Although they are a particularly rich source of these substances, many of which have been found have anti-cancer effects in animal studies, [64-71] wholegrain foods seem to afford protection that has not been found by any of these individual, isolated ingredients. Consequently, ensuring adequate wholegrain intake from a variety of sources would be far more protective than simply looking to identify specific “nutritional magic bullets” so to speak. 
There are several theories on why whole grains may protect against cancer, other than being rich sources of these beneficial substances. Because of their complex nature, it has been suggested that there may be many potential mechanisms responsible for the protective properties of whole grain foods, including their ability to bind carcinogens, thus limiting their absorption or contact with the intestinal lining. 
One prominent theory to explain their protective effect on colon cancer is due to their fibre content. High fibre foods tend to speed the passage of stools through the colon, thus decreasing the amount of time that any potentially carcinogenic substances would have to be exposed to the colon wall. This decrease in transit is especially true for insoluble fibre, such as that found in wheat bran. 
Another possible mechanism may be the effect that fermented fibre has on bile acids Fermented fibres tend to increase the acidity of the colon which inhibits primary bile acids from converting to secondary bile acids. This is thought to be significant because these secondary bile acids may promote cell proliferation, which would enhance the ability for mutations to occur and for tumour cells to replicate; thus allowing cancers to spread. Furthermore, a particular product of fermented fibre is a short chain fatty acid called butyrate, which has been found to protect against colon cancer in laboratory studies. 
An inverse association has been found between fibre intake, particularly from cereal/ grain foods, and the risk of colon cancer in population studies. One of the most evident associations was found in the European Prospective Investigation into Cancer and Nutrition (EPIC), which examined the incidence of bowel cancer among half a million Europeans, and found fibre, particularly cereal fibre to be protective. 
The Wheat Bran Fiber Study however found no difference in the reoccurrence of colon polyps in 1,429 men who were given either a high wheat bran fiber cereal supplement (13.5 grams of fiber in 2/3 cup cereal per day) or a low wheat bran fiber cereal supplement (2 grams of fiber in 2/3 cup cereal per day) for 3 years. 
There have been several criticisms of this study however. The first is that it examined colon polyps, not malignant cancer. Polyps may or may not become cancerous, but this study did not prove that wheat bran could not prevent cancer altogether. The second problem was that it involved men who were already at a high risk, as they had already been found to have colon polyps. Consequently, there may have still been some abnormal tissue remaining in their colon to begin with. Therefore, it does not prove that a more protective effect may have been found among people with healthy colon tissue to begin with. Thirdly, 13.5 grams of fiber may not have necessarily been adequate, as the results from population studies such as EPIC have found fibre intakes in excess of 30g/day to offer the most protection. Furthermore, the men in this trial were given “cups” of wheat bran. Presumably, this meant that the bran was finely ground; however finely ground wheat bran does not have the same stool-bulking effect that course wheat bran does, even at the same dosage,  which suggests that the particle size of the whole grain is an important factor in determining the physiologic effect.  Another problem was that it lasted only 3 years, whereas the protective effect seen in population studies has been consumption of high fibre diets over many years, between 6-12. Lastly, this study only examined one specific food (wheat bran) as apposed to a variety of fibrous foods. Although wheat bran has been shown to be the most protective fibre source in animal studies  the anti-cancer effect of whole grains may come from components other than that found solely in wheat bran.
Whole grains are among the foods most proven to decrease the risk of cardiovascular disease according to epidemiological investigations. Several large cohort studies have found that intakes of approximately 3 serves per day or more have a significantly protective effect.
Results from the Adventist Health Study revealed that among the 31 thousand adults involved, the risk of non fatal and fatal heart disease was 44% and 11% lower respectively among those who usually consumed whole wheat bread.  Similarly, every 10-g increase in cereal fiber per day decreased the risk of heart attacks in the U.S. Male health professionals study which followed 43,757 older men for 6 years.
The Iowa Womens Health Study followed 34,492 postmenopausal women for 9 years after which time 438 had died from heart disease. Consuming 1.2 servings of wholegrain food per day was associated with a 29% reduced risk whilst 1.9 servings decreased the risk by 36%.  An inverse association between wholegrain intake and heart disease was also found in the Nurses Health Study which recorded 761 cases of CHD throughout the 10 years that 75,521 women were studied. The results demonstrated a 25% reduction in CHD risk among women who consumed 3 wholegrain servings per day.
After following 16,933 men and 16,915 women for over 11 years, The Norwegian County Study recorded a 25% reduction in the risk of heart disease among those who consumed 9 slices (5.4 servings) of wholegrain bread per day compared with those who consumed only 1 or less sliced pre day. 
Compared with those who rarely or never consumed whole-grain cereal, men whose intake was greater than or equal to 1 serving per day, had a 17% lower risk of death from all causes and a 20% lower risk of death from Cardiovascular Disease (Both CHD and Stroke) in a 5 and a half year long cohort study of 86,190 US male physicians aged between 40 – 84 years.  In fact, wholegrain intake has also been found to decrease the risk of ischemic stroke in several population-based studies.
Among the 15, 792 adults followed for 11 years in the Atherosclerosis Risk in Communities (ARIC) cohort, those whose daily wholegrain intake was 1.5 servings had an 11%, 27% and 13% lower risk of ischemic stroke, coronary artery disease and all cause mortality respectively, whilst 3 serves per day decreased the risk by 28%, 25% and 23% respectively.  In the Nurses Health Study, an average daily intake of 3 servings of whole grains reduced the risk of ischemic stroke by 31% among the 75,521 US women aged 38 to 63 years studied for 12 years.  In a similar cohort of over 71,768 women, compared with the typical “Western” diet high in red and processed meats, refined grains and desserts, diets characterized by higher intakes of fruits, vegetables, legumes, fish, and whole grains reduced the risk of all strokes by 22% and ischemic strokes by 26%.
A daily cereal fibre intake of 6.3g or more decreased the risk of cardiovascular disease by 21% in an 8-year long cohort that studied 3,588 American men and women aged 65 years.  The authors also noted that
“… higher cereal fiber intake was associated with lower risk of total stroke and ischemic stroke and a trend toward lower risk of ischemic heart disease death .”
It is not entirely certain how wholegrain foods exert their protective effect on the cardiovascular system, however there are several theories which include the fact that they are a rich source of beneficial nutrients and non-nutrient substances such as Folate, tocotrienols (a form of Vitamin E), Magnesium, Potassium, Selenium, and certain phenolic phytochemicals, all of which are associated with a decreased risk of cardiovascular disease. 
Whole grains such as wheat bran also possess significant antioxidant activity, due to their considerable phenolic content. In the colon, bacterial enzymes release these phenolic acids thus allowing them to be absorbed, offering antioxidant protection.  Plant sterols such as beta-sitosterol are another component of whole grains, which are known to be able to lower serum cholesterol levels by inhibiting cholesterol absorption in the gut. [93,94]
Oats in particular, can improve gastro-intestinal function, improve glucose metabolism,  and are rich in a substance called beta-glucan, which reduces serum cholesterol levels.  Consumption of barley was also found to reduce total cholesterol, LDL-Cholesterol and triglyceride levels as well as increase HLD “good” cholesterol levels, in a recent trial conducted by researchers at the United States Department of Agriculture. 
Furthermore, whole grains are especially high in dietary fibre, resistant starches and oligosaccharides, which are fermented by bacteria in the gut to produce short chain fatty acids which are also known to lower cholesterol synthesis in the liver,  thus improving serum cholesterol levels. 
Other than their antioxidant and hypocholesterolaemic characteristics, wholegrain foods provide a good source of both macronutrients (such as carbohydrate and protein) and micronutrients (vitamins and minerals) with a relatively low Glycemic Index (GI) and a low Glycaemic Load ( which is [the GI of a food multiplied by the amount of carbohydrate] ÷ 100), thus improving insulin function. Diets with a low GL are associated with a reduction in the risk of heart disease and diabetes. 
In 2000, researchers from the University of Minnesota published the results of the Iowa Women's Health Study; a prospective cohort which studied the diets of nearly 36 thousand women between 55 and 69 years old, all of whom were initially free of diabetes. Throughout a 6 year follow up, 1,141 of these women had been diagnosed with diabetes. After studying their dietary intakes, it was found that wholegrain intake was inversely related to the risk of diabetes; that is, the greater the wholegrain intake, the lower the risk. There were 21% fewer cases of diabetes among those who consumed the most wholegrain foods (3 serves or more per day) compared to those who consumed the least. This study also found that total dietary fibre as well as dietary magnesium had a protective association. 
In the same year, researchers at the Harvard Medical School published the results of another cohort study which followed 75,521 women for 10 years, after which time 1,879 cases of diabetes were reported. Once again, wholegrain consumption decreased the risk of diabetes. Compared to those who had the least wholegrain intake, a higher intake reduced the risk by 38%. This study also found that a high intake of refined-grain cereals increased the risk by 31%. Moreover, those who had the highest ratio of refined grain to wholegrain intake had a 57% greater risk. These associations were particularly strong among overweight women. This study found no protective association between total fibre, magnesium or vitamin E intake. 
Similar results were obtained in a more recently published cohort study which followed both men and women for 10 years, after which time a 35% reduction in the risk of diabetes was observed among those with a higher wholegrain intake. 
A higher whole-grain intake was associated with a 42% reduced risk of diabetes in the Health Professionals follow-up study, which had followed almost 43 thousand men, 1,197 of whom were diagnosed as diabetic throughout the 12 years they were studied. 
Insulin is a hormone produced by the pancrease which allows glucose in the blood to be taken up by the bodies' cells and used for energy, as well as preventing blood glucose levels from rising too high. The most common form of diabetes, accounting for approximately 85% of all cases,  is Type II, also known as “late” or “adult onset diabetes”. In Type II diabetes, the pancreas produces insulin, however the insulin does not function properly. This leads to excessively high levels of glucose in the blood, which, over a long period of time, can lead to a multitude of serious health problems such as an increase in the risk of heart disease, stroke, glaucoma & blindness, as well as amputations  and even a reduced lifespan.  The risk increases progressively with age, in fact it is thought that in excess of 20% of the population aged over 60 have type II diabetes .
When we eat, depending on what we eat, our blood glucose levels will rise over a period of 1-2 hours, before falling again back to normal levels. In diabetics, glucose levels are abnormally high to begin with (fasting levels), rise way too far upon food consumption, and take longer than normal to return to fasting levels again. 
High fasting glucose levels are often an indicator of diabetes, however a more accurate diagnosis can be made by monitoring the rise of glucose after a high glucose meal/drink, and then monitoring how long it takes to return to fasting levels; a test known as a “ glucose tolerance test ”.
If we consumed a food high in pure glucose, our blood glucose levels would rise very sharply, peak, and then plummet very rapidly. The rise and fall of blood glucose levels that occurs after a specific reference glucose intake, was given a score of 100 on the “glycaemic index”.  The glycaemic index values of other foods, were then given a number that correspond to how similar their effect is on blood glucose by comparison with this reference intake. 
For example, certain foods such as white bread, potatoes and a lot of “junk foods” cause our blood insulin to rise sharply, similar to the way in which the glucose reference value does, because once they are in our gut, we absorb their sugars very quickly. These foods are said to have a high Glycaemic Index, or high “GI”. 
Other foods such as nuts, legumes and oats, release their energy very slowly, so that our blood glucose rise slowly, and fall slowly. These foods are said to have a low GI. Foods with a GI value of about 70 or higher, are generally considered high. Those with a GI of around 40 or lower are generally considered low, whilst those in between are considered moderate. More importantly however is the GI of the entire meal. For example, if you combined a high GI food such as white rice, with a low GI foods such as legumes, the GI of the entire meal would be somewhere in between.
Additionally, another important issue is the “Glycaemic Load” (GL), which is an indicator of glucose response or insulin demand that is induced by total carbohydrate intake. It is calculated by multiplying the GI of a food/meal, by the total number of carbohydrates, and then dividing that number by 100. 
The problem with high GI foods is not only that they provide only very short lasting energy followed by rapid declines, but they put a lot of strain so to speak, on our insulin. When our glucose levels “spike”, or rise very rapidly, our insulin has to work extra hard to try to modulate blood levels. If this happens constantly, it is thought that our insulin can start to become resistant. This decline in the ability of insulin to function properly may lead to type II diabetes.
One of the most prominent causes of insulin resistance is having excess body fat, which is why obesity is the most serious risk factor for developing diabetes.  Furthermore, obese type II diabetics who lose weight and maintain a healthy body weight can generally gain far better control over their condition, and expect a decrease in the risk of many of the more severe complications associated with the disease. 
Other than obesity, is the most strongly correlated risk factor for developing insulin resistance is a diet with a relatively high GI and more importantly, a high GL.  The importance of the GL is the ability to obtain adequate carbohydrate without having a high GI. For example, foods such as lean meat may have a low GI, but we need adequate sources of carbohydrate as well, which meat does not provide. Refined grain foods such as white bread are a good source of carbohydrate; however they have a very high GI. Foods such as whole grains however also provide a good source of carbohydrate, but do so without the high GI; thus they have a favourable GL. Therefore, obtaining our primary source of carbohydrates through whole grains (which provide excellent sources of other nutrients as well) ensures adequate intake without the high GI. This is important because low GL diets are associated with a decreased risk of heart disease and type II diabetes, as a low GL diet provides enough energy without causing insulin resistance. Several important studies have confirmed that whole-grain intake may prevent diabetes by protection against insulin resistance. 
In a randomized crossover trial conducted by the Harvard Medical School in 2002, insulin levels were significantly improved in overweight insulin resistant adults who consumed a wholegrain rich diet compared to those receiving a refined-grain rich diet, even though both contained the same percentage of fat and carbohydrate. 
These findings were supported by the results of the Framingham Offspring study, which revealed that wholegrain intake was inversely associated with fasting insulin levels among 2, 941 people. Furthermore, wholegrain intake was also inversely associated with body mass index (BMI), waist-to-hip ratio, Total Cholesterol (TC) and LDL “bad” cholesterol – all factors known to increase the risk of diabetes, as well as heart disease. These results shed further insight into the mechanisms by which wholegrain intake protects against diabetes. 
In 1997, researchers from the Harvard School of Public Health published the results of a prospective study which followed 42,759 men aged 40-75 years who were free of diabetes of cardiovascular disease to begin with. During 6-years of follow-up, 523 of these men had been diagnosed with type II diabetes. An analysis of their diets revealed that even after multivariate adjustments, compared to lower GI diets, those with the highest GI diets had a 37% greater risk of diabetes. Moreover, those with the greatest cereal fibre intake had 30% reduction in risk. 
The same researchers published the results of another prospective cohort study which followed 65, 173 US women, 915 of which were diagnosed with diabetes throughout the 6 year follow up. Once again, high GI diets increased the risk by 37%. Moreover, high GL diets increased the risk by 47%, whilst higher cereal fibre intakes decreased the risk by 28%. Most importantly, the combination of a high GL and a low cereal fiber intake increased the risk by a massive 150%.  More recently, the results of a cohort study following 91, 249 younger women for 8 years found a 59% greater risk for developing diabetes among those with a higher GI diet, whereas higher intakes of cereal fibre decreased the risk by 36%. 
The most recent study to support these findings found a 32% greater risk among the 31,641 men and women followed for 4 years who had the highest GI diets, despite greater intakes of total carbohydrates having a decreased risk. Higher intakes of white bread and other starchy foods however increased the risk by 37% and 47% respectively. 
It is commonly thought that foods containing “complex” carbohydrates or starches, are good and produce slow burning energy, whilst food with “simple carbohydrates” or sugars, have the opposite effect and increase the risk of diabetes. This is often not the case, as we can see in the case of foods such refined grains which have a very high GI, despite being high in complex carbohydrates.  Conversely, some foods high in sugars such as fruit and fruit juices, actually have a relatively low GI as they contain soluble fibres that delay the release of sugar into the blood.
This is a table which compares the GI and GL values of a variety of common Australian foods, particularly those which may cause confusion due their relatively high carbohydrate levels. Full list of the International Table of Glycemic Index and Glycemic Load Values can be downloaded here: http://www.ajcn.org/cgi/content/full/76/1/5/T1