The Scientific Report of the 2015 Dietary Guidelines Advisory Committee (DGAC) which was submitted to the US government in February 2015 provided a comprehensive review of the impact of diet on health. While its relationship on heart attacks and strokes have been summarised in my earlier article, the review also reviewed the evidence on diets and its relationship with diabetes, cancers, birth defects, brain and bones, all of which have been summarised below.

Unhealthy diets increase risk of developing diabetes

The DGAC was of the view that there is moderate evidence that indicates that adherence to a “Healthy Diet” can reduce the risk of developing type 2 diabetes (diabetes that is not dependent on the use of insulin injections). This conclusion was based on the pooled data of 15 studies which indicated that there was a 21% reduction in the risk of developing type 2 diabetes with adherence to a “Healthy Diet” with high consumption of whole grains, vegetables, and fruit.

The data also showed that for those unhealthy diets characterised by high consumption of red or processed meats, high-fat dairy, refined grains, and sweets, there was a 44 percent increased risk of developing type 2 diabetes. However, this evidence is not applicable to children.

Diet and the risk of cancer

The American Institute for Cancer Research and World Cancer Research Fund (AICR/WCRF) updated their “Report on strong evidence on diet, nutrition, physical activity, and cancer prevention” in 2014.  The key findings for those dietary factors which have convincing evidence of increasing the risk of certain cancer are summarised.

For large intestine (Colon) cancer, there is moderate evidence that a “Healthy Diet” is associated with a lower incidence of large intestine cancer compared to unhealthy diets. High consumption of red/processed meats, french fries/potatoes, and sources of sugars are associated with a higher risk of large intestine cancer.

For liver cancer, consumption of foods contaminated by Aspergillus fungi which produces the mycotoxins, aflatoxins, is associated with a higher risk of liver cancer. The fungi can be found in peanuts, peanut butter, tree nuts (such as pecans), corn, wheat and oil seeds (such as cottonseed).

Consumption of food contaminated by aflatoxin can be avoided by buying nuts and nut butters from reputable brands and not consume any nuts which appear mouldy, discoloured or shrivelled.

For breast cancer, there is moderate evidence that diets rich in vegetables, fruit, and whole grains, and lower in animal products and refined carbohydrate, are associated with decreased risk of breast cancer in post-menopausal women. For pre-menopausal women the data, although suggestive, is not definitive due to the limited studies available.

For lung cancer, there is evidence that excessive consumption of beta-carotene is associated with an increased risk of lung cancer. There is currently no evidence that there is any association between dietary factors and prostate cancer.

Although mild alcohol consumption has been considered as being beneficial for prevention of heart artery disease, there is convincing evidence that alcohol consumption is associated with an increased risk of mouth, throat, oesophagus, colon and breast cancer.

Diet and Congenital Anomalies

It is an established fact that adequate intake of folate, a Vitamin B, is essential in the prevention of birth defects affecting the brain, nervous system and the spine such as anencephaly and spina bifida.

The current recommendation is that females of reproductive age should take 400 to 800 micrograms of folic acid daily in addition to a healthy diet. For women who are at high risk of birth defects, 4 mg of folate daily should be considered.

Diet and brain

There have been studies on the effects of omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the brain as DHA is a major component of the grey matter and nerve junctions of the brain, and the light detecting cells of the retina in the eyes.

In addition, EPA is now being considered as having anti-depressive effects and as a result of the strong medical evidence, both EPA and DHA are now considered as complementary treatment for major depressive disorder by the American Psychiatric Association.

However, there is currently no strong evidence to determine whether any particular diet can have an impact on brain or psychological disorders.

Pregnant mothers may be interested to know that the 2010 DGAC concluded that there was evidence supporting an association between maternal dietary intakes of omega-3 fatty acids from seafood and improved cognitive ability in infants.

Diet and bones

It is not uncommon to see advertisements on mass media urging you to take more calcium to strengthen your bones.  Yet the 2015 DGAC concluded that “Only limited evidence is available on the relationships between dietary patterns and bone health outcomes in adults and other age groups. Although there is strong evidence on the roles of vitamin D and calcium in bone health across the age spectrum, further research is needed on dietary patterns that are most beneficial”.

In addition, in recent years, there has been evidence showing an association between calcium supplementation and an increased risk of heart attacks and stroke. Earlier studies have also shown an increased incidence of kidney stones with calcium supplementation.

Summary

It is not surprising that the DGAC did not make a specific recommendation on cholesterol as most of the cholesterol in our body is produced by our liver from various food components, including saturated fat. Hence, current emphasis on diet is focussed on reduction of saturated fats. The current evidence has shown that a healthy diet can reduce the incidence of type 2 diabetes, high cholesterol, strokes, heart attacks and death.

While taking supplements is very common, one must be careful that certain supplements may cause more harm than benefit, and sadly the effects may be irreversible. For example, calcium supplementation has not only been associated with permanent and irreversible calcium deposits in the arteries of the heart and brain but also associated with an increase in the risk of stroke and heart attacks.

Hence, before you tuck into your favourite food or swallow your daily dose of supplements, do remember that what you eat can have a significant impact on your health.

** This is the second of two parts on diet and health.