Evidence has shown that interventions which involve changes in a person's lifestyle, such as diet and physical activity, lead to weight loss and thus reduce the risk factors of cardiovascular disease. However, the effectiveness and necessary duration of specific interventions are unclear. The purpose of this research was to evaluate and compare the effect on weight of interventions based on diet, exercise and a combination of both. The research subjects were overweight and obese adults, at six and twelve months after the beginning of the intervention. First a systematic review was carried out, followed by a meta-analysis. Initially, 24 studies were selected which met the established criteria for inclusion; twelve of these demonstrated the required level of quality. The diet-based interventions resulted in reductions of-6.66 kilograms (95% confidence interval (CI): -9.04 to -4.28) and -3.80 kilograms (CI: -5.50 to -2.10) at six and twelve months, respectively. Those who engaged in exercise showed a loss of -2.21 kilograms (CI: -4.62 to -0.21) and -2.00 kilograms (CI: -5.70 to -1.70) at six and twelve months, respectively, while those who changed their diets and engaged in exercise showed a loss of -10.86 kilograms (CI: -13.22 to -8.49) and -6.50 kilograms (CI: -8.09 to -4.90) at six and twelve months. The combination of diet and exercise showed the best effect in reducing weight among overweight and obese people, followed by diet alone while exercise alone didn't reach significant results. The effect of these interventions was greater during the first six months of intervention.