## 1 Introduction  
According to the definition by the World Health Organization (WHO), obesity is an unhealthy condition resulting from the abnormal or excessive accumulation of fat in adipose tissue [1; 2]. The body mass index (BMI) is a person's weight (kg) divided by the square of their height (m); individuals with BMI ranges between 25 and 30 are considered to be overweight and obese [1; 2; 3]. According to the WHO report on obesity, over 1.9 billion adults aged over 18 years old are overweight, and 600 million are reported to be obese, which makes obesity a problem in both developed and developing countries [4; 5]. In Korea, according to the National Health and Nutrition Examination Survey, the prevalence of obesity among adults aged 19 years old and older is reported to be ~40% for men and ~26% for women. By 2030, the prevalence rate is expected to reach ~62% for men and 37% for women [6]. Among the factors that cause obesity are lack of exercise, stress, genetic factors, and endocrine disorders. Obesity is mainly caused by excessive body fat accumulation due to excessive calorie intake beyond consumption [7]. Adipocytes are endocrine organs that control several metabolic processes that regulate the body's physiology. Adipocytes secrete various hormones that regulate metabolism, such as leptin and adiponectin, which are essential for maintaining homeostasis [8]. When excessive fat accumulates in adipocytes, functions such as intracellular endoplasmic reticulum stress and mitochondrial dysfunction are lost, and the risk of complications of obesity, such as hypertension, cardiovascular disease, cancer, and fatty liver, increases through insulin resistance and inflammatory responses [9; 10]. Oxidative stress that affects obesity-related diseases increases the probability of various diseases by inducing chemical changes in the human body. Therefore, it is necessary to find antioxidant substances to protect cells in the human body from free radicals and prevent obesity. Obesity-related studies have used mouse fibroblast 3T3-L1 preadipocytes to analyze the life cycle mechanisms of adipogenesis, lipogenesis, and lipolysis. The production and differentiation of adipocytes are regulated by transcription factors, such as CCATT/enhancer-binding protein \(\alpha\) (C/EBP\(\alpha\)) and peroxisome proliferator-activated receptor \(\gamma\) (PPARy) [11]. C/EBP\(\alpha\) levels increase in the late stage of differentiation along with those of PPARy and induce the expression of genes that induce adipogenesis such as fatty acid synthase (FAS) and adiponectin (ADIPOQ). Their expression levels tend to markedly increase in differentiated adipocytes [12]. The regulation of genes related to adipocyte differentiation is one of the most important strategies for obesity prevention and treatment [13]. Therefore, therapeutic agents effective for adipocyte differentiation and the inhibition of adipogenesis are being released. The available drugs against obesity include oralistat, topiramate, sibutramine, rimonabant, and phenylpropanolamine; however, these drugs have side effects such as insomnia, anorexia, gastrointestinal disorders, increased blood pressure, and heart disease [14]. Overall, the only two drugs currently approved by the Food and Drug Administration (FDA) for long-term use are orlistat and sibutramine, while others are available only for short-term or limited use [15]. To address this problem, research on the development of treatments against obesity using safer natural products or phytochemicals that have fewer side effects is active [16]. Among such natural products, _Sargassum horneri_ (SH) and _Ulva australis_ (UA), used in the present study, are marine waste resources generated in large quantities on Jeju Island, Republic of Korea. SH, that belongs to the Sargassaceae algae, is a yellow-brown plant with thin, spatula-shaped leaves that grows mainly in winter. UA is a green alga in the Ulvacae family and inhabits the middle and lower intertidal zones.