Evidence of the role of cooking methods on inflammation and metabolic health is scarce due to the paucity of large-size studies.

Our aim was to evaluate the association of cooking methods with inflammatory markers, renal function, and other hormones and nutritional biomarkers in a general population of older adults.

Eight cooking methods were considered: raw, boiling, roasting, pan-frying, frying, toasting, sautéing, and stewing.

Marginal effects from generalized linear models were calculated and percentage differences (PD) of the multivariable-adjusted means of biomarkers between extreme sex-specific quintiles (Q) of cooking methods consumption were computed ([Q5 − Q1/Q1] × 100).

Significant PD for the highest vs lowest quintile of raw food consumption was − 54.7% for high sensitivity-C reactive protein (hs-CRP), − 11.9% for neutrophils, − 11.9% for Growth Differentiation Factor-15, − 25.0% for Interleukin-6 (IL-6), − 12.3% for urinary albumin, and − 10.3% for uric acid.

Concerning pan-frying, the PD was − 23.2% for hs-CRP, − 11.5% for IL-6, − 16.3% for urinary albumin and 10.9% for serum vitamin D.

Raw, boiling, pan-frying, and toasting were associated with healthy profiles as for inflammatory markers, renal function, thyroid hormones, and serum vitamin D.

Cooking methods not including added fats where healthier than those with added fats heated at high temperatures or during longer periods of time.

However, food preparation, including cooking methods, have only recently received attention.

Indeed, the study of cooking methods has been very limited.

One of the main reasons is the lack of data on cooking methods in large-sized studies, particularly when diet is collected using food frequency questionnaires.

Frying is the most frequently studied cooking method in large populations, both in the United States and in Spain.

In the United States, analyses from the Nurses' Health Study and the Health Professionals Follow-Up Study showed an association between fried food consumption and higher incident diabetes, cardiovascular disease and heart failure5,6.

However, in Spain no association was found between the consumption of fried food and incident cardiovascular disease8,9.

Other cooking methods have scarcely been investigated10.

Nevertheless, the study of cooking methods could provide interesting insights into the investigation of chronic diseases development given that many of these are related to changes in inflammatory, renal, and other nutritional biomarkers11,12.

Thus, in a previous large-sized study, cooking patterns showed a relationship with inflammatory and cardio-metabolic health biomarkers13.

More specifically, four cooking patterns were found: (a) The “Spanish traditional pattern”, characterized by boiling, sautéing, brining, and pan-frying, tended to be metabolically advantageous; (b) the “Health-conscious pattern”, based on battering, frying, and stewing, seemed to improve renal function; (c) the “Youth-style pattern”, with frequent consumption of soft drinks and distilled alcoholic drinks, and low consumption of raw food, was associated with metabolic benefits, except for a higher insulin and higher urinary albumin levels; and (d) the “Social business pattern”, rich in fermented alcoholic drinks, food cured with salt or smoked, and cured cheese, appeared to be detrimental for lipid profile, renal function and other metabolic biomarkers.

Therefore, the aim of the study was to evaluate for the first time in literature, the association of eight cooking methods with a wide array of inflammatory, renal, and other hormones and nutritional biomarkers in a population-based sample of older adults.

Our initial hypothesis was that frying could be a detrimental form of cooking, while boiling, pan-frying, and sautéing could be beneficial cooking methods.

The Seniors-ENRICA-2 study consists of 3273 individuals aged 65 years and older.

Information on cooking methods was collected by trained and certified non-medical interviewers through a validated dietary history, DH-ENRICA.

Information on habitual food consumption was collected considering all food consumed at least once every two weeks during the year prior to the interview.

DH-ENRICA comprises 860 foods along with their cooking technique or preservation method.

Then, food consumption according to their cooking method was summed up in grams per day.

Eight cooking methods with a minimum mean consumption of 15 g/d were included in this analysis: raw (not cooked), boiling (cooking in aqueous medium with heat transmission by water), roasting (dry cooking transmitted by air or radiation), pan-frying (high temperature dry cooking using a pan and a minimum amount of oil), frying (high temperature cooking with food immersed in hot fats or oils; breaded, floured or battered food were also considered in this category), toasting (browning food using heat radiation), sautéing (brief medium to high heat cooking in a low-fat pan), and stewing (low heat cooking, usually simmered food in liquids for a long period of time).

Mixed cooking methods (e.g. boiling + sautéing, frying + boiling, or sautéing + roasting) were not considered for this analysis since these are not frequent in Mediterranean diet16,17 and were barely consumed by participants in this study (less than 10 g per day).

Hormones and other nutritional biomarkers included thyroid-stimulating hormone (TSH), serum vitamin D, serum albumin, and total serum protein.

These biomarkers encompassed: hs-CRP, urinary albumin, serum uric acid, serum sodium, serum potassium, TSH, serum vitamin D, and serum albumin.

For each cooking method, analyses were performed in grams per kilogram (g/kg) of body weight to take into account body size.

Participants were classified into sex-specific quintiles for each cooking method.

Baseline characteristics of main sample according to quintiles of cooking methods are shown in Table 1.

Significant differences were found when comparing extreme quintiles of consumption for each cooking method.

PD for raw food consumption was − 54.7% for hs-CRP, − 11.9% for neutrophils, − 11.9% for GDF-15, − 25.0% for IL-6, − 12.3% for urinary albumin, and − 10.3% for uric acid.

PD for pan-frying were − 23.2% for hs-CRP, − 11.5% for IL-6, − 16.3% for urinary albumin, and showed an increase of 10.9% for serum vitamin D.

Also, a PD of 10.6% in serum vitamin D was observed.

Adjusted means (95% confidence interval), percentage difference, and p for linear trend across quintiles of cooking methods consumption are presented in Supplementary Tables 2–9.

In this large population-based study on cooking methods, raw, boiling, pan-frying, and toasting showed beneficial profiles for inflammatory markers, while frying and stewing showed detrimental inflammatory profiles.

Raw, boiling, and pan-frying showed beneficial differences in renal biomarkers; no other cooking methods influenced renal function.

Boiling was associated with lower TSH levels.

Pan-frying and toasting were associated with a higher serum vitamin D, while frying was associated with a lower level of this vitamin.

Cooking methods that included no added fats where healthier than those including added fats heated at high temperatures or during longer periods of time.

Thus, although some cooking methods are considered healthy, and in many cases older adults have gradually adopted them, comprehensive information that provides guidance on food preparation has never been provided in literature before.

In addition, some specific cooking methods are considered harmful.

Our findings support that, among the elderly, raw food consumption is associated with beneficial marker profiles (such as lower levels of hs-CRP, neutrophils, GDF-15, IL-6, urinary albumin, and serum uric acid).

In the second one, an exclusive raw food diet was associated with lower LDL-cholesterol and HDL-cholesterol levels, in addition to vitamin B-12 deficiency34.

Hence, along with general recommendations for a healthy diet (quality food choices and adequate daily consumption), it seems very reasonable to favor raw food consumption for this age group.

Regarding inflammatory markers, raw, boiling, and toasting food consumption were associated with lower hs-CRP levels.

A possible explanation could rely on the fact that these cooking methods usually do not imply the addition of unhealthy fat sources35.

In an analysis of the National Health and Nutrition Examination Survey (NHANES) conducted with 4900 adults, it was observed that saturated fat consumption was modestly associated with elevated hs-CRP36.

In addition, raw food has high fiber and antioxidants contents, which are also associated with lower levels of hs-CRP37.

On the contrary, frying and stewing were associated with higher hs-CRP levels in this study.

The addition of harmful fats or a longer cooking time might be associated with higher levels of this inflammatory marker.

Concerning other inflammatory markers, the association of raw food consumption (mainly fruit and vegetables) with lower counts of white blood cells and neutrophils was also examined in other studies.

Likewise, Mediterranean diet adherence, rich in raw food, was associated with lower leukocyte count levels42.

This is important since neutrophils regulate inflammatory function and it has been suggested that lower neutrophil levels are associated with lower atherothrombotic processes and cardiovascular diseases43,44.

In our study, raw food consumption was also associated with lower GDF-15 levels.

On the other hand, although cooking methods were not evaluated, a 6-month clinical trial found no association between GDF-15 levels and plant-based or protein-based diets48.

Raw and toasting food consumption showed decreasing differences in IL-6 levels.

These cooking methods involve dressing oils and hidden fats from fruits, nuts and seeds (usually consumed toasted)49 which in turn promote carotenoid absorption.

Similarly, in a cohort with 3075 older adults an association was found between lower IL-6 levels and a dietary pattern that included high consumption of fruit, vegetables, whole grains, and non-fried poultry52.

Concerning renal function, there was a lower mean urinary albumin with raw, boiling, and pan-frying consumption in this population.

A high renal excretion of albumin could reflect greater animal protein and fat intake, and, generally, these cooking methods do not involve the addition of fats.

On the other hand, lower uric acid levels occurred with raw food consumption.

This is in line with the literature as a lower purine content in raw food could be responsible for a lower uric acid formation55,56.

In relation to hormones and other nutritional biomarkers, our results suggest that boiling is associated with lower TSH levels and it could potentially prevent subclinical hypothyroidism.

A study demonstrated that boiled cauliflower had lower levels of progoitrin, a precursor of goitrin57.

Additionally, beneficial PD in serum vitamin D was observed with pan-frying and toasting.

Conversely, the exposure to high temperatures has been shown to cause adverse effects in vitamin D3 content in food, but knowledge regarding this matter is limited, and the stability of vitamin D3 in added vegetable oils during cooking has not yet been determined59,60.

The main strength of this study is the use of a dietary history that allowed to collect detailed information on food consumption and cooking methods.

Second, as when a specific food is studied, it is not possible to distinguish the effect of cooking from the consumption of the food itselfD

Similarly, we cannot distinguish the effect of a specific cooking method from the effect of added fats when occurring together.

This study suggests that among older Spanish adults, some cooking methods, especially, raw, boiling, pan-frying, and toasting, are associated with positive differences in several inflammatory markers, renal function, thyroid hormones, and serum vitamin D.

Therefore, these findings add information to current knowledge on the possible impact of cooking methods on health.

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Cooking methods are associated with inflammatory factors, renal function, and other hormones and nutritional biomarkers in older adults