Q&A: Are There Truly 'Negative-Energy' Foods?

Take-home messages:

Introduction

Food energy is commonly expressed in kilocalories (kcal, often simply called calories) or kilojoules (kJ), with kilojoules being the international standard unit for energy. To convert calories to kilojoules, multiply the number of calories by 4.2. The fascination with 'negative-calorie', 'negative-energy', or 'zero-net-calorie' foods and beverages emerges from their potential role in nutrition and weight management strategies [1]. If they exist, incorporating these items into a diet might lead to an energy deficit, thereby facilitating weight loss that may prove substantial over time. These are foods or drinks purported to create a unique metabolic scenario: the energy expended to process them (through digestion, absorption, transportation, metabolism and storage of nutrients) supposedly surpasses the energy they provide. This phenomenon hinges on the 'thermic effect of food', also known as specific dynamic action or energy or diet-induced thermogenesis, which is the energy required for the body's processing of food [1, 2]. The concept of negative-calorie foods has gained traction and popularity over the years, partly due to the publication of diet books such as "Foods That Cause You To Lose Weight" by Neal Barnard, first published in 1992 and then in 1999 [3], and "The Negative Calorie Diet" by Rocco DiSpirito, released in 2015-2016 [4]. These works have significantly contributed to the widespread recognition of the idea [1].

This examination delves into the reality of negative energy foods, focusing on water, chewing gum and celery to evaluate the scientific support for their classification as such. It seeks to clarify whether these items indeed result in a net energy loss and to differentiate between so-called negative-energy foods and 'free' foods. While 'free' foods are low in energy and carbohydrates, beneficial for managing certain health conditions, they may not achieve the same metabolic effect as negative-energy foods. Additionally, this exploration addresses the 'negative energy or calorie illusion', shedding light on the complexities of dietary perceptions and their implications for nutrition practices.

Water as a Possible Negative Energy Food: The Role of Temperature in Energy Expenditure

Water, while devoid of any energy content, can potentially act as a negative energy food due to the energy expended by the body to warm ingested water to its internal temperature, an effect known as water-induced thermogenesis. Drinking water at a temperature different from the body's will expend some energy to maintain the body’s internal temperature, a phenomenon known as the water-induced thermogenesis effect. Because it requires one kilocalorie of heat energy to raise the temperature of 1 kg of water by 1°C, heating 250 g (the amount in a normal glass) of water from 3 to 4°C (refrigerator temperature) to body temperature, approximately 37°C, requires 33.5 kcal/1000 g × 250 g, which equals 8.375 kcal or 35.18 kJ. Boschmann and colleagues [5] found that drinking 500 ml of water boosted the metabolic rate by 30%, a significant increase compared to other studies. In contrast, most studies summarised by Brown et al. [6] did not observe any increase, although two studies did report slight increases of 2.7% [7] and 2% [8], respectively.

In an effort to replicate Boschmann and team's findings [5], Brown and colleagues [6] explored the thermogenic effect of water and its potential variation with osmolality, a measure of solute concentration in a solution, or temperature. They concluded that neither distilled nor saline water increased energy expenditure. Interestingly, they observed an energy expenditure increase with sugar (sucrose) water. The study noted a modest 4.5% increase in energy use over 60 minutes when participants consumed water at 3°C in a quantity of 7.5 ml per kg of body weight. This finding suggests that while water, in general, may not have a substantial thermogenic effect, specific types of water or conditions may slightly elevate energy usage.

While water-induced thermogenesis or osmolality may not significantly increase energy expenditure, replacing high-energy containing beverages, such as sugary sodas (often termed 'empty calorie' or ‘empty energy’ drinks because they provide a high amount of energy with minimal nutritional value), with water can lead to weight loss [9]. One of the dietary guidelines recommended by nutrition experts, is the consumption of ample amounts of clean and safe water [10]. However, it's important to balance water intake, as excessive consumption can lead to water intoxication, a condition where the balance of electrolytes in the body is disrupted [11].

Chewing gum sweetened with artificial sweeteners as a possible negative energy food

Chewing gum, especially the sugar-free variety aimed at freshening breath and oral hygiene rather than bubble gum designed for bubble blowing, may be considered for its minor negative-energy potential due to the energy spent during mastication. However, its potential for energy expenditure is limited because the thermic effect of food is lessened when the food bypasses substantial chewing or digestion processes.

Table: How to distinguish between chewing gum and bubble gum

Footnote: The 'ol' at the end of xylitol and sorbitol indicates they are sugar alcohols.

Chewing, or mastication, incurs a modest energy expenditure of approximately 46.2 kJ (11 kcal) per hour [12], a figure considerably lower than the 1000–3000 kJ (240–720 kcal) per hour burned through traditional forms of exercise, depending on their intensity and type [11]. Considering that a single piece of gum—whether a strip, pellet or stick—contains about 42.0 kJ (10 kcal), one would need to chew continuously for over an hour just to neutralise the energy ingested from the gum itself.

This analysis positions chewing gum as a potential energy-negative food, provided it is chewed for a sufficient duration to burn more energy than it delivers. However, it's important to recognise that while mastication does not significantly contribute to energy expenditure, chewing gum can still offer benefits beyond energy balance, such as aiding in appetite control and reducing overall energy intake (as detailed in the table below). Also the sugar alcohols used to sweeten chewing gum might be beneficial, because xylitol seems to reduce the occurrence of dental caries (as summarised in a systematic review and meta-analysis [13]). A word of caution is necessary here, as case studies indicated that chronic diarrhoea and severe weight loss could be caused by the sugar alcohol sorbitol in chewing gum [14].

For those who do not find chewing gum appealing, evidence from a summary of 17 trials suggests that increasing the number of chews per bite of actual food can help control cravings by promoting a longer-lasting sense of fullness, thereby offering similar benefits. [15]. Nevertheless, it's important to emphasise that habitual gum chewing or increasing the chews of actual food per bite should not be considered substitutes for maintaining a balanced diet and engaging in regular physical activity, which are the cornerstone practices for effective weight management.

Table: The relationship between chewing gum and weight loss

Celery as a possible negative energy food

Celery is often mentioned as a food that might use more energy to digest than it provides (67 kJ or 16 kcal per 100 g), making it a candidate for a ‘negative energy’ food. Research in this area includes a study where one person ate raw and liquefied celery, finding they burned more energy digesting it than the celery had (as reviewed by [1]). Another study with fifteen women showed that after eating 100g of celery, the energy used for digestion (thermic effect of food) was 86% of the energy intake from the celery itself [22]. However, this did not lead to a negative energy balance. Further research using reptiles [1, 23], chosen for their similarity to humans being omnivorous and their willingness to eat celery, also found a net energy gain from eating celery. Given the findings that celery does not result in a negative energy balance but can support weight management efforts due to its high fibre and water content, it is more precise to categorise celery as a "free food" in dietary contexts.

Free foods

Individuals with diabetes or those monitoring their dietary intake are likely familiar with the concept of 'free foods,' which are considered nutritionally insignificant and were once thought to be energy-negative until more research became available. Defined as any food or beverage that contains fewer than 84 kJ (20 calories) or less than 5 grams of carbohydrate per serving, these items play a crucial role in dietary management for those needing to control blood sugar levels and overall caloric intake [24]. These include celery, berries, grapefruit, carrots, tomatoes, cucumbers, watermelon, apples, zucchini (baby marrow), broccoli, cabbage and lettuce among others. Reducing energy intake by including low energy containing foods are key to keep the dieter from feeling hungry, while keeping energy intake low. Foods considered to be free has such low energy content that dieters could consume them without risking weight gain. Note that one should not confuse these foods with empty energy/calorie foods which are on the opposite side of the spectrum providing huge amounts of energy while lacking in micro-nutrients (vitamins and minerals). Diets based on so-called negative-energy food or to use the more acceptable term 'free foods' do not work because they cause an energy deficit, but rather because these foods satisfy hunger with food that is not energy dense and coupled with exercise can lead to burning more energy than was ingested to create an overall energy deficit. However, a word of caution is needed because studies indicate that people would underestimate the energy content of a food/meal when a healthy food such as free food is present a phenomenon coined by Chernev as the 'negative calorie illusion'' [25-27].

Negative energy or calorie illusion

The ‘negative energy illusion’ is a psychological effect where individuals judge the energy content of a meal combining high-energy and low-energy foods to be lower than the high-energy food alone [25-27]. Chernev and colleagues' seminal research [26] and those by Wang et al. [28] found that participants estimated a meal with both a vice (e.g., cheeseburger) and a virtue food (e.g., celery sticks) as having fewer energy than the vice food by itself, indicating that adding healthy items might lead to a distorted perception of a meal's energy total. Paradoxically, Chernev [27] observed that those who were more concerned with their body weight and management thereof (the 'weight-conscious') were more prone to this bias. 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. However, further research has yielded inconsistent results regarding this illusion [25], suggesting it may depend on specific factors like contrast effects or the use of anchoring to address people's limited knowledge of energy counts, highlighting the complex nature of how we estimate and perceive energy content in food pairings.

Conclusion

Although the concept of negative-energy foods, including both cold or hot water and chewing gum, may seem appealing, their actual impact on weight management is minimal. There remains a need for further comprehensive research to conclusively determine the scientific validity of negative-energy foods and beverages.

While incorporating low-energy, nutrient-rich foods such as 'free foods', fruits and vegetables into one's diet is beneficial for weight management and overall health, it's important to understand that these should not be viewed as creating a negative energy balance. Caution is advised due to the negative energy illusion phenomenon, as it may lead to misunderstandings about diet and energy consumption that do not support healthy weight management practices. Ultimately, the most effective and healthful approach to weight management or maintenance is not through seeking out supposed negative-energy foods, but rather through maintaining a balanced diet and engaging in regular physical activity.

Reflections

·         Take a closer look at the 'free' foods within your diet—those low in energy and carbohydrates like vegetables and certain fruits. Assess how they fit into your overall energy intake and nutrition. Are they helping you feel fuller without significantly adding energy? Reflect on whether there are opportunities to enhance your dietary choices by incorporating more ‘free’ foods to improve satiety.

·         While the concept of negative-energy foods is intriguing, it's important to question their true impact on weight management. For instance, could substituting sugary drinks with cold or hot water help reduce your energy intake? If you are regularly snacking on higher energy dense foods such as sweets and chips consider chewing gum instead. If you're a regular gum chewer, ponder on whether it is affecting your appetite, hunger levels and your snacking behaviour.

·         Start paying closer attention to the nutritional information on the food packages, particularly the energy content per 100 g. This practice can improve your understanding of how different foods contribute to your daily energy intake.

References

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2.         Calcagno, M., et al., The thermic effect of food: a review. Journal of the American college of nutrition, 2019. 38(6): p. 547-551.

3.         Barnard, N., Foods that Cause You to Lose Weight: The Negative Calorie Effect. 1999: Harper Collins.

4.         DiSpirito, R., The Negative Calorie Diet: Lose Up to 10 Pounds in 10 Days with 10 All You Can Eat Foods. 2015: HarperCollins.

5.         Boschmann, M., et al., Water-Induced Thermogenesis. The Journal of Clinical Endocrinology & Metabolism, 2003. 88(12): p. 6015-6019.

6.         Brown, C.M., A.G. Dulloo, and J.-P. Montani, Water-Induced Thermogenesis Reconsidered: The Effects of Osmolality and Water Temperature on Energy Expenditure after Drinking. The Journal of Clinical Endocrinology & Metabolism, 2006. 91(9): p. 3598-3602.

7.         Komatsu, T., et al., Oolong tea increases energy metabolism in Japanese females. J Med Invest, 2003. 50(3-4): p. 170-5.

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28.       Wang, Z. and T. Begho, Investigating cognitive biases: Does halo effect from nutrition or health claims drive negative calorie illusion in food combinations? International Journal of Food Science & Technology, 2024. 59(1): p. 208-218.

In the process of creating this work, the author employed ChatGPT 4 to streamline the text and to perform language editing.