The human brain continues to grow in size, which may reduce the risk of dementia

Diseases often cause damage to the body, and brain diseases are more likely to cause more damage to the brain. The most typical condition is Zika disease, also known as microcephaly, which is caused by Zika virus infecting a person's fetus. It has been found in both mouse and human embryos that after Zika virus infection, the virus can cross the placental barrier and directly damage the neural precursor cells of the cerebral cortex of the embryo, inhibiting their growth, leading to the depletion of neural precursor cells and ultimately the reduction of the cerebral cortex area. Later, the researchers further found that Zika virus infection caused a disorder of gene expression networks in the brains of offspring animals, leading to poor brain development and dementia.

Decades of research have shown that the human brain continues to increase in size, white matter, hippocampus, and brain surface area. Visual China|Figure

Now, a new study confirms that it is possible to alleviate symptoms of Alzheimer's disease (senile dementia) and other dementias if changes occur in the brain, such as increased volume. This could be a boon for future human evolution, as there is no effective treatment for Alzheimer's disease, which currently accounts for about 55 million people worldwide, 60% to 70% of whom have Alzheimer's disease, according to the World Health Organization. The number of people with Alzheimer's disease in China is close to 10 million.

A study published on March 25, 2024 by a research team from the University of California, Davis, shows that the human brain is getting bigger, with people born in the 1970s having a 6.6% increase in brain volume compared to those born in the 1930s, while brain surface area has increased by nearly 15%. The research team speculates that the increase in brain size may contribute to an increase in brain reserve, which may reduce the overall risk of age-related dementia, including Alzheimer's disease.

The research team used magnetic resonance imaging (MRI) data from brain MRI data from participants in the Framingham Heart Study (FHS). The FHS study was initiated in 1948 in Framingham, Massachusetts, USA, with the aim of analyzing the causes and patterns of cardiovascular and other diseases and proposing appropriate prevention and treatment methods.

The original cohort of the study included 5,209 men and women aged 30-62 years, and the study continues to this day, expanding to second- and third-generation participants. However, the study only used magnetic resonance imaging of the brain between 1999 and 2019 in FHS participants born in the 1930s to 1970s. Brain studies were performed on magnetic resonance imaging in 3,226 people, of whom 53% were women and 47% were men, with an average age of about 57 years at the time of MRI examination.

After comparing the MRI results of these individuals, the researchers found that the participants experienced slow but continuous growth in several brain structures, and overall, their brain volume (intracranial volume) increased steadily every 10 years. Participants born in the 1930s had an average brain volume of 1,234 ml, but participants born in the 1970s had an average brain volume of 1,321 ml, an increase of about 6.6%.

Measurements of brain surface area (cortical surface area) showed a greater difference in brain surface area between the two age groups, with a significant increase over each 10-year period. Participants born in the 30s of the 20th century had an average surface area of 1933 square centimeters, but participants born in the 70s of the 20th century had an average surface area of 2222 square centimeters, an increase of about 15%. In addition, participants born in the 30s of the 20th century had an increase of about 7.7% in white matter volume and 5.7% in the volume of the hippocampus, which was 441.9 ml and 476.3 ml, respectively, compared to those born in the 70s of the 20th century, with a hippocampal volume of 6.51 ml and 6.89 ml in the latter.

In the human brain and cerebellum and brainstem, the outer part is gray matter, and the inner part is white matter. In the spinal cord and brain, the exterior is predominantly white matter, and the interior is gray matter. Gray matter is the nerve center that plays a innervating role (including the gray matter of the spinal cord, which can also control some low-level unconditioned reflexes), and white matter mainly plays a role in conduction, such as the white matter of the spinal cord, which mainly conducts nerve signals in the brain and spinal cord gray matter. The hippocampus has two bodies, located on the medial temporal lobes of the left and right cerebral hemispheres, which are part of the limbic system and are responsible for short-term memory, long-term memory, and spatial localization.

The decades-long study showed a sustained and significant increase in brain volume, white matter, hippocampus, and brain surface area in humans, as well as significant increases in brain volume after fully adjusting for the effects of height, sex, and age variables.

The researchers believe that the larger brain structure observed in this study may reflect improvements in brain development and health, while larger brain structures represent an increase in brain development and potentially greater brain reserves, which could explain the decline in the incidence of dementias such as Alzheimer's disease in the United States. According to the Alzheimer's Society of America, about 7 million Americans currently have Alzheimer's disease, and that number is expected to rise to 11.2 million by 2040. As the U.S. population ages, the number of people with Alzheimer's disease is on the rise, but the incidence of Alzheimer's disease has been declining for decades. Previous studies have also shown that the incidence of Alzheimer's disease in the United States has declined by 20% every 10 years since the 1970s.

Although there are many reasons for the decline of Alzheimer's disease in the United States, researchers believe that the gradual increase in brain volume shows a kind of "strategic depth" expansion, even if damage to brain neurons and tissues can cause dementia due to various reasons, but due to the expansion of brain volume, other uninjured parts can make up for the injured parts, so the brain has greater compensatory ability and error correction ability, which can slow down senile brain diseases, so that the incidence of Alzheimer's disease and other dementias is reduced.

Of course, this is only a corollary and hypothesis of the results of this study, and there is still direct evidence that the increase in brain volume and surface area plays a role in compensating and correcting dementia, thereby reducing the incidence of dementia and reducing symptoms.

ZHANG Tiankan

Editor-in-charge: Zhu Liyuan