For the many Americans genetically programmed to add pounds, the effort to lose weight can seem doomed from the get-go. Mix in other factors no one can change — age, race, birth order — and the struggle becomes even tougher.

Though innate biology does play a role in whether someone tends to gain weight, individuals can work with their biology to achieve weight goals, say obesity experts. Step one, however, is to know what you're up against. Step two is knowing how not to trip the genes you've been dealt.

In this four-part series, local and national obesity experts weigh in on 40 reasons Americans are fat — and what you can do about them.

Today's list explores 10 ways heredity plays a role.

Genetics: "About half our propensity to gain weight or not is inherited," says Dr. Steve Smith, an obesity expert and scientific director of Sanford-Burnham Translational Research Institute for Metabolism and Diabetes, in Orlando. Though genes haven't changed much in thousands of years, we have seen a rapid change in the environment, and that has interacted with our genetic propensity toward obesity. "Your genes won't make you fat unless you put them in the wrong environment." In other words, obesity is a result of the way certain genes interact with an environment stacked against them.

Ethnicity: "The prevalence of obesity is higher among African-Americans and Hispanics because of how their genes and cultures interact with our environment," Smith says. From a demographics standpoint, the rise in these minority populations has contributed to the uptick in the nation's obesity rate. Black women are at the highest risk of any group, according to the Department of Health and Human Services' Office of Minority Health. Four out of five black women in the U.S. are overweight or obese.

Why certain racial groups have more obesity is a question researchers at the National Institutes of Health have been working to answer. For 30 years, NIH researchers have studied the Mexican Pima Indians in Arizona, who have an exceptionally high rate of obesity. Their obesity problem didn't start until late in the 19th century, when American settlers diverted their water supply and upset their agricultural traditions. As their diets became more westernized, their obesity rates soared.

Hormones: "Obesity is indeed a hormonal disorder. That idea should never have been refuted," says science writer Gary Taubes, author of "Why We Get Fat." The main hormone involved is insulin, which regulates fat accumulation; however, a whole bevy of other hormones contribute to weight regulation, Smith says.

Metabolism: Largely determined by a person's lean-tissue mass, metabolism is what makes you burn calories even at rest. It's why some people can't keep weight on and others can't seem to lose it. A person whose body composition is high in fat and low in lean muscle will burn energy more slowly than someone who has a lot of lean muscle. Taubes pictures the differences in metabolism this way: Imagine everyone's body as having a needle on a gas gauge. On one end is E is for energy, on the other F for fat. Those whose needles point toward E burn more fuel and store less as fat. Those whose needles point toward F store more calories as fat and burn fewer.

Fat mothers: "If your mom was obese while she was pregnant with you, you're marked," says Philip Wood, professor at Sanford-Burnham Research Institute in Lake Nona. "You will have an uphill battle fighting off excess weight and its ill effects, even if you're adopted and raised by slim parents." The rising field of epigenetics is revealing how diet and the environment are reprogramming the genes we're dealt at conception. The mechanisms likely involve maternal circulating hormones and glucose levels, says Tracy Bale, associate professor of neuroscience at the University of Pennsylvania School of Veterinary Medicine. These effects can be handed down to the next generation, she adds. Women who have had sugar-laden diets, who are heavy and insulin-resistant create an environment in utero that programs babies to prepare for life in a high-carb environment. In response, the fetus overproduces pancreatic beta cells, which make insulin. To turn that around, pregnant moms need to keep circulating blood-sugar levels under control.

Birth order: Siblings born later in the lineup tend to be fatter than firstborns or those born earlier, Smith says. Researchers think this is because moms tend to weigh more with each pregnancy, and therefore have more circulating insulin with each subsequent pregnancy. This is in turn passed along to the baby and future generations.

Brown fat: Why some people get fat and others don't regardless of what they eat may lie in part in the color of their fat, says Sheila Collins, an obesity researcher at Sanford-Burnham Medical Research Institute. Bodies store two kinds of fat: white (fairly inactive tissue that acts as a storage bank) and brown (highly active tissue that actually burns calories and white fat), she says. How much brown fat we have is probably determined by our genes. It appears to play a role in body-fat composition and in keeping weight off. Collins and others are looking for ways to make the brown fat we do have more active and also to increase it. Based on new work from her lab, she thinks exercise will likely play a role.

Aging: Most Americans get heavier as they age. An aging population skews the overall percentage of obesity up, says Smith. Americans gain an average of a pound a year. Today the median age of Americans is 36.9, up from 32.9 years in 1990.

Bacteria: Microbes living in the gut appear to also play a role in how different bodies store fat. Researchers at Washington University School of Medicine in St. Louis compared the gut bacteria in genetically obese mice with their lean littermates, as well as those of obese and lean humans, and found that those that had more of two particular gut microbes were more likely to be obese. Those whose guts had an abundance of these bacteria were better at harvesting energy from the diet, leading to easier weight gain, said the researchers, whose findings were published in Nature.

Weight regain: As hard as it is to lose weight, keeping pounds off is even harder. Once again, biology works against you. Many mechanisms in the body kick in to encourage the body to regain lost weight. "The hormone leptin, for instance, which is produced in fat cells, stimulates appetite while slowing metabolism," Smith says. This leads many in the field to think that sustaining weight loss during the long haul may require hormone therapy.

What can we do about it? Differences in genetic makeup, metabolism and even bacteria levels in the gut explain why some people gain weight and others don't. But obesity experts are quick to add that just because you've inherited a tendency to put on pounds doesn't mean you have to feed that tendency. Everyone can build lean-muscle mass through exercise, especially weight training, to boost metabolism. That will help you burn more calories even at rest. Exercise also may give brown fat a boost. Pregnant moms need to be especially mindful of excess weight gain, sugar consumption and controlling blood-sugar levels through diet and exercise so they don't pass on problems to future generations. Losing baby weight between pregnancies also would help. While Americans do their part, scientists will continue to look at ways to manipulate gut bacteria and hormone levels to help prevent weight gain — and weight regain.

mjameson@tribune.com or 407-420-5158

Why we're fat: 40 reasons

Sunday: How diet contributes to obesity.

Today: The role played by heredity.

Tuesday: How lifestyle changes have affected our weight.

Wednesday: The influence of environment and government policies.