In the present study, we explored the relationship between BMI and the risk of stroke recurrence. The results indicate that BMI is associated with stroke recurrence, and obese and overweight patients have a higher risk of stroke recurrence compared to underweight patients. Our study further suggests that the 'obesity paradox' is possible.
Underweight is a risk factor for recurrent stroke, and the risk of recurrent stroke is increased in underweight patients by 1.45 times that of the normal BMI group. This may be due to BMI and the onset of stroke, which lead to various kinetics, such as a catabolic/anabolic imbalance and tissue wasting of both fat and muscle tissue[41], and form a vicious cycle that worsens the nutritional status[42], subsequently causing a poor outcome. On the one hand, it may be that patients with underweight, whether malnourished or not, have mild low-grade inflammation, and chronic inflammation may play a crucial role in the relationship between underweight and atherosclerosis[43]. One study suggests that underweight is associated with impaired endothelial function, leading to a decrease in the bioavailability of nitric oxide, which can regulate vascular tension, and inhibit platelet aggregation and adhesion[44]. On the other hand, low body weight can lead to a decrease in muscle mass, which is associated with excessive inflammation[45], insulin resistance, and various metabolic disorders[46], which may lead to myocardial infarction and stroke. Greater muscle mass is associated with better exercise ability and cardiovascular health, which may lead to a reduced risk of cardiovascular disease[47]. In addition, lower BMI can serve as an alternative marker for patient frailty, including malnutrition and non-cardiovascular comorbidities, and frailty itself increases the risk of stroke[48, 49].
Overweight and obesity are protective factors for stroke recurrence, with a reduced risk of stroke recurrence in overweight and obese patients of 0.91 and 0.88 times that in the normal BMI group, respectively. This conclusion is consistent with the "obesity paradox" found in previous studies[20, 23, 50, 51]. This may be related to overweight or obese patients having more metabolic reserves compared with normal weight patients, which helps them better cope with unbalanced catabolic status and impaired metabolic efficiency caused by stroke[52, 53]. Furthermore, a soluble tissue necrosis factor receptor produced by adipose tissue can neutralize the deleterious effects of tumors on the development and progression of stroke[54].
However, one study[32] suggests that obesity is associated with an increased risk of recurrent stroke compared to non-obese individuals, which may be related to several mechanisms in obese people, such as stress-related neuroendocrine autonomic nerve activation, proinflammatory cytokines, an increased load of oxygen-free radicals, and systemic hormone imbalance[41, 55]. Obesity can significantly aggravate the occurrence and development of atherosclerosis. On the one hand, adipocytes produce leptin, plasminogen activator inhibitor 1 (PAI-1), adiponectin, tumor necrosis factor (TNF-α) and other physiologically active substances that are directly involved in the formation of inflammation and atherosclerosis. On the other hand, obesity is one of the important risk factors for diabetes, hypertension and hyperlipidemia, which will promote arteriosclerosis and may increase the risk of stroke recurrence[56].
Although we have explained the relationship between obesity and stroke recurrence, there is still controversy, possibly due to the heterogeneity of the study subjects, differences in sample size, and inconsistent BMI criteria. Further research is needed to explain the potential mechanisms leading to the obesity paradox in stroke recurrence patients.
Strengths and limitations
Several strengths exist in this study. Firstly, this is one of the few studies that have conducted a systematic evaluation and a meta-analysis of the relationship between BMI and stroke recurrence. Secondly, this study gave a detailed explanation of the relationship between BMI and stroke recurrence in different, while the previous study mainly elaborated the relationship between one or several BMI categories and stroke recurrence. In addition, the recent new original studies on the relationship between BMI and stroke recurrence were included and most of the included studies had a large sample size. Finally, the literature included in this study was of good quality, with high hierarchy of evidence. 12 studies were of high quality, and 5 were of medium quality.
However, there are still some limitations that need to be addressed in this study. Firstly, the retrieval language was confined to English or Chinese; thus, publications in other languages may have been missed. Secondly, the small number of literature included in this study affects the reliability of the conclusion. Finally, the included subjects were mostly older and fewer younger, which may lead to biased results.