How to Know You’re Burning Fat (instead of carbs) Dr. Merav & Michal Mor

Merav and Michal invented this breath-hold hack because in normal breathing, when you burn carbs and produce more CO2, your body immediately increases ventilation to exhale it, so exhaled CO2 doesn't change. By holding the breath for 10 seconds, the CO2 in the bloodstream reaches equilibrium with the air in the lungs, and the exhaled air then accurately represents what's happening in the blood. This simple technique essentially replicates the principle of invasive arterial-venous blood gas measurement that was used historically to determine metabolic fuel, but in a non-invasive consumer device. They contrasted it with traditional indirect calorimetry that uses a metabolic cart, which can be fooled by ketone metabolism and gluconeogenesis.

Merav explained that for athletic performance, this is the measurement she uses most. The app gives tailored recommendations: if you're in a carb-burning state, it suggests high-intensity work or, if you still want to do your planned workout, it recommends consuming carbohydrates to fuel it and prevent muscle protein breakdown. For fat-burning states, the recommendation is to stay in low to moderate intensity to prolong fat oxidation. This approach eliminates guesswork and helps prevent the common mistake of forcing high-intensity sessions when the body isn't fueled for them, which can lead to muscle catabolism.

Merav recounted the story of their older sister who had demonised carbs for years while struggling with weight and sleep in perimenopause. When Merav suggested she try eating complex carbs before bed, the sister thought she was crazy, but it worked amazingly: she slept better, had more energy, and her metabolic readings improved. The mechanism is that during perimenopause, women become less insulin sensitive, so blood glucose drops at night even without symptoms like waking up, causing cortisol elevations that prevent deep sleep. Consuming slow-digesting carbs before bed buffers glucose levels, reduces nighttime cortisol, and fosters restorative sleep. Better sleep leads to healthier mitochondria and improved fat oxidation the next morning. Thomas DeLauer reinforced this by noting his own experience that strategic carb use lowered his chronic sympathetic tone, enabling better fat loss over time.

Michal presented data from Lumen showing a metabolic cyclic pattern across the entire cycle, not just two phases. In the follicular phase, increasing oestrogen makes cells more insulin sensitive, so they utilise carbs more efficiently; avoiding carbs during this time can cause the body to demand them later, leading to strong cravings and overeating. Incorporating complex carbs prevents this. In the luteal phase, progesterone rises and the body favours fat oxidation; intermittent fasting amplifies this effect, making it an ideal time for fat loss efforts. This protocol respects the endocrine physiology and can help women avoid the frustration of fighting their own hormones.

Michal described an experiment in the Lumen community where participants measured their metabolism before bed and upon waking. When they stopped eating earlier and avoided screens before bed, their pre-bed measurement shifted from nearly 100% carb burning to about 75% carbs, and they woke up with a stronger fat-burning state. This demonstrated that simple behavioural changes have measurable metabolic impacts. The circadian rhythm is an internal clock that signals mitochondria when to rest and repair; when we eat late or expose ourselves to light late at night, the clock is disrupted and mitochondria remain in 'work mode', leaving them tired and inefficient in the morning. This ties into the broader point that sleep and stress are as important as nutrition for metabolic health.

Merav noted that if you start a workout at 80% carbs and finish at only 30% carbs (i.e., more fat burning), it means you depleted glycogen and are now in a fat-burning state, so you can delay carbs to prolong that benefit. Conversely, if you shift further into carb burn, the workout intensity was high enough that glycogen was heavily used, and taking carbs and protein afterward is crucial for recovery and preventing muscle breakdown. This eliminates the one-size-fits-all approach to post-workout nutrition and lets users individually gauge whether they need to refuel or can ride the fat-burning wave. Michal emphasised that within-person variation is still high due to daily changes in stress, sleep, and for women, menstrual cycle, so repeated measurements build a personal picture.

Michal shared a personal experiment: despite measuring her metabolism daily and knowing she should be burning fat while resting, she was stuck in constant carb burning due to work stress. A 5-day Vipassana retreat (no talking, no phones, just sitting and breathing) completely reset her nervous system. After returning, her parasympathetic system dominated, and she finally began burning fat in resting states — something she could feel subjectively. This illustrates the profound effect of psychological stress on mitochondrial fuel selection and that sometimes the metabolic fix isn't about diet or exercise at all.

Michal Mor explained that both ketone oxidation and gluconeogenesis (when glycogen is critically depleted) produce high CO2, which can mislead traditional tools into reporting a predominantly carb-burning state. This is a significant blind spot in metabolic measurement, especially during fasting, keto diets, or menopause when muscle breakdown becomes more likely. By adding a sensor that captures acetone — a byproduct of ketone metabolism — Lumen aims to add a 'third dimension' to its measurements, giving users a true depth of fat-burning beyond just the momentary CO2/O2 ratio. She and Merav see this as a major innovation that will give deeper insight into how deeply someone is in fat-burning mode, not just what they're burning right now.

Merav and Michal explained that in metabolic disorders, people are unable to use fat properly because mitochondria are damaged, but they also can't use carbs effectively due to insulin insensitivity. This leaves them with high circulating fatty acids and glucose that the mitochondria can't oxidise, a state they call metabolic purgatory. This is why people with high BMI, as shown by Lumen's 75-million-measurement database, are less flexible and cannot shift into carb oxidation after a carbohydrate meal; the excess carbs linger in the blood longer, eventually leading to diabetes. Merav described it as a 'physiology conflict' where there's plenty of fuel but no ability to burn it, and the mitochondria becomes the 'hero' that needs help.

Michal shared that their published research shows a metabolic cyclic pattern where women tend to use more carbs in the first phase (follicular) and more fat in the second (luteal). However, the pattern is not a simple binary switch; the metabolism changes day by day. This means that blanket low-carb advice can be counterproductive in the first phase, leading to carb cravings (and fridge raids) because the body's demand for carbs is hormonally driven. In the second phase, intermittent fasting can be a powerful tool to optimise fat burning. Merav emphasised that each woman's pattern is unique, so personal measurement is key, but the underlying hormonal influence is consistent across the population.

Michal explained that while VO2max is well-known, it's useless for someone like their late father who had diabetes and high BMI — he couldn't have completed the test. Fat max, by contrast, can be estimated from submaximal exercise and Lumen's methodology. Improving fat max means you can exercise at higher intensities while still burning fat, which directly indicates improved mitochondrial health. This makes it a powerful clinical and wellness metric that could help a much larger population track and improve metabolic health, not just athletes.