You say you were born with a predisposition to a particular body size and weight
You say you were born with a specific predisposition to a disease(s)
You say it's pointless to eat right or exercise because you've already been pre-programmed to a particular weight or health status
Science says different. Though genetics do play a part in your health destiny you do have the power to delay if not prevent certain family health traits. You also have the power to defy certain genetic traits, though it may be a battle.
So what can you do to defy your genes and family health history?
First find out at the earliest age possible the diseases that are in your family. Get maternal and paternal information from parents, grandparents, aunts, uncles and cousins.
If heart disease is prevalent, then get screened for heart disease. The screening can include a history and physical, blood work that includes a cholesterol profile, as well as other cardiac markers. You may also be asked to do an EKG and other more invasive screening tests, again depending on findings from preliminary tests. Once these baseline results are discussed you should be willing to work with a dietician to either lose weight if you need to, or simply to learn the best eating habits to support heart health and to lower risk factors. That may include reducing the saturated fat and refined carbohydrates in your diet, lowering the amount of sodium you consume and increasing consumption of fruits, vegetables and fish. Also consider working with a personal trainer to create an exercise program. Daily exercise is a key habit to reduce cardiac risk factors.
If diabetes type 2 is prevalent in your family, then you may want to get yearly blood tests that can include a fasting blood sugar, a 2 hour post-prandial blood test, an insulin level and possibly an A1C blood test which offers a 3 month average of blood sugar levels. Based on the findings your doctor may recommend weight loss, lifestyle modifications and other habit changes. Using a dietician and personal trainer can again be hugely helpful to reduce your lifelong risk for this disease.
If colon cancer features prominently in your family, especially in younger members (even cousins), then you will need a colonoscopy every year, starting at a younger age than 50 (considered to be the age for a baseline screening). This is a familial disease and screening, as well as certain dietary habit changes, can help to prevent or lower risk of this disease. Early detection also improves survival rate dramatically.
Breast, ovarian and cervical cancer can also have familial components, so women with first degree relatives may be wise to begin screening for breast cancer at younger ages. Your OB/GYN can help you to decide which screening tests are appropriate and the age at which to start. Maintaining a healthy weight (losing excess weight), eating a diet rich in plant-based foods and exercising can also help to reduce risk of certain cancers including breast cancer. If you are sexually active, screening for HPV can help to identify early risk of cervical cancer. Using a condom and knowing the sexual history of your partner(s), which may mean a screening blood test for your partner, can also help to reduce the risk of HPV. You can also discuss whether you are a good candidate for the HPV vaccine.