Most of us want nutrition to be simple, where foods are either inherently ‘good’ or ‘bad’. While it’s more nuanced than that, there are some concepts to keep in mind when thinking about whether a food is healthy or not.
Tabloids feed off of the idea that foods are ‘good’ or ‘bad’, or ‘healthy’ or ‘unhealthy’. One week, they say that eggs will kill you; the next week, eggs are superfoods! The really unfortunate thing about this is that it causes the lay person to lose trust in nutrition science, where people often lament that the scientists just can’t make their minds up. The reality is that there are often reasons why findings are—on the face of it—in complete contrast to each other. But that wouldn’t really sell, would it?
Cartoon by Jim Borgman, first published by the Cincinnati Inquirer and King Features Syndicate 1997 Apr 27; Forum section: 1 and reprinted in the New York Times, 27 April 1997, E4.
Because our diets are made up of many different foods, it’s difficult to identify how any particular food relates to our health. While difficult on the surface, with a little bit of investigation, it becomes much easier to get to the bottom of this conundrum. You see, whether a food is healthy or not depends largely on the following three factors:
The independent health effects of that food (and its components)
How much of the food is eaten (the dose)
What you’re not eating (the replacement effect)
In other words, context is crucial. This is why Prof. Christopher Gardner—a Professor of Nutrition from Stanford University—answers the question of whether a food is healthy or not with: ‘it depends'. Let’s bring these concepts to life with some examples.
Independent Health Effects
Foods contain nutrients or compounds that have independent health benefits. Take fruits and vegetables, for example. They’re often high in fibre, they contain essential vitamins and minerals, and they contain a class of nutrients only found in plant foods, called polyphenols. Each of these components yield health effects, irrespective of what else makes up the diet.
Fibre (particularly soluble fibre) lowers risk factors of heart disease and is the favourite food of the microbes that live in our gut microbiome (1). Certain polyphenols like flavonols have been demonstrated to lower blood pressure, LDL cholesterol, and markers of diabetes risk (2). These effects are part of the reason why fruit and vegetable consumption is associated with a reduced risk of cardiovascular disease, diabetes, cancer, and death (3).
Components of fruits and vegetables that may have independent health benefits.
But what if I eat two blueberries and one small floret of broccoli per day? Can I expect the benefits of fruit and vegetables that I just laid out above? Of course not; how much you eat is a crucial moderating factor on any food’s health effect. Red meat is a great example of this. It is pretty well established that the risk of heart disease is increased when intakes of processed or unprocessed red meat are ≥50 or 100 grams per day (g/d), respectively (4). But you’ll see headlines that high red meat consumption is not associated with heart disease. How can this be?
It could be down to the dose. In a study from Japan, where high red meat eaters showed no increased risk of heart disease, their ‘high’ intakes weren’t high by our Western standards. In fact, we would class their intakes (i.e., median for men = ~78 g/d; median for women = ~60 g/d) as moderate, not high (5). So does this study suggest that red meat is not a concern for heart health? No—it actually agrees with the wider body of evidence that moderate or low intakes are probably not an issue, and highlights the importance of the dose.
Some foods and nutrients have a dose-response curve like this, where low intakes don’t have much effect on a particular outcome, and very high intakes don’t further increase the likelihood of that outcome. Rather, there is a specific range of intake where effects are seen. Other foods and nutrients have different types of non-linear relationships, or even linear (straight-line) relationships. Source: Toxicology Education Foundation.
The Replacement Effect
You must factor in the counterfactual scenario: if you’re not eating a particular food, what are you eating in its place? Researchers from Harvard demonstrated this concept beautifully in a 2015 paper, where they compared the health effect of swapping saturated fats (found in butter, red meat, coconut/palm oils, etc.) for a number of different foods and nutrients.
As you can see in the graph below, when 5% of energy (kcal) from saturated fats were swapped with the same amount of energy from refined starches and sugars (e.g., white bread, fizzy drinks), there was no cardiovascular benefit observed for reducing saturated fats. But when saturated fats were swapped for whole grains (e.g., oats, brown pasta), the risk of CVD was reduced by nearly 10%, and when swapped for mono- and polyunsaturated fats (found in nuts/seeds, avocados, vegetables oils, etc.), the risk was reduced by 15% and 25%, respectively (6).
Figure 1. Replacing 5% of energy (kcal) from saturated fats (SFA) with different types of foods/nutrients shows marked differences in the risk of cardiovascular disease (CVD) (Figure adapted from 6).
This isn’t meant to be an exhaustive list of the factors influencing whether a food is healthy or not. And of course, certain foods are going to be more often than not either ‘healthy’ or ‘unhealthy’, no matter the context. But understanding these three concepts allows us to go beyond the simple narrative that all foods must be either ‘healthy’ or ‘unhealthy’, and take contextual factors into account when trying to understand the health effects of food. So next time you see a headline about a food causing X or Y, ask yourself:
What are the independent health effects of the food?
How much of it was eaten?
What was it compared to (or what did it replace in the diet)?
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That’s it for this week, we hope you enjoyed it. Have a great weekend and see you here next week!
Patrick Elliott, BSc, MPH
Health and Nutrition Science Communication Officer at Training121
Health Disclaimer: this article is for informational and educational purposes only, and is not a substitute for professional advice. For health advice, speak to a physician or other qualified health-care professional, and for nutrition advice, speak to a qualified nutrition professional (e.g., registered dietitian). The use of information on this site is solely at your own risk.
(1) Ghavami A, Ziaei R, Talebi S, et al. Soluble Fiber Supplementation and Serum Lipid Profile: A Systematic Review and Dose-Response Meta-Analysis of Randomized Controlled Trials. Adv Nutr. 2023;14(3):465–74. Available at: https://www.sciencedirect.com/science/article/pii/S2161831323000054
(2) Raman G, Avendano EE, Chen S, et al. Dietary intakes of flavan-3-ols and cardiometabolic health: systematic review and meta-analysis of randomized trials and prospective cohort studies. Am J Clin Nutr. 2019;110(5):1067–78. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6821550/
(3) Aune D, Giovannucci E, Boffetta P, et al. Fruit and vegetable intake and the risk of cardiovascular disease, total cancer and all-cause mortality-a systematic review and dose-response meta-analysis of prospective studies. Int J Epidemiol. 2017;46(3):1029–56. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5837313/
(4) Shi W, Huang X, Schooling CM, Zhao JV. Red meat consumption, cardiovascular diseases, and diabetes: a systematic review and meta-analysis. Eur Heart J. 2023;44(28):2626–35. Available at: https://academic.oup.com/eurheartj/article/44/28/2626/7188739
(5) Nagao M, Iso H, Yamagishi K, Date C, Tamakoshi A. Meat consumption in relation to mortality from cardiovascular disease among Japanese men and women. Eur J Clin Nutr. 2012;66(6):687–93. Available at: https://www.nature.com/articles/ejcn20126
(6) Li Y, Hruby A, Bernstein AM, et al. Saturated Fats Compared With Unsaturated Fats and Sources of Carbohydrates in Relation to Risk of Coronary Heart Disease: A Prospective Cohort Study. J Am Coll Cardiol. 2015;66(14):1538–48. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4593072/
Polyphenols: These are a group of phytochemicals—or plant nutrients—that act as antioxidants and improve our health. Health benefits from polyphenol intake include reductions in LDL cholesterol, improved cognition, and other metabolic improvements.
Soluble fibre: This is a type of fibre that is soluble in water, which means that it can dissolve in water. Soluble fibre offers a lot of health benefits; it reduces cholesterol absorption in the gastrointestinal tract which decreases LDL cholesterol; it delays gastric emptying and can reduce blood sugar levels; and it feeds the microbes in our gut microbiome which in turn produce many healthful compounds for us. Soluble fibre may also reduce blood pressure and keep us feeling fuller for longer.
Gut microbiome: This is the community of microbes (i.e., bacteria, fungi, viruses, archaea) that live in our body (mostly in our large intestine, or colon). These microbes produce a range of healthful compounds for us, such as short-chain fatty acids (e.g., acetate, butyrate). Their favourite food is soluble fibre.
Median: The median value of something is the value bang smack in the middle of all other values, which divides the numbers so that an equal amount are below, and an equal amount are above. In other words, it is the middle value of a range of values.