The important role of genetic factors in the development of obesity

II. Genetic Factors and Obesity

Obesity has a genetic component.

Heredity refers to the phenomenon of the transmission of traits from parents or paternal generations to offspring.

The relationship between genetic factors and obesity has long been a focus of the medical community.

Studies have shown that genetics not only affects the amount and distribution of body fat, but also the body's energy intake, basal metabolic rate, physical activity habits, and nutrient utilization.

(I) Genetics and the Occurrence of Obesity

Studies have shown that when one or both parents are obese, the incidence of obesity in their children is significantly higher. American scholars, through a large-scale population survey, found that among adolescents aged 10 to 19, the incidence of obesity is about 35% to 45% if one parent is obese, and as high as 70% to 80% if both parents are obese.

In individuals where both parents are thin or of normal build, the incidence of obesity is only 10%.

The above data indicates that obesity has a clear familial clustering pattern, but in many cases, congenital genetic factors and acquired environmental factors are intertwined and difficult to separate.

Different scholars have used BMI to assess the heritability of obesity with great variation (ranging from 0% to 90%), so some people still have doubts about the importance of heredity.

Recent studies on twins and adopted children have provided strong evidence for the role of genetic factors. Among identical twins, one group grew up in the same environment after birth, while the other group lived in different regions. The incidence of obesity was very similar in both groups of twins.

The latter, since identical brothers (or sisters) grow up in different environments, largely eliminates the influence of environmental factors.

The adoption study found no significant relationship between the weight of adopted children and their adoptive parents, but a close correlation with the weight of their biological parents.

The above experimental results indicate that obesity is related to genetic factors.

(II) Genetic factors and body type

The role of genetic factors is not only reflected in the amount of fat in the whole body, but also in the local distribution of fat in the body.

Recent studies have found that the distribution of localized fat is closely related to certain diseases that affect human health, such as diabetes and cardiovascular disease.

The familial similarities in human fat distribution have also been further studied.

Selly et al. measured 173 pairs of identical twins and 118 pairs of dizygotic male twins and found that the heritability of subscapular skinfold thickness was as high as 77%.

Further research indicates that girls are generally more affected by the genetics of body type than boys.

(III) Genetics and Energy Intake

The intake of calories and nutrients is family-oriented.

This strong similarity is not only observed between spouses, but also between parents and children.

Several twin studies have shown that monozygotic twins are more likely to be affected.

Since monozygotic twins are more likely to spend time and eat together than dizygotic twins, the influence of shared environmental factors cannot be ruled out.

A study from the Quebec Family Institute shows that total energy intake across generations is almost unaffected by genetic factors, but heritability increases significantly when the proportions of carbohydrate, fat, and protein intake are calculated separately.

If the intake ratio of the three macronutrients is an indicator of food choices, then this result suggests that food choices are also regulated by genetic factors.

(iv) Heredity and Energy Consumption

When calorie intake remains constant, reducing calorie expenditure leads to excess energy, weight gain, and even obesity.

Therefore, individual differences in energy expenditure are essential considerations when studying the genetic role of obesity.

The factors affecting energy expenditure are quite complex, including the following: basal or resting metabolic rate, energy expenditure from the specific dynamic action of food, and energy expenditure during physical exercise and other physical activities.

1. Basal metabolic rate: Basal metabolism accounts for 70% of daily energy expenditure. Studies on monozygotic twins and dizygotic twins have found that, after adjusting for age, sex, BMI and body proportions, heritability has an effect of 80% on energy expenditure in monozygotic twins and 40% in dizygotic twins.

Other data shows that babies born to obese women have a 24% lower basal metabolic rate than babies born to underweight women. When the amount and quality of food consumed by both types of babies are roughly the same each day, there is a significant difference in their weight after one year. Most of the babies born to obese women become overweight, while babies born to underweight women with normal basal metabolic rates are mostly of normal weight.

After conducting cross-sectional surveys and prospective studies on a large population, Dr. Ravussin of the United States found that resting and 24-hour metabolic rates have obvious familial characteristics, with some families having higher than normal values ​​and others having lower than normal values.

A low metabolic rate is a risk factor for weight gain at a fixed body size and body composition.

2. Specific dynamic action of food: It has long been observed that when obese and lean individuals of the same weight consume the same amount of food, the energy expended by the specific dynamic action of food is far less in the former than in the latter. This difference becomes even more pronounced after adjusting for lean body mass.

Other scholars have reported that after ingesting 4184 kJ (1000 kcal) of carbohydrates, the specific dynamic effects of food were observed for 4 hours, and the correlation coefficient between parents and offspring was 0.30, 0.35 for dizygotic twins, and 0.52 for monozygotic twins.

Therefore, the authors believe that the heritability of the specific dynamic effects of food is 40% to 60%.

3. Physical labor and sports activities: There is very little research on the role of genetic factors in energy consumption during physical activities.

In a Canadian study, the authors recorded the activity levels of 18,073 people, including intensity and density, and then recorded their average daily calorie expenditure.

The correlation coefficients between parents and children were found to be 0.12, between spouses 0.28, and between siblings 0.21.

This suggests that genetic factors influence calorie expenditure during activity to some extent.

A study from the Birmingham Institute of Child Research showed that, based on accelerometer readings of 100 children aged 4 to 7 and their 99 mothers and 92 fathers for 9 days with 10 hours of activity per day, children whose parents were active were more active than children whose parents were not. Children whose parents were both active were 5.8 times more active than those whose parents were not.

While genetic factors influence the occurrence and development of obesity to some extent, the human obesity gene phenotype is not controlled by a single gene like in some animals (such as ob/ob mice), but is a complex polygenic system with gene-gene and gene-environment interactions.

It is currently believed that genetic and environmental factors work synergistically, and obesity can only occur when people with a predisposition to obesity are exposed to fattening environments.

Therefore, eliminating or controlling nutrient-producing environments is of paramount importance.