Metabolic flexibility is a process in which the body can switch energy substrates in different physiological states. This flexibility plays an important role in an individual's health because losing it increases the risk of obesity, metabolic syndrome, insulin resistance, and type 2 diabetes. Considering that humans spend most of their awakening hours in a postprandial (PP) state, an organism's metabolic flexibility (MF) to respond to a standardized meal's consumption would provide information on the individual's metabolic health. The PP response to glucose following an oral glucose tolerance test or consumption of a high-carbohydrate meal is well described; however, few studies assess the FM and PP metabolome using mixed meals with different macronutrients. The investigators address how metabolic flexibility and metabolome change after consuming standardized meals with different macronutrient ratios. Data collection includes clinical and diet information, indirect calorimetry, and capillary blood sampling during fasting and after consumption of standardized meals. Samples are collected weekly for one month. The data will determine the metabolic flexibility and metabolome after consuming standardized meals with different macronutrient ratios.

Enrolled subjects are followed every week for one month. At each visit, a questionnaire assesses daily time activity patterns relevant to energy expenditure, general health status, including infectious symptoms, and confirmation of basic social and demographic characteristics. Dietary intake is assessed by a food frequency questionnaire and a multi-step 24-hour dietary recall for quantitative analysis. If symptoms of infection are present, participants are treated with ad-hoc broad antibiotics. Anthropometrics are obtained. Subjects will then be randomized to receive the metabolic challenges in a different order. The procedures will be performed before (fasting; 8-10 hours) and after (postprandial) consumption of the metabolic challenges. Capillary blood samples (40µ) are obtained in the morning after an 8-hour fasting and after test meal consumption. To obtain the capillary blood sample, sterilize the ring finger with alcohol and allow it to dry. Then, puncture the area with a sterile 2 mm long lancet. Once the drop of blood is formed, it is placed directly into the CardioCheck Plus® cassette to determine glucose triglyceride, LDL-cholesterol, HDL-cholesterol, and total cholesterol. A second drop of capillary blood shall be placed on a filter paper (S&S 903) until a circle of filter paper is filled with blood to saturate the paper throughout its thickness. Insulin concentration shall be determined following the protocol for dried blood, which is standardized in the laboratory. Indirect fasting calorimetry is also performed. After the indirect calorimetry, the metabolic and hormonal response to the test meal is performed.

Each challenge should be consumed within 15 minutes. After 5 minutes of rest, indirect calorimetry will be postprandially, lasting 30 minutes. Capillary blood shall be obtained at the following times: 15-30-45-60-90 and 120 min after ingestion of food.

Nutrient composition of standardized meal and meal example.

• High carbohydrate challenge: Energy, 479.6 kcal; Carbohydrate, 83.6%; Lipids, 12.4% and Protein, 4.0% (70 g of hot-Cake, 100 g of mango, 270 ml of peach nectar, and 40 g of strawberry jam)
• High lipid challenge: Energy, 1043.4 kcal; Carbohydrate, 4.9%; Lipids, 86.8%, and Protein, 8.3% (60 g of manchego cheese, 25 g of egg, white, dried, 24 g of bacon, 5 ml of oil, 65 g of cream cheese, 70 g of cream, and 16 g of poblano pepper)
• High protein challenge: Energy, 441.3 kcal; Carbohydrate, 1.6%; Lipids, 5.1%, and Protein, 93.3% (2 scoop Isopure Zero Carb, 180 g of chicken breast, 20 g of lettuce and 24 g of ham turkey breast)

Determination of metabolites in dried blood To determine the concentration of metabolites (amino acids and acyl-carnitines), a circle of 3 mm diameter shall be punched out of the filter paper and placed in a 96-well plate. Add 100 µl of the acyl-carnitine and amino acid standards from the NeoBase PerkinElmer kit. Subsequently, follow the manufacturer's instructions for determining metabolites by liquid chromatography coupled to mass spectrometry (LC-MS).

Determination of the respiratory quotient (RQ) To determine the respiratory quotient (RQ= VmaxCO2/ VmaxO2) and lipid and carbohydrate oxidation examinations, the Cardio Coach CO2 Vmax Encore 29 System calorimeter software (Korr, Inc, UT, USA) will be used according to the instructions of the supplier. Examinations are invariably performed in the morning (8:00-9:00 am) in a thermoneutral environment with controlled pressure, humidity, and temperature, with the patient supine but awake. The investigators examine a maximum of two subjects per day. Oxygen consumption and carbon dioxide production were obtained using a canopy and were monitored continuously for 30 minutes. The initial 10 minutes of the measurement are discarded for the calculation to ensure greater data homogeneity. O2 consumption and CO2 production will be recorded continuously for 30 min. VO2 and VCO2 values will be used in the equation proposed by Weir (Energy Expenditure = [3.941(VO2) + 1.11(VCO2)] x 1440 min/day), considered as the standard method [34]. Additionally, one day before the test, subjects are instructed to fast for 8 hours and not engage in physical activity or consume caffeine the day before the exam. All participants will be asked to eat the same dinner the night before each test. Dinner provides 15% of the daily energy intake; 20 g of protein, 7 g of lipids, and 34 g of carbohydrates)

Inclusion Criteria:

• be over 18 years of age and under 30 years of age;
• without active drug addictions: smoking, alcoholism, and/or drug addiction;
• in female subjects: regular menstrual cycles;
• give written consent for their inclusion in the study.

Exclusion Criteria:

• subjects with any pathology that requires medication or special treatment, such as type 2 diabetes mellitus, high blood pressure, dyslipidemia, polycystic ovary syn-drome, autoimmune, thyroid, kidney, neurological diseases, and cancer;
• pregnant or lactating women;
• subjects with problems with chewing, salivation, and swallowing.

Elimination criteria:

• subjects who only have samples (blood and calorimetry) from one study period (fasting or postprandial);
• subjects who only have samples (blood and calorimetry) of a metabolic challenge;
• subjects who have an infection at the scheduled appointment;
• subjects who withdraw their informed consent

There will be a single-blind principal investigator (PI). All results will be coded and will remain closed until the study is completed. The PI will remain blind to the results until the codes are opened at the end of the study.