In recent months, several US fast food chains have replaced their usual frying oils with beef tallow, a decision rooted more in political messaging than in evidence-based nutrition. The shift has been accompanied by a social media narrative that seed oils are toxic, while beef tallow is “natural” and therefore healthier. The claim has been amplified by widely followed podcasts and public figures without a background in nutrition science.

A similar trend has emerged for exercise. Zone 2 training, a low-intensity form of exercise performed below the lactate threshold, is increasingly promoted as a key to longevity, with claims that it can reprogram mitochondrial function in ways that other exercise intensities cannot.

Beef Tallow

The argument for beef tallow rests largely on the belief that it is a “traditional” fat that has been part of the human diet for centuries, whereas seed oils have been used industrially for a much shorter period. However, history alone does not establish a physiologic or clinical benefit.

A 2026 review by cardiologists and nutrition specialists published in JACC: Advances specifically examined the controversy and classified beef tallow among dietary components with evidence of harm. The review cited evidence that beef tallow increases low-density lipoprotein (LDL) cholesterol compared with seed oils and other plant oils that contain more unsaturated fat. There is no clinical evidence that beef tallow provides a net health benefit over unsaturated fats.

Several clinical trials, including crossover studies, found that replacing vegetable oils with beef tallow increased LDL cholesterol by 9%-13%, depending on the study. The 2026 review also discussed concerns about oil oxidation with excessive or repeated heating and reported evidence related to endothelial function and the gut microbiome.

In contrast, seed oils, which are rich in unsaturated fatty acids, were classified in the same review as having evidence of benefit. A higher intake of linoleic acid, the primary polyunsaturated fatty acid in many seed oils, has been associated with lower cardiovascular (CV) mortality in cohort meta-analyses.

However, this distinction is not simply between one type of oil and another. Storage and cooking practices are also important. Unsaturated fats are more susceptible to oxidation, and repeated exposure to heat, light, or air can alter their composition. This does not negate evidence supporting unsaturated fats within a healthy dietary pattern.

The debate also needs cultural context. In the US, seed oils such as sunflower, canola, corn, and soybean oils have been widely used for cooking for decades. Olive oil is the predominant cooking fat in Spain and much of the Mediterranean region. Olive oil is primarily monounsaturated and has a substantial evidence base from cohort studies and clinical trials in Mediterranean populations.

The underlying dietary principle remains consistent: replacing saturated fat with unsaturated fat is supported by evidence. In Mediterranean countries, the traditional alternative to animal fat is dietary fat, with strong evidence for CV benefit.

Saturated Fat

Evidence supporting the replacement of saturated fat with unsaturated fat comes from several sources with strong methodology.

The 2017 American Heart Association presidential advisory on Dietary Fats and Cardiovascular Disease strongly reaffirmed the benefit of lowering dietary saturated fat and replacing it with unsaturated fats to prevent CV disease incidence by approximately 30%, an effect similar to that reported with statin therapy.

One study classified full-fat dairy among dietary components for which evidence of harm or benefit remains insufficient, rather than placing it in the same category as beef tallow. This evidence should be shared with the patients with the emphasis that it is not a minor dietary nuance but an effect of a similar magnitude to that of a pharmacologic intervention.

The 2020 Cochrane review provides an important qualification. Reducing saturated fat lowered the risk for combined CV events by approximately 17% but did not significantly reduce overall mortality or CV mortality. The number needed to treat was 56 in primary prevention, meaning that 56 people would need to reduce their saturated fat intake for approximately 4 years to prevent one CV event.

A large US cohort study published in 2024 provided additional evidence. The study included 407,531 participants with a 24-year follow-up period. Replacing 5% of energy from animal fat with an equivalent amount of plant fat was associated with a 4%-24% lower risk for overall mortality and a 5%-30% lower risk for CV mortality, depending on the source of animal fat and plant fat. The strongest associations were observed when plant fat from grains or vegetable oils were used to replace animal fat. Because this was a prospective observational study, the findings cannot establish causality.

The same cohort study found that a higher intake of animal fat, including dairy and egg fat, was associated with higher overall CV mortality. However, these findings should be interpreted in the context of broader evidence.

Taken together, the available evidence does not support beef tallow as a safer or more “natural” alternative to seed oils. It supports replacing saturated fat with unsaturated fat, including fat from seed oils, within an overall healthy dietary pattern, rather than focusing on the substitution of one individual food for another.

Zone 2 Training and Fitness Benefits

The second trend concerns exercise and follows a similar pattern. Popular science communicators have transformed a reasonable physiologic concept into a categorical recommendation that is not supported by experimental evidence to the same degree.

Zone 2 is exercise performed below the first lactate threshold. It has been widely promoted as the optimal intensity for improving mitochondrial capacity and long-term metabolic health. The concept has a plausible physiologic basis. Elite endurance athletes perform substantial amounts of training at this intensity and have a high mitochondrial capacity.

According to a 2025 narrative review in Sports Medicine, the association has been turned into a recommendation for the wider population without sufficient experimental evidence. Studies directly assessing the effects of Zone 2 training on mitochondrial capacity have found mixed results. Some reported improvements after training at this intensity, whereas others, including several week programs in healthy adults and prolonged low-intensity exercise during skiing expeditions, did not show an increase in mitochondrial capacity. In some studies, mitochondrial capacity decreased.

The available evidence does not establish Zone 2 as superior to higher-intensity exercise for improving mitochondrial capacity or fat oxidation. The authors also raised a practical concern: Individuals with limited time for exercise who replace higher intensity activity with Zone 2 training could miss some of the cardiometabolic benefits associated with higher intensity exercise.

The possibility that the mitochondrial claims surrounding Zone 2 have been overstated does not mean that improving cardiorespiratory fitness (CRF) lacks merit. CRF, typically assessed through maximum oxygen consumption, has some of the strongest and most consistent evidence among modifiable health measures.

A 2024 overview of systematic reviews synthesized evidence from 26 reviews representing more than 20.9 million observations from 199 unique cohort studies. High CRF was associated with a 53% lower risk for all-cause mortality than low CRF. Each metabolic equivalent of task increase in CRF was associated with an 11%-17% lower risk for all-cause mortality. The review also found consistent dose-response relationships between higher CRF and lower risks for heart failure, hypertension, and other chronic conditions. However, because the underlying evidence comes from cohort studies, these associations do not establish causality. The key clinical question, therefore, is not whether CRF should be improved but how to improve it efficiently.

A 2024 systematic review and meta-analysis of polarized training included 17 studies involving 437 participants and found a small advantage of polarized training in improving maximum oxygen consumption (VO₂ peak) compared with other types of endurance training intensity distribution on athletes’ endurance performance. The advantage was observed primarily in interventions lasting less than 12 weeks and among highly trained athletes.

For other endurance performance measures, including time trial performance, time to exhaustion, and speed or power at the second ventilatory or lactate threshold, the review found no evidence that polarized training was more effective than other training intensity approaches.

Therefore, the evidence does not support Zone 2 as a unique approach for improving mitochondrial health, nor does it establish a single training intensity distribution as superior for the wider population. Evidence supports the importance of CRF, while the optimal approach for improving it may depend on the individual, training status, goals, and available time.

Conclusion

Both trends offer the same lesson for clinical practice; science rarely supports the most categorical claims made by viral trends, but it does not necessarily disprove them either. Beef tallow is neither a safer nor a more “natural” alternative to seed oils. The available evidence supports replacing saturated fat with unsaturated fat as one of the most well-established dietary approaches to CV prevention, although the effect on overall mortality is more modest than the effect on CV events.

In exercise, CRF deserves the same attention as other modifiable risk factors. Improving CRF does not require prescribing a specific exercise intensity. Any training approach that improves CRF when performed consistently and at a sufficient level can contribute to better health outcomes.

The authors’ conflicts of interest are reported in the original study publications.

This story was translated from Univadis Spain, part of the Medscape Professional Network.