Background
Obesity and overweight currently affect about a third of the world’s population [
1]. In total, 38% of adults are expected to be overweight and 20% to be obese by 2030 [
2]. Obesity is linked to all metabolic-related diseases and is also associated with more than 200 different complications. For example, more than 80% of cases of diabetes can be attributed to obesity, and obesity is also associated with diabetes-related deaths. Overweight and obesity also increase tumor risk, and 40% of tumors in the United States were estimated to be caused by overweight and obesity in 2014 [
3]. Obesity significantly increases the risk of death. The relationship between obesity and mortality has been evaluated in several large-scale epidemiological studies. For example, a meta-analysis of more than 30 million people in 230 cohort studies showed a significant increase in all-cause mortality rate with overweight and obesity [
4,
5]. The treatment of obesity and obesity-related diseases also carries a huge economic burden. In addition to direct economic expenses, there are other costs such as loss of labor and decline in household income. It is therefore important and urgent for overweight and obese people to lose weight [
6]. Weight loss can significantly decrease the incidence of obesity- and overweight-related diseases and the associated mortality, reduce the incidence of cardiovascular diseases [
7], significantly delay the progress of impaired glucose tolerance to diabetes mellitus [
8], and improve urinary incontinence, sleep apnea, depression, quality of life and physical function.
Obesity is a complex polygenetic condition that is affected by genetic, behavioral, socioeconomic and environmental factors, but it is preventable and treatable [
9]. Medication and surgery are only recommended for specific groups, and the first-line treatment for obesity is diet management combined with exercise [
10]. Many studies have shown that the right diet and exercise guidance can help obese and overweight people to achieve short-term weight loss. However, the recommended dietary guidelines for weight loss vary significantly and have been revised several times in different social groups, reflecting uncertainties in the field of obesity nutrition management and the difficulty of establishing a uniform recommendation for all social groups. Unfortunately, most people who successfully achieve short-term weight loss (about 70%) tend to regain at least 50% of the weight over the next 2 years and return to their previous weight levels within 3 to 5 years [
11]. It is therefore very important for clinicians and dieticians to explore reasonable weight management strategies for obese people that address both short-term weight loss and subsequent weight maintenance [
12‐
15].
A high-protein diet can provide sufficient satiety and food taste to resist the hunger caused by calorie restriction and can therefore increase compliance. Compared with a high-carbohydrate diet, a high-protein diet has a more significant thermic effect, which increases energy consumption [
16]. A high-protein diet combined with resistance training can reduce the loss of fat-free body mass during weight loss and maintain a relatively high basal metabolic rate. It can also develop good dietary management habits and improve self-efficacy in the initial weight loss stage. This improvement in self-efficacy comes from enthusiasm for diet intake management and exercise. People with low self-efficacy tend to be more likely to give up and regain weight [
17]. There have been a few international studies on short-term weight loss and long-term weight maintenance linked to a high-protein diet [
18], but high-quality research on the effect of a high-protein diet on weight loss in the Chinese population is very limited [
19]. More importantly, there are only a few international and Chinese studies on the role of resistance training in medium- and long-term weight maintenance, and the results are inconsistent [
20‐
23]. We hypothesized that resistance training combined with a high-protein diet would result in similar short-term weight loss but better long-term weight maintenance than either a conventional low-fat diet control or a high-protein diet alone.
The objectives of this study were therefore: 1) to verify that a high-protein diet has a similar short-term weight loss effect as a traditional low-fat diet; 2) to verify that a high-protein diet combined with resistance training can achieve better medium- and long-term weight maintenance; and 3) to explore whether the improvement in self-efficacy and the ability for diet management during weight loss is a predictive factor for medium- and long-term weight maintenance.
Discussion
Generally speaking, an energy imbalance causes obesity or overweight. Only a negative energy balance can result in weight loss. To lose weight, therefore, people require an energy intake that is always lower than their energy consumption [
29]. Diet and exercise are ways to control energy intake and consumption, and the basic strategy to lose weight is diet control, exercise, or a combination of the two. In addition to calorie restrictions, a low-fat diet is also considered to be important in weight loss because different nutritional ratios have different effects on satiety, fat-burning and maintaining fat-free mass (muscle) with metabolic activity. As well as aerobic exercise, resistance training has been found to reduce fat, which plays an important role in maintaining fat-free mass and high metabolic activity.
There have been some international studies on short-term weight loss and long-term weight maintenance associated with a high-protein diet [
18]. The meta-analysis by Clifton et al. included 32 studies and compared high-protein and low-carbohydrate diet with a normal-protein weight-loss diet. The results showed that most intensive interventions based on a short-term high-protein diet were accompanied by better weight loss and fat content reduction. A protein content difference of more than 5% led to three times more weight being lost (0.9 versus 0.3 kg) in 12 months, and the high-protein diet had better effects on fasting triglycerides and insulin. Unfortunately, there is very limited high-quality research on the effect of a high-protein diet on weight loss in China. Chen et al. [
19] observed the effect of a high-protein diet on weight loss in an obese or overweight Chinese population with hyperlipidemia. The average weight loss and BMI decrease percentage in the high-protein group was higher than in the normal-protein group at week 12, but the difference was not significant. There was a significantly larger decrease in the waist to hip ratio in the high-protein group than in the normal-protein group. The triglyceride level decreased in both groups and showed no significant difference between the two groups. These results suggested that a high-protein diet for overweight or obese Chinese people with hyperlipidemia could lead to a more significant reduction in waistline size. However, the study only observed the effect on short-term weight loss and did not assess the effect of a high-protein diet on medium- and long-term weight maintenance [
19].
More importantly, both internationally and domestically, few studies have focused on the role of resistance training in medium- and long-term weight maintenance, and the results are inconsistent [
20‐
23]. Some studies have reported that resistance training contributed to the maintenance of weight loss [
22], some that it has an effect on the maintenance of fat reduction [
30], and one that there was no significant difference in the maintenance of weight loss and fat loss with resistance training [
23]. The heterogeneity of study populations and study designs may partly explain the differences between the results. For example, one study included men aged 35–50 years old, with a 6-month intervention period, while another included premenopausal women aged 21–46 years with a weight loss of 12 kg, and a follow-up period of 1 year.
Regaining weight after effective short-term weight loss is often a problem, especially in participants of studies providing centralized meals in the early stage. These studies have found that, after the centralized diet was stopped, a significant proportion of the participants were unable to adhere to the original diet. Studies have shown that the establishment of self-management of dietary intake seems to be a key factor, and the formation of good dietary management habits in the initial weight loss stage will help participants to avoid regaining weight. The weight regained was higher for participants receiving a lower initial calorie intake [
31]. One study showed that weight lost and regained depended more on high-protein than on low-carbohydrate levels in the diet and had no significant association with dietary fat [
32].
Few studies have explored whether a combination of resistance training and diet will achieve better results than just one method [
22,
30,
33]. For obese people, sports, especially resistance training, are difficult because they require extra money and time, and may require participants to attend particular locations [
34]. Studies have found that guided exercise could improve weight maintenance among obese people who would otherwise take no exercise [
30].
To achieve successful long-term weight loss, the intervention must effectively improve the individual’s self-efficacy. Improvements in self-efficacy are strongly affected by the management of dietary intake and the enthusiasm for exercise; participants who regain weight are more likely to give up and lose motivation [
17]. One of the core requirements of resistance training is to persist, so it may have a better effect on perseverance than traditional aerobic exercises such as fast walking or jogging. In this study, we assumed that resistance training can increase the proportion of subjects who adhere to the original diet (or decrease the degree of noncompliance), and therefore ultimately improve the long-term weight loss effect to avoid regaining weight.
Overweight and obesity are worldwide problems, and the exploration of scientific weight management and weight loss schemes has been an important area of research and has attracted a lot of investment. However, appropriate weight loss and weight maintenance strategies in specific populations (such as different ethnic groups) and social environments are unclear. We hope that the PROMOTE study will provide a basis for obesity management in China and promote the development of exercise- and diet-related studies. These are important to improve nutritional status, promote healthy development, prevent obesity-related diseases, and improve the health and quality of life of the population.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.