My friend's father has diabetes (who was diagnosed with diabetes long after my friend's birth). The doctor told the friend that he has more chances of getting diabetes than a normal person and he must make lifestyle changes as he is more likely to get the disease. Does it mean that diabetes is genetic (hereditary)?
Does it mean that diabetes is genetic (hereditary)?
Yes, Type 2 Diabetes most definitely contains a genetically inherited component. However, there is also a very strong environmental influence, which is the reason why your friend's doctor recommended that your friend change his lifestyle (because the onset of T2D is heavily determined by environmental factors, and is by no means 100% genetically predetermined).
According to the World Health Organization...
Genetics in the Development of (Type 2) Diabetes
It has long been known that T2D is, in part, inherited. Family studies have revealed that first degree relatives of individuals with T2D are about 3 times more likely to develop the disease than individuals without a positive family history of the disease (Flores et al., 2003; Hansen 2003; Gloyn 2003). It has also been shown that concordance rates for monozygotic twins, which have ranged from 60-90%, are significantly higher than those for dizygotic twins. Thus, it is clear that T2D has a strong genetic component.
Environmental Risk Factors
The major environmental risk factors for T2D are obesity (> 120% ideal body weight or a body mass index > 30 k/m2) and a sedentary lifestyle (van Dam, 2003; Shaw and Chisholm, 2003). Thus, the tremendous increase in the rates of T2D in recent years has been attributed, primarily, to the dramatic rise in obesity worldwide (Zimmet et al., 2001). It has been estimated that approximately 80% of all new T2D cases are due to obesity (Lean, 2000). This is true for adults and children. In the Pima Indians, 85% of the T2D children were either overweight or obese (Fagot-Campagna et al., 2000). Another study in the US reported that IGT was detected in 25% of obese children age 4-10 years, and in 21% of obese adolescents (Sinha et al., 2002). Undiagnosed T2D was detected in 4% of the adolescents.
The other major T2D risk factor is physical inactivity. In addition to controlling weight, exercise improves glucose and lipid metabolism, which decreases T2D risk. Physical activity, such as daily walking or cycling for more than 30 minutes, has been shown to significantly reduce the risk of T2D (Hu et al., 2003). Physical activity has also been inversely related to body mass index and IGT. Recently, intervention studies in China (Pan et al., 1997), Finland (Tuomilehto J et al., 2001) and the US (Diabetes Prevention Program Study Group, 2002) have shown that lifestyle interventions targeting diet and exercise decreased the risk of progression from IGT to T2D by approximately 60% . In contrast, oral hypoglycemic medication only reduced the risk of progression by about 30%.
That being said, I would strongly nudge your friend in the right direction, and start asking them to go on walks or bike rides (with you), and remind them from time to time to drink less sodas and processed foods (assuming that they do).
Just as a side note, the American Heart Association recommends that only an additional 36g of sugar be added to your daily diet (for an adult male; 25g for adult female).
The trouble with questions like this--"is this disease genetic?"--is that diseases are complicated phenotypes that arise from interactions with both individuals' genetic makeup and also the changes in gene expression that happen as a result of the environment of that individual over their lifetime.
To put it another way: neither nature without nurture nor nurture without nature are particularly interesting, because the first amounts to a little vial of DNA in a microcentrifuge tube and the second amounts to, uh, a whole lot of nothing.
In the case of Type II diabetes, there is both a strong environmental component--persistent obesity in humans is so likely to lead to diabetes that one of the two original mouse obesity model strains was named 'db' for diabetes--and also a genetic component that affects the likelihood of developing Type II diabetes in a number of ways. Some genetic variants may be less likely to develop obesity even under the high-fat diets that typically are used to create obese phenotypes in mice. Some variants may be more or less likely to develop insulin resistance even in the presence of high levels of adipose tissue. Both of these variations would affect your likelihood of getting Type II diabetes... depending on your environment and the level of body fat you carry, and that last one is going to be dependent on not only your genetic makeup but also your environment and (almost certainly) epigenetic influences from your parents.
With respect to Type II diabetes and actual specific genetic variations that have been identified, there's the twin study cited in another answer here, which was done in Danish twins and really just looked at whether there was a genetic component done at all. GWAS studies have, predictably, resulted in several candidate loci, all with relatively small effects on risk--which makes sense, because metabolism is a complex thing and many players are probably involved in competing roles. (I'd link more, but I'm new here--hitting up a Google Scholar search for "type ii diabetes genetic variance" nets you a whole bunch more GWAS studies talking about specific types of variation noted.)