Whether for health, environmental concerns, or ethical reasons, more and more people are opting to cut meat from their diets. But what happens to the body when meat or animal products disappear from the plate?

Recent research suggests that even short stints without meat—whether weeks, days, or even just a few meals—can trigger surprising changes. The effects range from weight loss and signs of heart health improvements to more subtle impacts on the gut and immune system.

Weight loss and heart benefits

One of the most noticeable effects of cutting out meat is weight loss, but it’s not just about removing animal products—it’s about what takes their place. When people replace calorie-dense meats with fiber-rich plant foods like vegetables, legumes, whole grains, and fruits, “we end up just consuming more foods that are fewer calories,” says Matthew Landry, a registered dietician and health scientist at the University of California, Irvine. Fiber-rich foods also help us feel satiated for longer, which can reduce overeating.

For instance, a 2018 study tracking people over three months found that those who shifted from an omnivorous to a vegetarian diet lost roughly 4 pounds.

The benefits extend beyond weight. Several studies have found that vegetarian diets lasting six weeks or more can lead to lower blood pressure, a key factor in reducing the risk of heart disease and stroke. This effect is likely linked to weight loss and greater consumption of minerals like potassium and magnesium that help reduce blood pressure, says Luigi Fontana, an expert on nutrition and healthy aging at the University of Sydney. Vegetarian diets often also lower blood sugar, reducing the risk of developing type 2 diabetes, according to a recent review by Fontana and his colleagues.

Other studies on vegetarians have seen drops in low-density lipoprotein (LDL) cholesterol—the “bad” cholesterol abundant in meat and dairy products that clogs up blood vessels and raises the risk for heart disease and stroke.

A study on 11 pairs of twins, led by Landry and his colleagues at Stanford University, found that those who followed a vegan diet saw their LDL cholesterol drop to optimal levels below 100 milligrams per deciliter after four weeks. Even though participants started with relatively healthy levels, the decrease was still notable.

Changes in the gut

Switching to a plant-based diet doesn’t just change what’s on your plate—it can alter the ecosystem of bacteria living in your guts. New vegetarians tend to develop more bacteria specializing in chopping up fiber in plant-based foods, and fewer bacteria that target meat, explains Veronica Witte, a neuroscientist at Germany’s Max Planck Institute for Human Cognitive and Brain Sciences and the University of Leipzig Medical Center. In a 2014 study that used stool samples to analyze the gut bacteria of people who shifted to a plant-based diet, the makeup of their gut bacteria changed, “even after several days,” says Witte.

Although the precise effects aren’t consistent across studies, some changes may be beneficial. Preliminary data from the Stanford twin study found that Bilophila wadsworthia, a gut bacterium that chops up specific proteins in animal products and has been linked to disease-driving inflammation, declined in the twins who went vegan. It seems, “that B. wadsworthia can be very problematic to have in your gut microbiota at high abundance, and one way to help get rid of this microbe is to switch to a vegan diet,” says co-author Matthew Carter, a microbiologist at Stanford.  

Scientists are also investigating the potential benefits of fiber-fermenting bacteria and possibly harmful effects of meat-processing ones. Meanwhile, Witte is investigating whether changes in gut bacteria can somehow affect the brain and influence food cravings. But, she cautions, we need more research “to really show that these [processes] are at play.”

Immune health and beyond

Scientists are still unraveling the details, but early research suggests a plant-based diet may also influence the immune system. A 2024 study reported that people on a two-week vegan diet developed higher activity in parts of the immune system involved in defense against viruses. And, the vegans in the Stanford twin study experienced a decrease in cytokines, signaling molecules that coordinate immune activity. “It was amazing to see such striking changes in the immune system…after just a few weeks,” Carter says.

Still, the long-term implications remain unclear. It’s also a mystery whether removing meat is strictly necessary for health. Studies on classical Mediterranean diets—including ample vegetables, whole grains, beans, and meat and fish up to twice a week—show similar benefits to being vegan. “If you’re consuming a small amount of meat once a week, does this impair health?” Fontana asks. “The answer is, we don’t know.”

Cultivating healthy habits

While vegan and vegetarian diets can offer health benefits, Fontana emphasizes that these depend on smart food choices, not just cutting out meat. Swapping meat for high quantities of ultra-processed foods like vegan frozen pasta dishes, sugary snacks, or some ultra-processed substitutes can be just as detrimental.

“You can be a vegetarian and be as unhealthy—or more unhealthy—than someone on a typical Western diet,” Fontana says.

And, while nutrient deficiencies aren’t a big concern for people ditching meat for just a few weeks or months, it’s important in the long term to ensure you’re getting enough essential nutrients that are scarce in plant-based foods, like vitamin B12, iron, zinc, calcium, and omega-3 fatty acids. While many of these nutrients are readily available in diverse plant-based foods, like tofu and lentils, Landry suggests that those following a vegan diet should consider taking vitamin B12 and vitamin D supplements.

For people beginning vegetarian diets, Landry recommends starting gradually and experimenting with new foods and recipes. “Use this as a big opportunity to really explore your palate and try some global flavours.”