein and advised to engage in 30-45 minutes of PA a week. The LCM and TM diets included only low glycemic index carbohydrates, with LCM having a lower %. The TM and ADA diets had the same % of carbohydrates but the ADA also included mixed glycemic index carbohydrates.
FFQ were used asking the participants to recall the last 24hour food intake. It is felt this data will be more accurate than those studies asking participants to recall food intake from the previous year, making results more reliable. Data was collected 3 times over a 12 month period. Results showed all groups had reduced weight and BMI with no significant difference. All 3 dietary interventions reduced factors that increase glycemic control such as HbA1c and triglyceride levels. The LCM diet was the most effective in increasing glycemic control.
Esposito et al (33) compared a LCM and a low fat calorie restricted diet (LFD) on glycemic control and the delay on needing to commence antihyperglycemic medication in people newly diagnosed with T2D. The LCM diet was rich in fruit vegetables and whole grains and low in red meat. There was also a requirement that no more than 50% of calories was from carbohydrates. Complex carbohydrates rather than low GI carbohydrates were stipulated. Some complex carbohydrates can have high GI levels, which are associated with increased risk of Diabetes. (10) The LFD was based on American Heart Association guidelines. (12) Participants were randomized into 1 of 2 groups asked to keep food diaries and given guidelines on increasing physical activity.
Data was collected through reviews of the diary. Food diaries could provide more reliable information than FFQ if they are filled out daily. There is still a risk that they could be filled out inaccurately, and that participants may modifies their intake as a result of keeping a diary. The study reported that both groups lost weight, but reduction was greater in the LCM. Overall there was a significant difference in the need to commence antihyperglycemic medication between the LCM and the LFD; 44% and 70% respectively. Potentially this result could have shown more significance if low GI carbohydrates were stipulated instead of complex carbohydrates.
The findings coming out of this review indicate that certain dietary interventions maybe helpful in reducing the risk of developing T2D and may also help with glycemic control after diagnosis.(26-33) Some food groups and dietary patterns provide more consistent evidence than others. The results regarding fruit intake and risk of T2D are inconsistent within this review with Villegas et al (26) reporting no association between fruit and risk reduction, where as Bazzano et al (27) did. High consumption of fruit and vegetables are a major component in the Mediterranean diet and the Prudent dietary pattern, both of which were associated with an inverse association. Vegetables are more consistently associated with a reduced risk, in this review and a recent meta-analysis, (3) particularly green leafy vegetables. (27) More research studying the effects of fruit and vegetables separately is needed. This inconsistency is reflected in other studies and systematic reviews. (7) High intake of fruit and vegetables has often been associated with higher PA levels within this review, (26-29,31) which is a risk reducing factor in itself.
Part of the problem could be that many studies that look at dietary interventions are prospective cohort studies and although they can provide an association they can not prove whether the cause is due to the dietary item or another factor such as PA or weight loss. More experimental designed research is needed so that a direct cause between diet and the reduction of T2D can be established. (13) While it is unclear the exact role that fruit and vegetables play in reducing the risk of T2D there is an abundance of evidence that a diet rich in fruit and vegetables is beneficial in overall health (34-35) and weight reduction, (23) so including them as part of a healthy diet may indirectly help to reduce the risk of T2D.
Red and processed meat has been more consistent in its positive association to developing T2D. Fung et al (28) and VanDam et al (29) found strong associations between consumption and increased risk of T2D. One of the characteristics of The Mediterranean diet is the absence or reduction of red and processed meats, and this diet has been associated with reduced risk. These findings were backed up in a recent meta-analysis paper (18) studying 3 cohorts who’s conclusion suggests that red meat consumption, particularly processed red meat is linked to higher risk of developing T2D.
Meta-analysis produces level 1 evidence, providing increased confidence in the conclusions and good grounding for providing evidence based information such as dietary guidelines.(13) Diets high in red and processed meats are linked to high cholesterol (14) which is one of the leading causes of death in people with T2D (15) and other serious health conditions such as cancer and cardiovascular disease. (36) So while increasing fruit and vegetable intake may have a more beneficial and holistic effect on health, the same could be said for reducing the intake of red and processed.
Salas-Salvado et al (28) produced higher level evidence being a randomised control trial giving more weight to their findings;(5) following a MedDiet can reduce the risk of T2D. One of the studies that looked at the effect of diet after diagnosis, (32) was also a randomised control trial providing the same level of evidence and weight to their findings that LCM can delay the need for hyperglycaemic medication therefore having a positive beneficial effect on T2D. Although these two studies looked at the effect of diet after diagnosis it could be reasonable to suggest that following the dietary patterns associated with these findings, prior to diagnosis could help prevent T2D as they are similar dietary patterns to two of the other studies.(28-29)
While the findings from this review indicate that making changes to ones diet may help to reduce the risk of Diabetes occurring, it also suggests that implementing the same kind of dietary changes may help with glucose control after Diabetes has been diagnosed, preventing the further development of the disease and the need for antihyperglycemic medication. (32-33) One study has implied that dietary changes in the form of energy restriction can actually reverse beta cell failure and insulin resistance, symptoms found in T2D.
(37) It is a very small study and the dietary intervention is severe energy restriction making it difficult to generalise to the wider population, but it is a controlled study and could provide interesting findings that further research could be built upon, that dietary interventions may be able to reverse the symptoms of T2D. Collating this information together it seems reasonable to suggest that making positive changes to dietary habits could have benefits pre and post T2D diagnosis.
The findings from the 8 studies in this review can not all be discussed in detail due to word limitation. However recurring themes seem to be occurring, providing information on which to base a set of guidelines. Going by these findings the following guidelines are recommended:
Reduce intake of red meat and processed meat (all colours)
Substitute these with white meats and fish
Reduce intake of other processed foods
Reduce intake of refined grains
Reduce intake of high sugar foods and drinks including fruit juices Carbohydrate intake should be based on items with a low GI score Increase intake of olive oil
Increase nut intake
Increase intake of pulses
Increase wholegrain intake
Increase vegetable intake especially green leafy vegetables
Increase fruit intake
A Mediterranean style diet is characterised by much of this advise and is therefore a recommended diet style to follow.
The prevalence of T2D is growing around the world. It has been associated with many other health problems and reduces quality of life and life expectancy. It is a preventable disease and diet is one of the ways in which this disease can be combated. Dietary guidelines have been recommended from the findings of this review, based on following a Mediterranean diet, reducing intake of red and processed meats and other processed foods and increasing intake of foods such as fruit and vegetables, wholegrains and olive oils and nuts. While it has been acknowledged that more research needs to be carried out to further examine the cause and effect between diet and T2D, it is reasonable to suggest that one may find these dietary changes beneficial in helping to reduce the risk of T2D and other areas of health, possibly helping to indirectly reduce risk of T2D. It is also reasonable to suggest that a change in diet may bring beneficial changes once diagnosis has been given.
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