Prioritizing plant-based foods in our diet can benefit climate and health, but Forbes article missed some key details

On 28 August 2024, Forbes published an article by climate economist Simi Thambi**, with a number of claims about the climate and health impacts of adopting a more plant-based ‘flexitarian diet’, such as the Planetary Health Diet. Thambi claims that these diets focus on reducing overconsumption of certain foods – such as animal-based proteins – and replacing them with plant-based options, which can improve human health and reduce greenhouse gas emissions. Given the interdisciplinary nature of these claims, the climate team and health team at Science Feedback collaborated* to investigate them below. 

In the Forbes article, Thambi shares findings from peer-reviewed papers published in Nature and Science journals to support several claims about overconsumption and greenhouse gas emissions. Science Feedback decided to contact authors from each paper to verify if their research findings were accurately represented in the Forbes article. We did not receive any responses about the health-related claims; however, several researchers provided us with the comments about the climate-related claims. Their full quotes are included in a later section, but in short, two of the three researchers who responded explained that several of the claims citing their research had some inaccuracies.  

However, these misconstrued details do not necessarily make the central claim about climate and health benefits inaccurate. As such, we will investigate this claim using clarifications we received from researchers and findings from other reputable sources. 

Notes:

*This article was led by Darrik Burns (Climate Editor) in collaboration with Flora Teoh (Health Editor), who researched and wrote about the health-related claims. 

**Science Feedback reached out to Simi Thambi on 13 September 2024 with recommendations that could improve the accuracy of the Forbes article. Thambi responded the same day and explained that the article will be updated with the recommended changes.

Reducing overconsumption of animal-based proteins can decrease dietary greenhouse gas emissions

There is plenty of scientific evidence that our dietary choices impact the environment^[1-3]. As described in a Nature Climate Change paper from 2024 “The food system is responsible for about one-third of global anthropogenic GHG [greenhouse gas] emissions^[2,3] and climate goals become unattainable without efforts to reduce food-related emissions”^[1]. However, a key finding is that not all food choices have the same impact because dietary emissions are much higher for certain food categories than others^[1] (Figure 1). So what dietary choices can positively impact your health and the climate?

A group called the EAT-Lancet Commission sought to answer this question and “convened 37 leading scientists from 16 countries in various disciplines including human health, agriculture, political sciences and environmental sustainability to develop global scientific targets for healthy diets and sustainable food production”, their summary report explains. The result was the Planetary Health Diet, which the EAT-Lancet Commission describes as a “plant-forward diet where whole grains, fruits, vegetables, nuts and legumes comprise a greater proportion of foods consumed”. They explain that while meat and dairy are still important parts of the diet, they make up a much smaller portion of the diet than the food categories listed above (Figure 2). So what impact can someone make by switching to this diet? In short, it depends on one’s current diet. 

The Forbes article is titled “2 Simple Changes To Your Diet Can Help Your Body And The Planet”. However, whether or not these changes would decrease their dietary emissions depends on their current diet. This is because some populations are presently overconsuming high-emission food categories, while others are underconsuming. 

As shown in Figure 1, dietary greenhouse gas emissions vary both by country and by food category^[1]. As researchers explain, “Food consumption and associated emissions differ as a result of disparities in consumer choices guided by social and cultural preferences, wealth and income^[4]”^[1]. As a result, a worldwide adoption of the Planetary Health Diet would lead to different climate impacts across populations, depending on their current consumption.

For example, by adopting this diet, underconsuming populations would have to increase their intake of certain food categories, leading to a rise in their greenhouse gas emissions. Whereas overconsumers – 56.9% of the global population – would reduce their intake, thus lowering their emissions^[1]. However, the net result of these changes would be still be a 17% reduction in global dietary greenhouse gas emissions if nearly all* of the global population adopted the diet^[1]. 

But why is this considered a positive outcome for Earth’s climate? There is robust evidence that human greenhouse gas emissions have driven global temperature increases in recent times^[5] – which we have covered extensively in the past (see links here, here, and here). Therefore, decreasing these emissions through dietary changes would help limit global warming and its negative effects^[6]. 

Note:

*These researchers analyzed the dietary emissions of 139 countries, which account for 95% of the global population.

To gain another perspective about these climate claims, we also reached out to scientists who were not linked to papers cited in the Forbes article. Below is a comment we received from Dr. Harry Aiking, Visiting Fellow at the Vrije Universiteit Amsterdam, who studies sustainable food production and consumption. 

Dr. Aiking explains that the central claim in the article is accurate, but as we explained earlier, certain details were misconstrued. Below, researchers from the studies cited in the Forbes article explain why. 

How do the claims in the Forbes article compare to the cited research?

Based on the evidence above from peer-reviewed research and comments we received from scientists, the main claim in the Forbes article about the Planetary Health Diet reducing emissions is accurate. However, some of the supporting evidence presented in the article was not an accurate summary of the cited research. With better context – such as the details shared earlier by the researchers themselves – the main claim is well-supported. This addresses the first half of the claim, but another question remains: is there sufficient evidence that the Planetary Health Diet is good for your health? To answer this question, we turned to our health-science team at Science Feedback who investigated the evidence below.

Consuming more healthy plant-based food is associated with a lower risk of metabolic disorders

The Forbes article presents the flexitarian diet not only as a climate-friendly diet, but also one that’s beneficial to our health. It contrasts the composition of the flexitarian diet with that of the typical Western diet—the model for this diet being the standard American diet. Specifically, it highlights the reduced consumption of animal-derived protein in the flexitarian diet, claiming this feature improves metabolic health, blood pressure, and reduces the risk of type II diabetes.

We will examine the scientific credibility of these claims by taking a look at the studies cited in the article and other literature on this subject.

How is a healthy diet defined and how does it contrast with the “Western diet”?

The exact ingredients of what constitutes a healthy diet will vary depending on a few factors, such as age, gender, level of physical activity, and geography. However, the basic principles are the same, the World Health Organization explains.

For example, the Dietary Guidelines for Americans (2020 – 2025), issued jointly by the U.S. Departments of Agriculture and of Health and Human Services in December 2020, states that the core elements of a healthy diet include vegetables of all types, whole fruits, whole grain, low-fat dairy, and protein, particularly lean meats and seafood. It emphasizes consumption of nutrient-dense foods and beverages, and staying within calorie limits.

However, this model of a healthy diet is a far cry from what the average American consumes. In 2010, researchers at the U.S. Department of Agriculture reported that the majority of the U.S. population “did not meet recommendations for all of the nutrient-rich food groups, except total grains and meat and beans”^[11]. 

Moreover, the average American tended to overconsume “empty calories” (food with calories but few or no nutrients). The analysis concluded that “nearly the entire U.S. population consumes a diet that is not on par with recommendations”.

In a later report related to the Dietary Guidelines for Americans (2020 – 2025), experts noted that this unhealthy trend continues: 

“[W]hat Americans eat has not changed appreciably for the past decade. Throughout their life span, Americans overconsume calories, saturated fats, sodium, added sugars, and alcohol. In addition, they underconsume fruits, vegetables, dairy”.

The World Health Organization also observes this trend in general, finding that people now consume more foods high in energy, fats, added sugars and salt, but not enough fruit, vegetables, and dietary fiber.

Indeed, the typical American diet represents what is broadly termed the “Western diet”, which researchers characterize as “a modern dietary pattern” containing large amounts of processed and refined foods, red and processed meats, added sugars, and saturated and trans fats, but low intakes of fruits, vegetables, whole grains, and nuts^[12]. This dietary pattern can now be found in many countries around the world, not just in the U.S.

With regards to a healthy level of protein consumption, the Recommended Dietary Allowance for protein in the U.S. is 0.8 grams of protein per kilogram of body weight, or 0.36 grams per pound, per day. As this Vox article pointed out, this means the average American man needs 72 grams of protein and the average American woman needs 61.5 grams. 

However, data from the National Health and Nutrition Examination Survey show that the majority are consuming much higher quantities. The average amount of protein consumed by men aged 20 to 69 ranged from 92 to 100 grams. For women in the same age group, this ranged between 62 to 73 grams.

In brief, the evidence demonstrates that excess protein consumption, particularly of animal-based protein, and underconsumption of fruits, vegetables, and whole grains are typical features of the “Western diet” consumed in the U.S. and other parts of the world. What makes this concerning is that this dietary pattern is associated with an increased risk of various diseases, including cardiovascular disease, leading to heart attack and stroke, and type II diabetes^[13].

The flexitarian diet emphasizes plant-based food, which corrects for deficiencies in the “Western diet”

The Forbes article proposed the flexitarian diet as a healthier alternative to the standard Western diet. The Cleveland Clinic defines the flexitarian diet as the following:

“[E]ssentially a flexible alternative to being a vegetarian. So you’re still focusing on fruits, veggies, whole grains, legumes and nuts, but you occasionally still enjoy meat.”

In brief, it emphasizes plant-based foods in the diet, thereby reducing the amount of meat consumed, although it doesn’t strictly exclude meat. This clearly establishes a distinction between the flexitarian diet and the standard Western diet.

But there are some caveats to consider. For example, there is no strict rule about how much meat should be excluded in a flexitarian diet, unlike vegetarian and vegan diets which exclude certain food groups entirely. Therefore, exactly how much more plant-based food and how much less animal-based food a person decides to consume is up to them. 

In other words, one person’s idea of a flexitarian diet may look very different from that of another person. The benefits of a flexitarian diet will likely depend on how much more plant-based food and how much less animal-based protein a person consumes. This means that claims about its benefits need to be contextualized by a common understanding of what a flexitarian diet means in a given discussion.

There’s evidence that consuming more plant-based food is linked to lower metabolic and cardiovascular risk

The Forbes article claimed that “Studies on clinical medicine led by researchers at the University of Cambridge found that high protein intake has adverse effects, such as raising blood insulin levels”. However, it only linked to a single study, a randomized clinical trial examining how dietary protein intake affected the generation of fat tissue^[14]. 

The study found that the level of a certain type of lipid increased after a high-protein meal compared to a control or high-fat meal, suggesting that high protein intake fuels lipogenesis (fat formation). One of the study’s limitations is that just nine individuals were included in the clinical trial, meaning that larger studies are needed before the results can be generalized to the broader population.

While it did report that blood insulin levels rose 30 minutes after a high-protein meal, this rise also occurred in the control group, as shown by the study’s Supplemental Figure 2. Moreover, in both the control and high-protein meal groups, insulin levels declined after the 30-minute peak in similar fashion. 

Therefore the article’s representation of the study’s findings, implying that only high protein intake led to a rise in insulin, is inaccurate. 

Furthermore, the statement’s implication that any rise in blood insulin levels is “adverse” is incorrect. Insulin level naturally rises in response to a rise in blood glucose level, as occurs after a meal. Therefore, a rise in insulin level on its own isn’t necessarily harmful, although insulin resistance, which is typically accompanied by higher than normal insulin levels, is indeed an indicator of diabetes. But the study wasn’t designed to examine insulin resistance, thus it cannot be used to draw conclusions about the relationship between protein intake and diabetes.

The Forbes article also claimed that “Evidence supports the benefits of a flexitarian diet for improving metabolic health and blood pressure, reducing the risk of Type 2 diabetes”. The evidence provided to support this claim is a review of the scientific literature, published in the journal Frontiers in Nutrition^[15].

To examine the relationship between semivegetarian diets – like the flexitarian diet – and markers of metabolic health, the review looked at six studies. While these studies did find associations between a semivegetarian diet and a lower risk of metabolic disorders like type II diabetes, they come with important caveats that weren’t acknowledged in the Forbes article.

Firstly, the way a semivegetarian diet was defined depended on the study. This observation was also highlighted in Table 1 of the review. Notably, one of the six studies, which surveyed adults in India, defined a semivegetarian diet as “those who excluded fish, but still consumed meat and chicken, and eggs”^[16]. 

It’s questionable whether this study provides evidence for the Forbes article’s claim. This is because it’s unclear whether the semivegetarian diet, as defined in that study, is representative of the flexitarian diet as portrayed in the Forbes article. It’s also unclear how well this dietary pattern matches that of the Planetary Health Diet developed by the EAT-Lancet Commission. 

Secondly, three of the six studies were part of the Adventist Health Study^[17-19], a long-term medical research project conducted by scientists at Loma Linda University. The project looks at “links between lifestyle, diet and disease among members of the Seventh-day Adventist church”. This religious group is also noted for encouraging a vegetarian diet and abstinence from alcohol and tobacco.

Another of the six studies examined dietary mineral content and blood pressure in 26 nuns living in enclosed convents in Spain, 12 of whom were omnivorous and 14 of whom were semivegetarians.

In short, these four studies were conducted in populations that are unlikely to be representative of the general population. The study in Spain was also relatively small. While they all provide positive results suggesting that reducing animal-based food consumption reduces the risk of metabolic disorders like diabetes, larger studies conducted in a group representative of the general population are needed to provide stronger evidence for the article’s claim.

We were able to identify such studies, notably one that included more than 200,000 people in the U.S. Individuals included in the study were part of the cohorts in the long-term Nurses’ Health Study and Health Professionals Follow-Up Study. 

The results of the study showed that a diet emphasizing healthy plant foods – as opposed to less healthy plant foods like refined grains and potatoes – were associated with an approximately 20% reduction in diabetes risk^[20]. 

Another study that examined more than 208,000 individuals, also from the same cohorts, observed that a greater amount of healthy plant foods consumed was associated with a lower risk of coronary heart disease^[21].

In summary, the Forbes article’s broader claim that a flexitarian diet – defined as the diet that prioritizes plant-based food over animal-based food – comes with benefits for metabolic health is supported by large-scale studies. However, the evidence that the article chose to present to support this claim were misinterpreted or contained important caveats that limit its generalizability.

REFERENCES

• 1 – Li et al. (2024) Reducing climate change impacts from the global food system through diet shifts. Nature Climate Change.
• 2 – Crippa et al. (2021) Food systems are responsible for a third of global anthropogenic GHG emissions. Nature Food. 
• 3 – Tubiello et al. (2022) Pre-and post-production processes increasingly dominate greenhouse gas emissions from agri-food systems. Earth Systems Science Data.
• 4 – IPCC (2019) Food Security. In: Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems.
• 5 – IPCC (2021) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.
• 6 – Humpenöder and Popp et al. (2024) Food matters: Dietary shifts increase the feasibility of 1.5°C pathways in line with the Paris Agreement. Science Advances.
• 7 – Van Dooren et al. (2017) Proposing a Novel Index Reflecting Both Climate Impact and Nutritional Impact of Food Products. Ecological Economics.
• 8 – Aiking and Boer (2020) The next protein transition. Trends in Food Science & Technology.
• 9 – Xu et al. (2024) Global greenhouse gas emissions from animal-based foods are twice those of plant-based foods. Nature Food. 
• 10 – Armstrong McKay et al. (2022) Exceeding 1.5°C global warming could trigger multiple climate tipping points. Science.
• 11 – Krebs-Smith et al. (2010) Americans Do Not Meet Federal Dietary Recommendations. Journal of Nutrition.
• 12 – Clemente-Suárez (2023) Global Impacts of Western Diet and Its Effects on Metabolism and Health: A Narrative Review. Nutrients.
• 13 – Adolph and Tilg (2024) Western diets and chronic diseases. Nature Medicine.
• 14 – Charidemou et al. (2019) A randomized 3-way crossover study indicates that high-protein feeding induces de novo lipogenesis in healthy humans. Journal of Clinical Investigation Insight. 
• 15 – Derbyshire EJ (2017) Flexitarian Diets and Health: A Review of the Evidence-Based Literature. Frontiers in Nutrition. 
• 16 – Agrawal et al. (2014) Type of vegetarian diet, obesity and diabetes in adult Indian population. Nutrition Journal.
• 17 – Tonstad et al. (2009) Type of Vegetarian Diet, Body Weight, and Prevalence of Type 2 Diabetes. Diabetes Care. 
• 18 – Tonstad et al. (2013) Vegetarian diets and incidence of diabetes in the Adventist Health Study-2. Nutrition, Metabolism and Cardiovascular Diseases.
• 19 – Rizzo et al. (2011) Vegetarian Dietary Patterns Are Associated With a Lower Risk of Metabolic Syndrome. Diabetes Care. 
• 20 – Satija et al. (2016) Plant-Based Dietary Patterns and Incidence of Type 2 Diabetes in US Men and Women: Results from Three Prospective Cohort Studies. Public Library of Science Medicine.
• 21 – Satija et al. (2017) Healthful and Unhealthful Plant-Based Diets and the Risk of Coronary Heart Disease in U.S. Adults. Journal of the American College of Cardiology.