Why do intervention studies with animal-sourced foods fail to suggest harm on aggregate?

Randomized controlled intervention trials are useful for establishing cause-and-effect relationships and are considered high up in the evidence hierarchy. However, such trials are not infallible. Problems may arise when, for instance, the normal dietary context is overlooked, or when biomarkers are not robust enough to evaluate the actual health impact.

The evidence from randomized controlled trials with respect to the impact of a low or high intake of animal source foods on cardiometabolic outcomes or mortality is overall of low to very-low certainty. The causality of the associations between certain animal source foods and morbidity or mortality outcomes obtained from observational studies remains therefore highly uncertain.

Intervention trials have shown that consuming animal-source foods, including red meat, poultry, fish, eggs, and dairy, generally has neutral to beneficial effects on cardiovascular and metabolic health markers when integrated into a well-formulated diet. Red meat, even when consumed regularly, does not significantly impact blood pressure, inflammation, oxidative stress, or markers of glycemic control, though a modest increase in LDL cholesterol (often with a more benign particle profile) may occur with higher saturated fat content. Notably, reducing or avoiding red meat has shown limited benefits for weight, metabolic outcomes, or inflammation. Comparative studies reveal no significant differences in lipid profiles between red meat, poultry, and fish, while oily fish may offer added benefits such as increased HDL cholesterol and reduced triglycerides. Eggs show no adverse effects on vascular or metabolic markers, and they may enhance satiety, muscle synthesis, and body composition. Moreover, Palaeolithic diets, rich in meat, fish, and eggs, improve cardiovascular risk factors and anthropometric measures. Dairy products, particularly among those with metabolic disorders, exhibit anti-inflammatory properties and favorable impacts on cardiovascular markers, though minor increases in LDL cholesterol and triglycerides may occur. Overall, incorporating animal-source foods into a health-conscious dietary context supports positive health outcomes without significant harm.

Evidence from intervention trials on the impacts of vegan and vegetarian diets on the risk of chronic diseases remains insufficient to draw conclusions. While these diets may lower caloric intake and LDL cholesterol, they can also reduce HDL cholesterol, increase triglycerides, and elevate postprandial glucose and insulin levels, depending on the specific type of ‘plant-based’ approach. Comparisons with animal-based diets in individuals with type-2 diabetes have have not consistently demonstrated clear advantages, and substituting animal proteins with plant proteins may even negatively affect bone health markers, potentially due to lower protein quality and reduced vitamin D and calcium intake. Meta-analyses suggest some benefits, such as reductions in LDL cholesterol, apoB, and fasting glucose, but evidence on broader metabolic outcomes is inconsistent and often of low certainty. Reductions in LDL cholesterol alone may be less predictive of cardiometabolic health than maintaining a favorable balance of HDL cholesterol and triglycerides. Studies on plant-based imitation meats have yielded mixed results, with some indicating neutral or adverse effects on glycemic control and muscle protein synthesis. For instance, diets incorporating lean beef were equally satiating but more well-liked than those with plant-based alternatives and led to voluntary reductions in carbohydrate and sugar intake among overweight women. Additionally, an intervention trial comparing a soy-based meat alternative to beef patties showed a lower anabolic response to protein synthesis with the former. While certain plant-based diets may support caloric reduction and fat loss, they fall short of the overall health benefits offered by well-formulated omnivorous diets.

Intervention studies with experimental animals do not provide consistent evidence for the alleged harmful effects of red and processed meats. The International Agency for Research on Cancer (WHO) concluded that there is inadequate evidence in animals for the carcinogenicity of consuming red meat, processed meat, and haem iron.