David Ludwig often uses an analogy when he talks about weight loss: Human beings are not toaster ovens. If we were, then the types of calories we consumed would not matter, and calorie counting would be the most effective way to lose weight.

Dr. Ludwig, an obesity expert and professor of nutrition at the Harvard T.H. Chan School of Public Health, argues that weight gain begins when people eat the wrong types of food, which throws their hormones out of whack and sets off a cycle of cravings, hunger and bingeing. In his new book, “Always Hungry?,” he argues that the primary driver of obesity today is not an excess of calories per se, but an excess of high glycemic foods like sugar, refined grains and other processed carbohydrates.

Recently, we caught up with Dr. Ludwig to talk about which foods act as “fertilizer for fat cells,” why he thinks the conventional wisdom on weight loss is all wrong, and long-term strategies for weight loss. Here are edited excerpts from our conversation.

Q. What is the basic message of your book?

A. The basic premise is that overeating doesn’t make you fat. The process of getting fat makes you overeat. It may sound radical, but there’s literally a century of science to support this point. Simply cutting back on calories as we’ve been told actually makes the situation worse. When we cut back on calories, our body responds by increasing hunger and slowing metabolism. It responds in an effort to save calories. And that makes weight loss progressively more and more difficult on a standard low calorie diet. It creates a battle between mind and metabolism that we’re doomed to lose.

Q. But we’ve all been told that obesity is caused by eating too much. Is that not the case?

A. We think of obesity as a state of excess, but it’s really more akin to a state of starvation. If the fat cells are storing too many calories, the brain doesn’t have access to enough to make sure that metabolism runs properly. So the brain makes us hungry in an attempt to solve that problem, and we overeat and feel better temporarily. But if the fat cells continue to take in too many calories, then we get stuck in this never-ending cycle of overeating and weight gain. The problem isn’t that there are too many calories in the fat cells, it’s that there’s too few in the bloodstream, and cutting back on calories can’t work.

Q. That’s very different from the conventional wisdom that weight loss boils down to calories in, calories out.

A. Yes. An analogy would be like trying to treat a fever with an ice bath. Imagine going to the hospital with a very high fever, and the doctor says, “Fever is just a problem of heat balance – too much heat in the body, not enough heat leaving the body.” That’s true from a physics standpoint. So the doctor decides to put you in an ice bath. That will work temporarily. An ice bath will break your fever. But imagine what’s going to happen. Your body is going to fight back furiously with severe shivering and blood vessel constriction and you’ll feel miserable. You’ll want out of that ice bath as soon as possible. For that reason, ice baths are not a popular treatment for fever.

Q. So in this analogy, how do you treat the underlying cause?

A. A much more effective approach is to lower the body’s temperature set point, which is done with medicine like aspirin. Put biology on your side by eating the right way, and weight loss occurs naturally as a fever would break if you treat the underlying cause of the fever.

Q. If it’s not overeating, then what is the underlying cause of obesity?

A. It’s the low fat, very high carbohydrate diet that we’ve been eating for the last 40 years, which raises levels of the hormone insulin and programs fat cells to go into calorie storage overdrive. I like to think of insulin as the ultimate fat cell fertilizer.

When someone with Type 1 diabetes first comes to attention, their blood sugar is high because they can’t make enough insulin. They invariably have lost weight. They may be eating 5,000 calories a day, and they’re still losing weight. You can’t gain weight without insulin. The opposite is also true. If you give someone with diabetes too much insulin, they will inevitably gain weight. Insulin programs the body to store calories, and most of those calories get stored in the fat cells. If you’ve got too much insulin, you’re going to store too many calories. This has been very well established scientifically.

Q. How do you get your obese patients to lower their insulin?

A. The quickest way to lower insulin is to cut back on processed carbohydrates and to get the right balance of protein and fat in your diet. A high fat diet is really the fastest way to shift metabolism. It lowers insulin, calms fat cells down and gets people out of the cycle of hunger, craving and overeating.

Q. It sounds like the program you’re advocating is the Atkins diet.

A. No, this is different. The Atkins diet is a very low carbohydrate diet, which in its classic form means you can’t eat much fruit let alone any other carbohydrate. Many people don’t require – and are probably unwilling to sustain – the rigors of a very low carbohydrate diet. I think that except for people with already very severe metabolic problems like Type 2 diabetes, such restrictive regimens are not usually necessary.

Q. How does this program work?

A. Our program has three phases. First we tell people to give up processed carbohydrates, added sugars and all grain products for two weeks. The carbohydrates you eat should be a range of nonstarchy vegetables, fruits and beans. After two weeks, we reintroduce whole kernel grains, potatoes – except for white potatoes – and a little bit of added sugars. You do this until your weight comes down to a new, lower set point. It could be a few weeks, or it could be many months for someone who has a more significant weight problem.

Q. Where do you stand on saturated fat?

A. I think throughout the public health establishment we’re recognizing that saturated fat isn’t Public Health Enemy No. 1. But it’s also not necessarily a health food. There are many different kinds of saturated fats, and each of these has different effects on the body. But when you reduce processed carbohydrates and your insulin levels decrease, the saturated fat you consume burns faster and it doesn’t stick around the body as long.

We have to stop thinking about saturated fat as either just good or bad. Our meal plan is based on whole natural foods, which include saturated fat. But we make sure to balance that with lots of mono- and polyunsaturated fats like olive oil, nuts, avocado and flaxseed oil.

Q. Ultimately, what do you want people to take away from this book?

A. Until we address the underlying drivers of weight gain – which are fat cells stuck in calorie storage overdrive – we are going to be in a battle between mind and metabolism that we just can’t win. Cutting back on calories won’t do it. That doesn’t change biology. To change biology, you have to change the kinds of foods you’re eating.