Abstract: Associations between food and beverage groups and the risk of diet-related chronic disease (DRCD) have been the subject of intensive research in preventive nutrition. Pooled/meta-analyses and systematic reviews (PMASRs) aim to better characterize these associations. To date, however, there has been no attempt to synthesize all PMASRs that have assessed the relationship between food and beverage groups and DRCDs. The objectives of this review were to aggregate PMASRs to obtain an overview of the associations between food and beverage groups (n = 17) and DRCDs (n = 10) and to establish new directions for future research needs. The present review of 304 PMASRs published between 1950 and 2013 confirmed that plant food groups are more protective than animal food groups against DRCDs. Within plant food groups, grain products are more protective than fruits and vegetables. Among animal food groups, dairy/milk products have a neutral effect on the risk of DRCDs, while red/processed meats tend to increase the risk. Among beverages, tea was the most protective and soft drinks the least protective against DRCDs. For two of the DRCDs examined, sarcopenia and kidney disease, no PMASR was found. Overweight/obesity, type 2 diabetes, and various types of cardiovascular disease and cancer accounted for 289 of the PMASRs. There is a crucial need to further study the associations between food and beverage groups and mental health, skeletal health, digestive diseases, liver diseases, kidney diseases, obesity, and type-2 diabetes.
Alex’s Notes: It is estimated that about 26% of deaths in the U.S. are the result of dietary factors. That is a huge number. The nutrition-transition from a traditional diet towards the modern Western diet composed of industrialized, refined, and energy-dense foods has no doubt played a role in the burden of chronic disease. However, although it seems that a new observational study is published daily examining the relationship between dietary patterns and disease, there is an overwhelming focus on individual foods and food groups within the dietary patterns. Many researchers attempt to attribute the benefits or detriments of certain diets (i.e. Mediterranean diet; Western diet) to these food groups without appreciating that the dietary patterns are more of a lifestyle than a diet, per se.
Nonetheless, it is definitely worth asking which food groups appear to be associated with certain health outcomes. The current study, and lead article in Nutrition Reviews, attempts to pool the results of all published meta-analyses and systemic reviews (PMASRs) from 1950 to 2013 in order assess such associations. Since “diet-related chronic diseases” (DRCDs) can encompass a range of health parameters, the focus was on those that may occur following a chronically unbalanced diet that encompass the main physiological functions of the organism. Similarly, food/beverage groups were selected on the basis of their study frequency and representativeness within the Western diet. Both can be seen below.
A total of 304 PMASRs were analyzed for the 170 possible associations between the DRCDs and foods and beverages. I don’t want to go through every single item as that would take forever, so instead I am going to summarize some more general findings.
First, however, there are some honorable mentions in the crowd. For instance, all the studies looking at tea and/or coffee either found a protective effect or no effect, whereas those looking at sweetened beverages found either no effect or a deleterious effect. More impressively, every single plant-food category with the exception of refined cereals demonstrated either positive or neutral effects on health. For animal products, only dairy and fish were either protective or neutral.
Overall trends by diet-related chronic diseases and food groups and beverages
The DRCDs mostly commonly studied in association with the 17 food groups and beverages via PMASRs were cancers (191 different references), followed by CVD (61 references), type-2 diabetes (29 references), overweight/obesity (20 references), mental health (9 references), skeletal health (6 references), digestive diseases (2 references), and liver disease (1 references). As we can see, there is a lot of lacking information on incredibly important issues such as sarcopenia, digestive problems, liver and kidney health, skeletal health, and mental health. Even obesity has very few references in comparison to cancer and CVD despite the world obsessing about an apparent obesity epidemic.
When looking at the DRCDs together and the top four in isolation, the researchers created radar plots that summarize the number of studies for each food group showing a protective, neutral, or deleterious effect. Since I can’t copy their images, I summarized the notable information into the table below. I don’t list every food category because not all show positive or negative effects. Thus, foods not mentioned can be considered neutral. Also, food categories on the same line separated by a slash are relatively equal in terms of effect.
Nuts & seeds
Fruits & vegetables
Fish / Coffee / Wine / Dairy
Red & processed meat
|Overweight / Obesity||
Nuts & seeds
Whole-grains cereals / fruits & vegetables
Coffee / Nuts & seeds
Sweetened beverages / Eggs
Red & processed meats
Fruits / Whole-grain cereals
Fruits & vegetables / Dairy / Fish / Tea
Legumes / Wine / Vegetables
|Red & processed meats|
Fruits / Fruits & vegetables
Vegetables / Whole-grain cereals
Coffee / Fish
|Red & processed meats|
When looking through the above table, something that should become blatantly obvious is the protective nature of plant-based foods. This is further supported by the fact that 94% of the references for plant-based foods had either a protective or neutral effect, and I would bet that a good portion of the negative 6% is owed to refined cereals. In comparison, 27% of animal-based foods showed deleterious effects. I do find it odd that the researchers separate whole and refined grains into two separate categories but leave red and processed meats as a single group. Regardless, it is worth mentioning again that dairy and fish had no deleterious studies.
There is one huge limitation to this study and to all meta-analyses and systemic reviews in general, and that is the fundamental bias of publishing research. This study relied entirely on the PMASRs to draw its results, just as the PMASRs relied entirely on the published literature to synthesize their conclusions. However, it has been suggested that positive study results are published more frequently than negative results, and that there is a strong association between the significance of outcomes and their publication. Thus, in the current study, a larger number of studies demonstrating a protective effect does not necessarily mean that a food is more protective than another; it only reflects that more positive studies have been published in this area. The lack of published research could be due to numerous factors, not the least of which are the current scientific trends.
To illustrate the above point, did you notice a trend in the most protective foods? Fish, whole-grain cereals, nuts & seeds, legumes, and fruits sound a lot like the dietary pattern of the Mediterranean diet and the DASH diet. Whole-grain cereals are a food category in general that most governmental agencies promote consumption of in large quantities. While I’m not saying that the evidence is wrong, nor am I trying to discount the current study results, I do want us to remain critical thinkers and appreciate that every published study has a bias.
Another limitation is that there are obviously more than the ten researched DRCDs, and that the study relied only on PMASRs. For example, only seven meta-analyses and systemic reviews were found for skeletal health despite there being countless isolated studies on the subject, which may reflect the greater amount of heterogeneity (differences) between the studies that prevents or makes more difficult their synthesis into tertiary research. Similarly, no research was found for sarcopenia or renal diseases, with very little research for digestive, liver, and mental health. Regarding sarcopenia, it has been shown that higher protein intake is protective, especially when coupled with resistance training, and if more research was published in this area it would not be far-fetched to assume that animal-based products would excel in protection due to their superior protein content and quality.
Bottom line and other random information
Overall, this study does provide strong evidence in favor of consuming a plant-based diet. Additionally, tea, coffee, fish, and dairy products would be prudent additions to the daily menu. I would be hesitant to eschew red meat, however, simply because it is a very nutritious food and the current study failed to separate it from processed meats. It would be asinine to compare a hot dog to a pastured bison steak.
This is more of a list of some of the random pieces of information this study found.
- With the exception of esophageal cancer, tea is globally neutral or protective, most notably against CVD. The deleterious effect on throat cancer is from hot tea.
- Coffee is especially protective against type-2 diabetes, some mental illnesses, and liver disease.
- Wine consumption is associated with a CVD risk following a “J” shaped curve, with protective effects seen for moderate consumption (150-270 mL/day or about 1-4 drinks).
- Full-fat milk appears to be slightly less neutral than low-fat or skim milk towards DRCDs.
- With the exception of fermented soy foods, which increase the risk of stomach cancer, legumes may be particularly protective against cancers.
- High egg consumption tended to be associated with a slightly higher risk of DRCDs, but the limited number of studies collected did not allow definitive conclusions to be reached. With regard to CVD, the harmful effect of eggs might be due to the generally negative dietary patterns often associated with high egg intake.