Your weight depends on a simple but easily unbalanced equation: weight change equals calories in minus calories out over time. Burn as many calories as you take in and your weight won’t change. Take in more than you burn and your weight increases. Dieting explores the other end of the spectrum: burning more calories than you take in.

 Your diet. What and how much you eat affects your weight. I’ll talk about that in the future.

Chalk up why you’re the weight you are to a combination of what and how much you eat, your genes, your lifestyle, and your culture.

• Genes. Your parents are partly to blame or to thank, for your weight and the shape of your body. Studies of twins raised apart show that genes have a strong influence on gaining weight or being overweight, meaning that some people are genetically predisposed to gaining weight. Heredity plays a role in the tendency to store fat around the chest, waist, or thighs. It is possible that some people are more sensitive to calories from fat or carbohydrates than others, although the evidence for this is thin. I must stress the phrase “partly to blame,” however because genetic influences can’t explain the rapid increase in obesity seen in the United States over the last thirty years or the big differences in obesity rates among countries.
It’s likely that our prehistoric ancestors shaped our physiological and behavioral responses to food. Early humans routinely coped with feast-or-famine conditions. Since it was impossible to predict when the next good meal might appear—like a patch of ripe berries or a catchable antelope—eating as much as possible whenever food was available might have been a key to surviving the lean times. This survival adaptation means that complex chemical interactions between body and mind that evolved eons ago in response to routine periods of starvation may drive us to eat whenever possible. In this era of plenty, that means all the time.

• Lifestyle and physical activity. If eating represents the pleasurable, sensuous side of the weight change equation, then metabolism and physical activity are its noseto- the-grindstone counterparts. Your resting (basal) metabolism is the energy needed just to breathe, pump and circulate blood, send messages from brain to body, maintain your temperature, digest food, and keep the right amount of tension in your muscles. It typically accounts for 60 to 70 percent of your daily energy expenditure. Physical activity makes up most of the rest. If you work a desk job and do little more than walk from your car to your office and back again, you may burn ridiculously few calories a day..


Ours is a culture of living large, of Texas-size appetites where quantity often edges out quality. Indulgence is tolerated, even revered. Love is food, and food is love: Imagine your grandmother urging you to have another helping or the pleasurable groans and belt loosening that end many holidays and regular meals. These are not universal tendencies. In France and throughout much of Asia, the cuisine emphasizes quality and presentation, not how much food can be crammed on a plate or into your belly. People in many cultures also believe it is inappropriate or downright rude to eat until you are full, and teach their children to eat to 70 percent of capacity.

Family and friends

Our family and friends, where we work and play, and other social factors strongly influence what and how much we eat. Making healthy choices can be challenging when everyone around is filling up on sugary soda and pizza and no healthful foods are insight. In the next article, we also describe how individuals can change or circumvent the factors working against them, for their own well-being and for those around them.

Your microbiome

Billions of bacteria, fungi, viruses, and other microbes— collectively called your microbiome—live quietly inside your intestines. They help digest your food; protect you against microbes that can cause disease; make vitamins such as vitamin B12, thiamine, and riboflavin; and more. It’s possible but unproven that your microbiome helps regulate your weight. Some types of gut bacteria seem to be better at releasing calories from food or causing inflammation, either of which can lead to weight gain. But we still don’t know if the microbiome is causing weight gain or weight gain is changing the microbiome


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