Metabolism, explained

Understanding Metabolism: A Broader Perspective

Metabolism is the conversion of food into energy, but it’s not only about what we eat. Our body also relies on stored fat and oxygen, meaning even the air we breathe plays a role. Environmental factors, toxins, and lifestyle habits all contribute to metabolic health. Metabolism represents the sum of all energy reactions in the body and is influenced by factors such as sleep quality and insulin response. The energy produced is ATP, the body’s energy currency. Recent discussions on the Zoe podcast with Dr. Tim Spector help clarify the complexity of these processes and their impact on health.

How Widespread Is Metabolic Dysfunction?

A shocking fact: only 12% of the US population is metabolically healthy, with similar figures across Europe. This statistic comes from studies analyzing triglyceride levels, body fat percentage, and HDL/LDL ratios. A helpful analogy is to think of metabolism like a home heating system: some houses maintain a stable temperature efficiently, while others must work harder to achieve the same result.

However, efficiency doesn’t always mean health. Overconsumption of refined carbohydrates—white bread, white rice, or orange juice—especially without exercise, causes sharp spikes in blood sugar and insulin. This is an efficient process metabolically but is unhealthy. Complex carbohydrates and low-glycemic foods release glucose more slowly, resulting in a lower insulin response. But each person has a unique metabolic fingerprint and gut microbiome. Some people struggle with whole grains, while others spike in insulin after brown rice. Monitoring how your body reacts to different foods or checking blood sugar levels can help understand personal metabolic responses.

Interestingly, around 20% of people seem unaffected in terms of body composition despite poor diets or environmental factors. Timing is also important: refined carbs during or after exercise are absorbed efficiently without spiking insulin. Pairing carbs with fats or proteins slows digestion. True metabolic health is what works best for you at this moment, allowing efficient food processing, good energy levels, and healthy body composition.

Ultra-Processed Foods and Gut Health

Ultra-processed foods negatively affect metabolism. A crossover study compared a real sandwich with wholegrain bread and cheese to an ultra-processed version with white bread and cheese product containing the same calories, fats, proteins, and carbs. The processed sandwich caused a 50% lower energy expenditure, showing that breaking down ultra-processed foods is less efficient. This illustrates that metabolism is more than calories in versus calories out.

Ultra-processed foods are pervasive. In the US, 60% of adults and 70% of children eat diets high in these foods, with even higher figures in the UK. These foods disrupt the gut microbiome. Emulsifiers, artificial sweeteners, and preservatives hinder bacteria, sometimes damaging gut lining and causing inflammation in the body and brain. Neuroinflammation is linked to poor metabolic health and cognitive decline, sometimes called “type 3 diabetes.” Cold-pressed olive oil and omega-3-rich fish can help reduce this inflammation.

Manufacturers also design ultra-processed foods to bypass natural satiety signals. Soft textures, intense flavors, and rapid dissolution trick the body into overeating, which contributes to receding jawlines in children due to insufficient chewing.

The Role of Mitochondria and Exercise

Mitochondria, the “energy factories” in cells, are central to metabolism. About 10% of body mass consists of mitochondria, which break down fats, carbohydrates, and proteins into energy. However, metabolism also requires nutrients to be broken down, absorbed, transported, and delivered to mitochondria. Stored fat must be dismantled into free fatty acids before utilization. Increasing mitochondrial numbers significantly enhances metabolic health.

Training with two Zone 2 sessions and one high-intensity interval session per week for 3–4 months can double mitochondrial density in muscle fibers, from 2% to 4%, with a maximum potential of 8%. Considering there are 250–300 million muscle fibers, this represents a huge metabolic advantage. Exercise also triggers exokines that stimulate mitochondrial biogenesis throughout the body, even in cells like those in the cheeks.

Resistance training doesn’t significantly increase mitochondria (unless over 70) but builds lean muscle, which burns more energy at rest than fat tissue. More muscle mass increases daily energy expenditure.

Sleep: A Crucial Factor

Sleep dramatically impacts metabolic health. A University of Chicago study used a controlled crossover design: participants followed the same calorie-restricted diet under two conditions—8.5 hours of sleep per night versus 5 hours. Participants with ample sleep lost 50% more body fat. Sleep-deprived individuals also lost weight but lost up to 60% more muscle, which is key for metabolic health.

Sleep quality matters as much as quantity. Blue light from screens before bed suppresses melatonin and disrupts circadian rhythms. At least 30 minutes of screen-free time before sleep improves metabolic outcomes.

Chemicals, Genetics, and Environmental Factors

We are exposed to billions of man-made chemicals worldwide. One infamous “forever chemical” is Teflon—98% of people tested have these compounds in their blood. The long-term effects on metabolism are still uncertain.

Genetics also influence metabolism, including fat storage and energy regulation. However, the sharp rise in metabolic diseases over the past 50 years cannot be explained by genetics alone. Our ancestors’ genes were adapted to environments with natural diets and scarce food, while modern ultra-processed foods and calorie abundance overwhelm these adaptations.

Research on ultra-processed foods continues, but the food industry often slows progress to protect cheap, processed products from scrutiny. Combined with chemicals, plastics, air quality, and poor sleep, these factors create a challenging environment for maintaining metabolic health.

Conclusion: Building Resilient Metabolic Health

Metabolic health is much more than calories or exercise alone—it is the result of a complex interplay between diet, lifestyle, sleep, genetics, environmental exposures, and even the quality of the air we breathe. Ultra-processed foods, sleep deprivation, low physical activity, and exposure to “forever chemicals” all challenge our body’s ability to efficiently produce energy and maintain optimal function.

The good news is that many aspects of metabolism are within our control. Prioritizing whole, minimally processed foods, building lean muscle through resistance and aerobic exercise, protecting sleep quality, and incorporating anti-inflammatory nutrients such as olive oil and omega-3s can dramatically improve energy efficiency, gut health, and cognitive function. Understanding your unique metabolic fingerprint—how your body responds to different foods, activity, and environmental factors—allows you to make smarter, personalized choices that support long-term health.

Ultimately, metabolism is not just a biological process—it’s a lifestyle. The actions we take every day, from what we eat to how we move, sleep, and protect ourselves from environmental toxins, shape our energy, vitality, and resilience for decades to come. By focusing on these foundational factors, we can empower our bodies to function at their best and reduce the risk of chronic disease, while enhancing overall well-being.