These days, many health practitioners and healthy lifestyle advocates are praising the benefits of a plant-based diet. As the famous quote from Michael Pollan, author of The Omnivore’s Dilemma, wrote, “Eat food. Not too much. Mostly plants.” He also noted that “You are what you eat eats.” This simple statement, highlighting the connection between human health and the consumption of animal products, essentially recognizes that when we eat meat products, we are also consuming what those animals consume. This fact, coupled with the sad state of animal husbandry practices in the United States and other countries whose commercial operations received a failing grade in the Animal Protection Index from the global organization, World Animal Protection, suggests that consumption of animal products can carry serious health risks. But does it mean that diets that are meat-heavy are inherently bad for you?  We explored some of the science-backed evidence to find out.

Meat’s bad reputation

There are many reasons why meat-heavy diets have gotten a bad reputation. First, the commercial system of animal husbandry in the United States is essentially a feedlot system where animals are raised in horrendous and inhumane conditions, such as being separated from their young soon after birth; fed diets that make them grow unusually large, making free movement difficult even in cage free environments; forced to reproduce at unnatural rates, significantly shortening their lives; and being forced to endure suffering and pain before being killed for human consumption. Second, the practices of vegetarianism and veganism have become increasingly popular, largely because of the first reason above. Third, the invention of meat-like products made from plants has captured the attention of consumers who are interested in turning away from meat but need a “bridge” to help them do that. Fourth, there is growing awareness about the stress that raising livestock places on the environment, including animal waste runoff pollution and global warming. Fifth, meat is perceived to be inherently unhealthy, with the most alarming reports suggesting that all meat causes cancer, without qualification. It’s worth exploring these last two reasons in greater detail.

Raising livestock places more stress on the environment

While both vegetable and livestock farming alike rely heavily on the fossil fuel industry for their main energy source, raising livestock for human consumption is more expensive and places more stress on the environment. When compared with the resources used for the average lacto-ovo-vegetarian diet, the average meat-heavy diet uses up more resources of land and energy. For example, in the United States, over 9 billion livestock are raised to fulfill yearly consumer demand for animal protein. Livestock animals such as poultry and hogs only consume grains, while cattle and lambs consume both grains and forage: together this population of livestock eats more than 7 times the amount of grain that is consumed by the entire American population. This means that most of the grain that is grown in the US is being produced to fuel the livestock industry.

Moreover, animal waste, if not adequately managed, travels over land (through runoff) or underground (through leaching) and makes its way into local lakes and streams, contributing to the degradation of the nation’s surface water. Animal waste enters into groundwater, soil, and even the air and has been associated with a range of harmful impacts to human health and the environment. Finally, while all livestock farming contributes to global warming, the beef industry is especially implicated, since cattle produce a large amount of greenhouse gases, namely methane, which – when coming from cattle -- is shorter lived in the atmosphere but 28 times more potent than carbon dioxide in its capacity to warm the planet.

The (supposed) links between red meat and cancer

Ever since the 2015 publication of findings by the World Health Organization’s International Agency for Research on Cancer (IARC), which linked the eating of red meat and cancer, some organizations and individuals have taken things several steps further and erroneously claimed that eating meat causes cancer. In fact, the research results did not make a simple or direct connection between eating red meat and the incidence of colorectal cancer.

The 22 scientists from 10 countries who convened in Lyon, France to conduct this research found a 17% increased risk of colorectal cancer per 100 grams of red meat consumed daily (equal to ½ cup of cooked ground beef or 1/5 pound of T-bone steak) and an 18% increased risk per 50 grams of processed meat consumed daily (the equivalent of one hot dog or six slices of bacon). The scientists classified red meat as “probably carcinogenic to humans” and processed red meat as “carcinogenic to humans.” The distinction is an important one.

“Probably carcinogenic to humans” means that there is strong scientific evidence available to suggest that a substance can cause cancer in humans, but the evidence is not conclusive. “Carcinogenic to humans” may also be translated as “known to be cancer-causing to humans.” And yet even known carcinogens don’t always cause cancer in every person, no matter the circumstances surrounding exposure. Some known carcinogens only cause cancer in people who have a certain kind of genetic makeup. Other known carcinogens may lead to cancer after minimal exposure, while others might do so only after acute exposure over a period of many years. And known carcinogens also include alcoholic drinks, engine exhaust, wood dust, the sun, and outdoor air pollution, all of which are substances that most of us are exposed to on a regular basis, willingly or not.

There is also the question of risk: the increased risk that comes into play from eating meat is relative to other factors that must be taken into consideration, such as how much red meat is being consumed on a daily or weekly basis, what other lifestyle factors are present that may affect this risk (such as exercise, alcohol consumption, smoking, healthy eating habits, and exposure to other toxins on a regular basis), and what other underlying pathologies may be present in the body. In short, consuming meat – even on a regular basis -- will not cause cancer, but the evidence strongly suggests that reducing your consumption of meat, especially processed meat, will significantly reduce your relative risk of developing colorectal cancer later in life.

What about the Paleo Diet?

If the science still strongly suggests that eating meat will not cause cancer, but that it’s still better for your health to reduce your meat consumption and eat more plant-based foods, what about diets that are heavy on meat? I’m talking especially about the Paleo Diet, a controversial diet first created in the 1970s that excludes highly processed foods, but includes vegetables, fruits, nuts, roots, meat, and organ meats. The Paleo diet (which includes the “Carnivore Diet”) features foods that contain higher levels of cholesterol, saturated fats, and protein (most of which comes from meat).

Some studies have shown that the Paleo diet may have beneficial health effects: in one study of 25 participants, 14 were on the Paleo diet and 10 were on a diet based on recommendations from the American Diabetes Association (ADA). Both groups showed improved metabolic indicators at the end of the study, but the Paleo group saw greater benefits in terms of its blood glucose control and improvements in its lipid profile. Despite these promising results, there are several reasons to be wary of the benefits of the Paleo Diet, including these three:

The question of definition

The term “Paleo diet” is deceptive. Although the modern “Paleo diet” does have some commonalities with the diets of Paleolithic humans, it has many more differences. Namely, proteins may not have comprised a majority of actual prehistoric diets, and in fact, those diets would have varied according to location. Desert dwellers would have little access to meat and fish; instead, they would likely have consumed nuts, seeds and even insects. Groups living in colder climates would not have had much access to fresh fruits and vegetables and would have mostly eaten meat. Notably, the products they ate did not have the connection to industrial processes, as they do for Paleo diet aficionados today.

The question of “healthy”

There is little evidence to suggest that Paleolithic humans were healthy (most died before the age of 40, so never developed the end-stage degenerative diseases that older people today tend to develop). At the same time, even if Paleolithic humans were healthier, it’s not clear that this was directly, let alone solely, related to their diets. The factors for determining the “healthy” status of Paleolithic humans are complex, and we have to consider their different lifestyles, including hunting and gathering, which involves more physical activity than most people today routinely get.

The question of microbes

Some studies suggest that human gut bacteria have co-evolved with humans over millions of years to help shape our immune systems and development. This means that microbes are well adapted to the human digestive tract as it exists today, helping to train our immune systems, guide gut development, and even regulate our emotions and behavior. By the same token, humans have experienced a depletion of their gut microbiota in comparison to Paleolithic populations, which also means we do have much less microbial diversity – i.e. intestinal bacteria -- than our ancestors possessed. This means that large amounts of meat may be harder for our bodies to digest.

So what does the science suggest about eating meat?

Overall, the scientific data on consuming a meat-heavy diet is mixed. While most studies suggest that eating a diet that is rich in a variety of foods (i.e. plant-based foods and meats) that limits or even eliminates the consumption of highly processed food is best, there are no reliable scientific studies that suggest that you should give up meat altogether for health reasons. In fact, most of the scientific evidence advocates for an omnivore diet in which the majority of foods consumed are plant-based, and lean meats and fish make up the majority of meat included.

Paleo, full-vegetarian, and vegan diets may offer some health benefits to those who are diligent in their attention to replacing the nutrients that are lost by giving up certain foods altogether, and each of these diets has the potential to fix mild nutritional deficiencies or solve food intolerances. Any eating pattern, though, especially one that involves completely giving up a food group, has the potential to be unhealthy, so the bigger indicators of a positive correlation between diet and health are the practices we’ve consistently been advised to adopt: monitor your health; get enough sleep (your body will tell you if you’re not); minimize your intake of highly processed foods; get some exercise; and aim to eat a little better than you are now.

As is so often the case with adopting a healthier lifestyle, small actions taken now and pursued consistently yield big results over time.

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