The concept of "negative calorie" foods has long been a topic of interest in the realm of nutrition and weight management. These foods are purported to require more energy to digest, absorb, and process than they actually provide, resulting in a net calorie deficit and, theoretically, weight loss. While the idea of eating your way to thinness is appealing, a closer look at the science reveals a more complex reality.
Understanding Calorie Restriction and Energy Balance
Before delving into the specifics of negative calorie foods, it's crucial to understand the fundamental principles of calorie restriction (CR) and energy balance. Calorie restriction, defined as a sustained reduction in energy intake while maintaining sufficient nutrient supply, is a well-studied intervention known for its potential to attenuate aging and prevent chronic metabolic diseases. The primary rationale for CR extends beyond simple weight loss; it aims to reduce metabolic rate, the energy expended by an organism at rest to maintain vital body functions.
The First Law of Thermodynamics dictates that energy intake during an observation period equals the sum of total daily energy expenditure (TDEE) and changes in body energy stores. TDEE comprises resting energy expenditure (REE), diet-induced thermogenesis (DIT), and activity-related energy expenditure (AEE). Resting EE is the energy expended at rest. Diet-induced thermogenesis can be measured with the same methodologies performed before and after consumption of a meal, yet is routinely assumed to equate to 10% of total daily EE. Activity-related EE then is calculated as the difference between total daily EE and resting EE plus diet-induced thermogenesis.
Debunking the Negative Calorie Myth
The claim that certain foods possess negative calories is largely a myth. Although it’s true that most of these foods are nutritious, it’s unlikely that any of them are negative-calorie. Foods commonly cited as negative calorie, such as celery, grapefruit, broccoli, tomatoes, and cucumbers, are indeed low in calories, with approximately seven to 30 kcal per 100g. However, the energy required to process these foods is less than the energy they provide.
One small trial investigated the effects of celery - often referenced as the ultimate negative calorie food due to its low net calorie content, providing just 16 kcals per 100 g. Of the 16 kcal provided by the celery, only 2.24 kcal were absorbed. The hypothesis behind negative calorie foods proposes that, for certain food items, the energy required for your body to process them is greater than the energy the food itself provides.
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The thermic effect of food (TEF), the energy expended to digest, absorb, and metabolize nutrients, is a real phenomenon. However, it accounts for only about 10% of the energy we take in. For example, the energy used to process foods is about 5-10% of the calories the food contains for carbs, 0-5% for fat and 20-30% for protein. Most alleged negative-calorie foods are composed primarily of water and carbs, with very little fat or protein. It’s unlikely that the energy used to digest these foods is dramatically higher than for other carb-based foods, though this has not been studied specifically.
The Role of Water and Chewing
Water, being calorie-free, is sometimes touted as a negative-calorie beverage. Drinking cold water does expend some calories. However, such small effects will not substantially increase the calories your body burns. Some research supports small increases in metabolism for a short period after drinking cold water, ranging from about 3-24 calories over one hour.
Chewing gum (sugar-free) has also been suggested as a negative-energy food. Mastication burns approximately 11 kcal (46.2 kJ) per hour. Given that one stick of gum contains around 10 kcal (42.0 kJ), it would need to be chewed for an hour or more to burn off the energy it provides.
Low-Calorie Foods: A Realistic Approach to Weight Loss
While negative-calorie foods may be a myth, incorporating low-calorie, nutrient-rich foods into your diet remains a sound strategy for weight management. These foods allow you to consume a large volume of food without excessive calorie intake, promoting satiety and preventing overeating.
Examples of nutritious, low-calorie foods include:
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- Vegetables: Celery, lettuce, spinach, kale, broccoli, cucumbers, tomatoes, carrots, zucchini, cabbage.
- Fruits: Grapefruit, watermelon, apples, berries (blueberries, raspberries).
These foods are not only low in calories but also rich in vitamins, minerals, and fiber, contributing to overall health and well-being.
The Importance of a Balanced Diet and Lifestyle
Focusing solely on a select list of alleged negative-calorie foods can lead to nutritional imbalances. A well-rounded diet should include a variety of whole foods, encompassing protein, healthy fats, and complex carbohydrates.
Furthermore, lifestyle factors such as physical activity play a crucial role in weight management. Regular exercise increases energy expenditure and helps maintain a healthy body composition.
The "Negative Calorie Illusion"
It's important to be aware of the "negative calorie illusion," a psychological phenomenon where people underestimate the energy content of a food or meal when a healthy food is present. This bias can lead to overconsumption of less healthy foods, ultimately hindering weight loss efforts. Dietitians should educate their patients on how to estimate a food’s energy value more accurately and not to be deceived by the presence a healthy food that could lead to a perceptual bias and ultimately weight gain.
Metabolic Adaptation: A Key Consideration
Studies on calorie restriction have revealed a phenomenon known as metabolic adaptation, where the body reduces energy expenditure beyond what would be expected from weight loss alone. This adaptation can make it more challenging to maintain weight loss in the long term.
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A decline in EE which is beyond what would be expected relative to the changes body mass (or composition) is termed metabolic adaptation. Metabolic adaptation was the primary outcome of the CALERIE studies and was investigated after 3 and 6 months of 25% prescribed CR in CALERIE Phase 1, and after 12 and 24 months in CALERIE Phase 2. As hypothesized, after 6-24 months of CR, metabolic adaptation was observed during sleep, at rest, over 24 h in the confined environment of the room calorimeter, and over 14-days in free-living conditions. The metabolic adaptation observed during sleep, which is arguably the most reproducible measure of metabolic rate, was 8% at 3 months, 7% at 6 months, 6% at 12 months, and 5% at 24 months. In contrast the metabolic adaptation in free-living conditions was almost double at each time point across the 24-month period (13% at 3 months, 7% at 6 months, 8% at 12 months and 9% at 24 months, respectively). The larger metabolic adaptation in free-living conditions suggests that the culprit of such adaptations is not only changes in metabolic processes but also compensations in behaviors likely occurring in an effort to conserve energy. Importantly, body weight loss occurred only during the first 6-12 months, after which time it was maintained. This implies that metabolic adaptation is not exclusive to periods of chronic energy deficiency, but that it persists in energy balance. The cause of metabolic adaptation appears to be specific to calorie restriction because studies that demonstrated exercise-induced weight loss did not observe metabolic adaptation.