Objective
To test the hypothesis that an AHA diet enriched in MUFA improves lipoprotein lipids, insulin resistance, inflammation and endothelial function to a greater extent than a diet enriched in PUFA in middle-aged men and women with MetS.

Methods
A prospective, open-label, parallel group design with randomization to a hypocaloric MUFA or PUFA enriched diet following weight stabilization on an AHA Step I diet. Participants consumed 3 MUFA or PUFA enriched muffins daily with additional supplementation as required to ensure 25-50% increases in dietary fat intake from these sources at the expense of SFA and the opposing unsaturated fat. Changes in MetS components were measured at baseline and after 6 months of dietary intervention.

Results
Thirty-nine participants (mean age 60.8 years, 79% African-American, 60% women) with MetS completed the 6-month study. Compared to baseline, assignment to either MUFA (n=23) or PUFA (n=16) both were associated with weight loss (WL) (MUFA: −2.3±1 kg, P=0.06; PUFA: −4.6±2 kg; P=0.002), but PUFA was also associated with reductions in triglycerides (TG) (−30±18 mg/dL, P=0.02), systolic blood pressure (BP) (−7±3 mmHg, P=0.01), diastolic BP (DBP) (−4±2 mmHg, P=0.01) and improved flow mediated dilation (FMD) (7.1±1.8% vs. 13.6±2%, absolute increase; P=0.0001). When compared to MUFA treatment, PUFA intervention was associated with reduced TG (P=0.04) and DBP (P=0.07) as well as increased FMD (P=0.04) even after adjustment for changes in weight. There was no effect on total cholesterol, low-density lipoprotein cholesterol (LDL-C), glucose, high-sensitivity C-reactive protein (hs-CRP) or other inflammatory proteins. Overall, 25% (4 of 16) assigned to PUFA and 13% (3 of 23) to MUFA converted to non-MetS status.

Conclusion
Substitution of SFA with PUFA in patients with MetS is associated with greater reductions in TG and improvement in endothelial function than MUFA that is independent of WL. These preliminary findings raise the possibility that PUFA may be the unsaturated fat of choice to reduce cardiometabolic risk in patients with MetS.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5010036/

The conflict you’ve highlighted is the central battleground between Bioenergetic (Ray Peat) principles and Mainstream Lipid Science.

To understand why a study like the MUFFIN study shows positive results for Polyunsaturated Fats (PUFA) while Ray Peat viewed them as metabolic toxins, we have to look at the different metrics they prioritize: short-term blood markers versus long-term cellular stability.

1. The Chemical Difference: Why Peat Opposed PUFA

Ray Peat’s primary argument against PUFAs (like those found in soybean, corn, and sunflower oils) was based on their chemical structure. PUFAs have multiple "double bonds."

• Saturated Fats (SFA): Have no double bonds. They are straight, stable, and resistant to heat and oxygen.
• Polyunsaturated Fats (PUFA): Have multiple double bonds. These bonds are "open" sites where oxygen can attack, leading to lipid peroxidation.

According to Peat, when these unstable fats are incorporated into human tissues (cell membranes and mitochondria), they make the body more susceptible to oxidative stress, inhibit thyroid function, and promote the formation of age pigments (lipofuscin).

2. Analyzing the MUFFIN Study through a "Peat" Lens

The MUFFIN study found that PUFAs were superior to Monounsaturated Fats (MUFA) for improving triglycerides and blood pressure in patients with Metabolic Syndrome. A follower of Ray Peat would likely critique these findings using the following arguments:

A. The Weight Loss Confounding Factor

In the study, the PUFA group lost significantly more weight (-4.6 kg) than the MUFA group (-2.3 kg).

• The Mainstream View: PUFAs might improve insulin sensitivity, leading to better weight loss.
• The Peat View: PUFAs are known to be "pro-metabolic" only in the sense that they can be toxic to the mitochondria, sometimes causing a "wasting" effect or a stress-induced increase in metabolic rate. Peat often argued that PUFAs interfere with the body's ability to store energy efficiently, which might show up as weight loss but at the cost of increased systemic stress.

B. The "Blood Clearing" Effect vs. Tissue Accumulation

The study focuses on Triglycerides (TG) and Flow Mediated Dilation (FMD).

• The Mainstream View: Lowering triglycerides in the blood is a sign of improved cardiovascular health.
• The Peat View: PUFAs lower blood lipids because they are easily oxidized and "pulled" out of the blood into the tissues, or because they inhibit the liver's ability to release glucose and fats. Peat argued that having lower fats in the blood isn't a benefit if those fats are being stored in your cell membranes, where they remain vulnerable to turning into toxic byproducts like acrolein and 4-HNE.

C. The Randle Cycle (Glucose Competition)

Ray Peat’s work heavily emphasized the Randle Cycle, a metabolic process where the oxidation of fats inhibits the oxidation of glucose.

Peat argued that PUFAs are particularly effective at blocking the "burning" of sugar. While the MUFFIN study suggests improved "metabolic management," Peat would argue that over the long term, high PUFA intake forces the body into a state of "fat-burning" that mimics diabetes at a cellular level, suppressing the more efficient oxidative metabolism of glucose.

3. Why the results differ

The MUFFIN study and Ray Peat are essentially looking at two different things:

Summary

The MUFFIN study provides evidence that for obese individuals with metabolic syndrome, replacing saturated fats with PUFA can lead to rapid improvements in clinical biomarkers (blood pressure and triglycerides).

Ray Peat would counter that these improvements are superficial. He would argue that the participants are trading "good-looking" blood tests for long-term "rotten" cellular membranes. From a Peat perspective, the "metabolic syndrome" is caused by a lack of sugar oxidation, and adding PUFAs—even if they lower blood pressure in the short term—further suppresses the thyroid and the body's ability to use oxygen properly.