OBJECTIVE: The current meta-analysis aimed to examine the heritability and familial resemblance of dietary intakes, including energy and macronutrients in both twin and family-based studies.
METHODS: The online literature databases, including PubMed, Scopus, and Web of Science were searched comprehensively until 2023 to identify the relevant studies. The heritability index in family studies was h2 and the heritability indices for twin studies were h2, A2, and E2. Three weighted methods were used to calculate the mean and SE of heritability dietary intakes.
RESULTS: Eighteen papers including 8 studies on familial population and 12 for twin population studies were included in the present meta-analysis. The heritability of dietary intakes in twin studies (range of pooled estimated h2, A2, and E2 was 30-55%, 14-42%, and 52-79%, respectively) was higher than family studies (range of pooled estimated h2 = 16-39%). In family studies, the highest and lowest heritability for various nutrients was observed for the fat (%Kcal) (h2 range:36-38%) and carbohydrate in g (h2 range:16-18%), respectively. In twin studies, based on mean h2, the highest and lowest heritability for various nutrients was reported for the fat (%Kcal) (h2 range:49-55%) and protein intake in g (h2 range:30-35%), respectively. Also, based on the mean of A2, the highest and lowest heritability was observed for carbohydrates (% Kcal) (A2 range:42-42%), and protein (% Kcal) (A2 range:14-16%), respectively. Furthermore, in twin studies, the highest and lowest mean of E2 was shown for saturated fats (E2 range:74-79%) and energy intake (E2 range:52-57%), respectively.
CONCLUSIONS: Our analysis indicated that both environmental factors and genetics have noticeable contributions in determining the heritability of dietary intakes. Also, we observed higher heritability in twins compared to family studies.

The current study was motivated by an interest in deepening understanding of Brazilian twin research, which is underrepresented internationally, in an effort to rectify this situation. Our aim was threefold: (1) to carry out a comprehensive investigation of Brazilian research on twins according to the area of knowledge; (2) to evaluate the representation of research in the field of psychology in comparison with other areas; (3) to evaluate characteristics of the research that may have contributed to its exclusion from the comprehensive meta-analysis of 50 years of twin research. A scoping review was performed according to PRISMA guidelines. Titles and abstracts were searched up to 2022 in six databases: CAPES, BDLTD, PePSIC, PubMed, Google Scholar, and SciELO, using selected keywords both in Portuguese and in English (e.g., \'twins\' and \'Brazil\'; \'twinning\' and \'Brazil\'; \'gemelaridade\' [twinning], and \'gêmeos\' [twins]). Three hundred and forty publications were included in the review. Approximately half (53.8‰) used the classic twin design to investigate the heritability of several traits, and the other half (46.2%) used other research designs. The scoping review showed that the number of publications doubled approximately every 10 years. Most publications were from the health area, with medicine accounting for approximately half of the studies, followed by psychology, odontology, and biology. We found that the interest in studying twins among Brazilian scientists is increasing over the years and there are reasons to be enthusiastic about the potential impact of this trend in the global scenario.

Learning to read is a dynamic and cumulative process beginning from birth and continuing through the school years. Empirical data showed a decrease of additive genetic (A) and shared environmental (C) components and an increase of non-shared environmental (E) components from preschool to middle school. However, our understanding of the aetiology of continuity and change of reading skills across this developmental period is limited. Following the PRISMA guidelines, we reviewed the results of behavioral genetic research on reading-related neurocognitive skills of 13 longitudinal twin and adoptive sibling studies spanning from preschool/kindergarten to middle/high school. Our findings suggested that continuity was mainly explained by A components throughout the study periods, and, although to a lesser extent and less consistently, by C components during the early years; change was explained by new E components throughout the years, and also by new A components in the early years. As we are interested in models relevant to traits with early onset during development, it is crucial to deepen the investigation of how developmental time can moderate the genetic and environmental variation.

Twin samples allow to conduct a quasi-experimental co-twin case-control approach that can control for genetic and environmental confounding in brain-cognition associations, being more informative on causality compared with studies in unrelated individuals. We conducted a review of studies that have utilized discordant co-twin design to investigate the associations of brain imaging markers of Alzheimer\'s disease and cognition. Inclusion criteria encompassed twin pairs discordant for cognition or Alzheimer\'s disease imaging markers and reporting of within-twin pair comparison on the association between cognition and brain measures. Our PubMed search (2022 April 23, updated 2023 March 9) resulted in 18 studies matching these criteria. Alzheimer\'s disease imaging markers have been addressed only by few studies, most with small sample size. Structural magnetic resonance imaging studies have indicated greater hippocampal volume and thicker cortex in co-twins with better cognitive performance compared with their co-twins with poorer cognitive performance. No studies have looked at cortical surface area. Positron emission tomography imaging studies have suggested that lower cortical glucose metabolism rate and higher cortical neuroinflammation, amyloid, and tau accumulations are related to poorer episodic memory in within-twin pair comparisons. Thus far, only cross-sectional within-twin pair associations of cortical amyloid and hippocampal volume with cognition have been replicated.

BACKGROUND: Physical activity has been included in the list of twelve modifiable risk factors for dementia, despite conflicting results from observational and controlled studies. In particular it is not clear whether physical inactivity near the time of dementia diagnosis is a consequence or cause of dementia. We review all available studies reporting the possible association between having engaged in PA before 60 years of age and the risk of dementia.
METHODS: We performed a systematic review based on the methodology reported in the Cochrane handbook for systematic reviews and following the PRISMA statement. Bibliographic searches were carried out on the databases PubMed, ISI Web of Science and the Cochrane Database of Systematic Reviews. Further references were retrieved from published systematic reviews on the same topic. Included studies were assessed using the Newcastle Ottawa scale.
RESULTS: The bibliographic search yielded 1381 records. A total of 11 studies were included. Three of the included studies were case control studies, while the remaining 8 were cohort studies The overall quality of included studies was high. However, clinical criteria for the diagnosis of dementia, criteria to define and measure and PA and time-reference of exposure were heterogeneous, with some studies considering specific age range of exposure, and other reports dealing with more generic \"adult age.\"
CONCLUSIONS: This review suggests that there is insufficient evidence to conclude whether PA in early life may affect the incidence of dementia in later life. Studies in this field are very complicated and recognizing the impact of PA in early life given all the confounding factors is very difficult. Further studies are warranted. In these studies, it will be crucial to define the type, quantity and intensity of PA as well as to stratify analysis by sex, cultures and social classes.

Research into the origins of twinning has focused mostly on contributions from the female side of the family. A review of current findings suggests that possible male contributions to twinning events have been overlooked. This section is followed by brief reviews of twin research concerning monozygotic twins discordant for Parkinson\'s disease, fetal loss in twin pregnancies following prenatal diagnosis, uterine rupture and repair in an early twin pregnancy and a twin study of affectionate communication. The concluding portion of this article presents human interest stories involving twins that are both informative and poignant, namely conjoined twins in a triplet set, identical twin nurses who delivered identical twins, identical twins discordant for COVID-19 recovery course, identical twins who passed away from COVID-19 and archeological finds of the oldest identical twins.

Studies of twins can reduce confounding and provide additional evidence about the causes of disease, due to within-pair matching for measured and unmeasured factors. Although findings from twin studies are typically applicable to the general population, few studies have taken full advantage of the twin design to explore the developmental origins of cardiometabolic health outcomes. We aimed to systematically review the evidence from twin studies and generate pooled estimates for the effects of early-life risk factors on later-life cardiometabolic health.
An initial search was conducted in March 2018, with 55 studies of twins included in the review. Risk of bias was assessed using the Newcastle-Ottawa Scale, and eligible studies were included in a meta-analysis, where pooled estimates were calculated. Twenty-six studies analysed twins as individuals, and found that higher birthweight was associated with lower SBP (β = -2.02 mmHg, 95%CI: -3.07, -0.97), higher BMI (β = 0.52 kg/m2, 95%CI: 0.20, 0.84) and lower total cholesterol (β = -0.07 mmol/L, 95%CI: -0.11, -0.04). However, no associations were reported in studies which adjusted for gestational age. Few of the included studies separated their analyses into within-pair and between-pair associations.
Early-life risk factors were associated with cardiometabolic health outcomes in twin studies. However, many estimates from studies in this review were likely to have been confounded by gestational age, and few fully exploited the twin design to assess the developmental origins of cardiometabolic health outcomes.

We review the findings of systematic reviews and meta-analyses of case-control studies that examine brain functioning and cognitive correlates of adolescent cannabis use using structural and functional neuroimaging tools and standardised neuropsychological tests. We also examine prospective epidemiological studies on the possible effects of adolescent and young adult cannabis use on cognitive performance in adult life and the completion of secondary education. We summarize the findings of studies in each of these areas that have been published since the most recent systematic review. Systematic reviews find that adolescent cannabis use is inconsistently associated with alterations in the structure of prefrontal and temporal brain regions. Meta-analyses reveal functional alterations in the parietal cortex and putamen. Differences in the orbitofrontal cortex predate cannabis use; it is unclear if they are affected by continued cannabis use and prolonged abstinence. Longitudinal and twin studies report larger declines in IQ among cannabis users than their non-using peers but it is unclear whether these findings can be attributed to cannabis use or to genetic, mental health and environmental factors. Several longitudinal studies and a meta-analysis of cross-sectional studies suggest that there is some cognitive recovery after abstinence from cannabis. Longitudinal studies and some twin studies have found that cannabis users are less likely to complete secondary school than their non-using controls. This association might reflect an effect of cannabis use and/or the social environment of cannabis users and their cannabis using peers. Cognitive performance is altered in some domains (e.g. IQ, verbal learning) in young people while they are regularly using cannabis. There are two important messages to adolescents and young adults: First, cannabis has potentially detrimental effects on cognition, brain and educational outcomes that persist beyond acute intoxication. Second, impaired cognitive function in cannabis users appears to improve with sustained abstinence.

Monozygotic (MZ) twin studies constitute a key resource for the dissection of environmental and biological risk factors for human complex disorders. Given that epigenetic differences accumulate throughout the lifespan, the assessment of MZ twin pairs discordant for depression offers a genetically informative design to explore DNA methylation while accounting for the typical confounders of the field, shared by co-twins of a pair. In this review, we systematically evaluate all twin studies published to date assessing DNA methylation in association with depressive phenotypes. However, difficulty to recruit large numbers of MZ twin pairs fails to provide enough sample size to develop genome-wide approaches. Alternatively, region and pathway analysis revealed an enrichment for nervous system related functions; likewise, evidence supports an accumulation of methylation variability in affected subjects when compared to their co-twins. Nevertheless, longitudinal studies incorporating known risk factors for depression such as childhood trauma are required for understanding the role that DNA methylation plays in the etiology of depression.

Pain often co-occurs with depression and anxiety, and together cause considerable social and economic burden. Twin studies have investigated the aetiology of these covariations. However, to our knowledge, no systematic review examining the covariation between these conditions has been conducted. We searched the Cochrane Database of Systematic Reviews, EMBASE, MEDLINE, and PsychInfo for twin studies examining the covariation between pain with depression and/or anxiety published from inception to May 16, 2019. Included studies reported: original data on twin samples using the classical twin or co-twin control designs; investigated the association between pain and depression and/or anxiety; were published in peer-reviewed journals; used validated measures; included ≥100 twin pairs. Of 359 retrieved articles, 23 met our inclusion criteria. Most studies found that the covariation of pain with depression and/or anxiety was explained by genetic (n = 6) or both genetic and environmental (n = 16) factors. Most studies were cross-sectional, and all were led in adult populations. Future research requires the use of more standardized measurement tools, including quantitative sensory testing as well as assess child-adolescent cohorts.