Today, more than 2.5 billion people worldwide are overweight or have obesity1, excessive fat accumulation, posing risk factors for various diseases like type II diabetes, dyslipidemia, cardiovascular disease, and non-alcoholic fatty liver disease (NAFLD). The World Health Organization (WHO) defines people with overweight as a body mass index (BMI) over 25 kg/m² and people with obesity as over 30 kg/m². However, in Asia, individuals with a BMI < 25 face increasing prevalence of hyperglycemia, impaired glucose tolerance, lipid metabolism disorder, and metabolic syndrome due to excessive visceral fat accumulation2,3. Most obesity cases are associated with lifestyle disorders, such as overeating and lack of exercise. Moreover, low-income countries witness a rapid rise in people with obesity due to high-carbohydrate diets centered on staple crops1. Weight loss can prevent or slow down the progression of obesity-related diseases, thus, improving dietary habits holds potential to reduce health issues and associated medical costs4.
In recent years, various functional ingredients with anti-obesity and lipid metabolism improvement properties have been discovered in natural products; specifically, there are considerable reports on polyphenols found in plants. Tea polyphenols are known to inhibit lipid absorption, while tea catechins, chlorogenic acid in coffee, capsaicin in capsicum, and curcumin in turmeric inhibit fat accumulation and promote its degradation5. While these pharmacological activities have mostly been verified through experiments on mammals, there is a growing movement to abolish animal testing due to animal welfare concerns. Consequently, alternative methods to animal testing are being advocated6. The primary alternative method is cell culture systems, but accurately predicting biological reactions involving interactions between tissues and organs is challenging. Insects, however, have attracted attention as pathological models with no animal ethics issues.
Insects may appear vastly different from humans, but they possess tissues and organs crucial for glucose and lipid metabolism, with evolutionarily conserved cell signaling pathways. For instance, insects feature a midgut analogous to the mammalian digestive tract, and a fat body analogous to the mammalian liver and adipose tissue. Drosophila melanogaster serves as a representative model organism that demonstrate diet-induced obesity similar to humans when fed high-sugar or high-fat diets, exhibiting characteristics, such as hyperglycemia, increased triglyceride (TG) levels, activation of inflammatory signals, and insulin resistance7,8,9,10,11,12. Consequently, Drosophila are utilized as models for obesity and fatty liver disease to evaluate the effects of plant extracts and lactic acid bacteria13,14,15,16. However, their extremely small body size poses challenges in measuring food intake and collecting tissues. In mammals, the liver, muscle, and adipose tissue play crucial roles in lipid metabolism, underscoring the importance of assessing metabolic changes in each tissue in an insect model. Moreover, since food intake significantly influences fat accumulation and degradation, precise quantification becomes imperative. The silkworm, Bombyx mori, has attracted attention as an insect model due to its larger body size.
Fifth instar silkworms are large, measuring about 4–6 cm, facilitating the easy collection of hemolymph, fat bodies, and muscle. Moreover, these silkworms, daily, consume food equivalent to their own body weight, experiencing significant growth from 1 g after molting to 5 g within three days during which fat accumulates in the fat body. This makes silkworms ideal for verifying the effects of diet-induced inhibition of fat accumulation and degradation, a process that typically spans 10–25 days in Drosophila, within a few days. Furthermore, when fed an artificial diet containing (10GD) for 18 h, silkworms display symptoms similar to human lipid metabolism disorders, including hyperglycemia, increased fat body weight, and increased TG levels in fat bodies and hemolymph17. Despite these similarities, currently, there are no established evaluation systems for lipid metabolism enhancers utilizing silkworms. Additionally, Drosophila fed a high-fat diet supplemented with fenofibrate (FBT), a drug used to treat hyperlipidemia, exhibit lower TG levels in their bodies16, through the precise mechanism remains unclear.
In this study, we established a silkworm with obesity and lipid metabolism disorder (SOLD) model by feeding the worms with a 10GD diet for 72 h. Then, we evaluated the usefulness of the model using the therapeutic effect of FBT and epigallocatechin gallate (EGCG), a functional anti-obesity ingredient.