Our bodies are not built for modern times. We are optimized for a hunter-gather lifestyle where activity was plentiful but food - particularly carbohydrate-rich food - was scarce. We were thin, fast, and lived in constant fear of starvation. Popular keepsakes for men in those days were little statuettes depicting the rarest of treasures- a fat woman.

Those who didn't starve were usually those who had the best ability to store fat. We are descended from them.

Today in the developed world - particularly in the United States - we are incredibly sedentary. We get out of bed, drive to work, sit at a desk, drive home, sit in front of the T.V., and go to bed. That is the default lifestyle. If you want to, you know, do something crazy like move your body, you have to plan to do it. Exercise - even just taking a few extra steps - is not on the normal itinerary for adults in this country at this time.

And we have food... lots and lots of carbohydrate-rich food. One of the funniest critiques of the American preoccupation with food is in the kid's movie Over the Hedge. That movie effectively points out that our species just doesn't have the built-in ability to say "enough is enough" about food.

So, we have stone age bodies with stone age appetites. But we live in a world where - particularly for adults - movement is rare and food is abundant. Basically that means we have two options:

• Get fat.

  Obviously few in the modern world want to look fat. But its much worse than that. List off the most prevalent modern health problems: heart disease, cancer, Type 2 diabetes, hypertension, low back and other joint pain, arthritis, and other inflammatory diseases. The obese are at higher risk of suffering from all of these problems. These diseases rob us of our youth and then kill us. This is not an acceptable option for a Speculist who wants to "live to see it."

• Or, make a plan.

  Not an easy thing to do. Modern life is hectic. We engage our minds to do certain tasks that we have to do (our jobs, family... maybe a hobby) - the rest is autopilot. The tendency is to work on improving our health only when there is a crisis.

  Well, I had my crisis recently and it woke me up. In the last five weeks I've lost about 15 pounds of fat while gaining several pounds of muscle. Some experts may say that it's not possible to lose fat while gaining muscle. I disagree.

Before you start do as I did and consult your doctor and ask whether you can handle vigorous weight training and a low carb diet.

If you can, then I recommend two books: The TNT Diet and Body-for-Life. Both books present complete plans for improving your health with both diet and exercise. Both work. For my personal plan I've adapted elements of both. (and here are the websites for TNT and Body-for-Life).

The TNT plan is a low carb diet with weight training - cardio training is optional. The Body-for-Life diet is less low carb than controlled carb with weight training and cardio training. I'm basically using the diet recommendations of TNT coupled with the very useful Body-for-Life workout forms (which are available free here).

Even though the forms are free, I still recommend buying the Body-for-Life book. It's well thought out, easy for a lay person to understand, the exercises are pictured, and the book is inspirational. A good portion of the book is dedicated to the stories of people who've changed their lives. Testimonials may seem like fluff, but on days when you're tired and would like to skip the gym, it helps to read the stories and see the the pictures of normal people who've made fantastic improvements through dedication.

The TNT Diet is the more scholarly of the two books. Men'sHealth Magazine is behind this project, and its obvious that a lot of time and effort went into it. They cite study after study explaining their recommendations. The authors also lean heavily on their own considerable experience.

As with Body-for-Life, you will want to buy the TNT Diet book. There's a huge amount of important detail that I can't cover in a blog post. But here, briefly, is my understanding (gained mostly from the TNT book) of why low carb dieting coupled with heavy weight training is the best method for getting back in shape.

Losing (and gaining) fat is mostly a matter of controlling the intake and usage of sugar within the body. There are three important reservoirs of sugar in the human body: our bloodstream, our muscles, and our fat stores. When we eat carbohydrates like sugar and starch (starch is quickly converted to sugar), the sugar moves quickly from our digestive system to the bloodstream. It is critical that the sugar in our bloodstream not get too high or fall too low. So, the body carefully monitors and controls blood sugar with the hormone insulin. When sugar creeps up above the norm, insulin preferentially moves sugar to the second reservoir - our muscles.

It is easy to understand why insulin prefers putting sugar in our muscles over putting sugar in fat. We were made for constant action. Energized muscles were more important to our stone age ancestors than even the vital fat stores.

Muscles keep sugar on hand in a form called glycogen. But the muscle's ability to absorb sugar is strictly limited. Once the muscles have a full glycogen load - that's it. The second reservoir is full. The body must rid excess sugar from the bloodstream another way.

Insulin's "Plan B" is to move excess sugar to the liver for conversion to a fat called triglyceride. If you are over 35 you've no doubt heard of triglycerides. When you have a blood test this is one of the big things the doctor looks for - and with good reason. Excess triglyceride in the blood is a big risk factor for heart disease and other nasty problems.

Triglyceride in the bloodstream is eventually deposited in your fat stores - your gut and your butt. Unlike the bloodstream and muscle reservoirs, there isn't a hard and fast limit on how much sugar you can store in fat. Its usually possible to get fatter.

So, we Americans are fat for two reasons:

• We're inactive. We don't use the glycogen that's sitting in our muscles, so when we eat carbohydrates insulin must always respond with "Plan B" - sugar goes to the liver to make fat.

• Plus we eat way too much carbohydrate.

The TNT plan addresses both issues.

First, you will workout your entire body three times a week... hard. The object, primarily, is to deplete your glycogen load. Then, when you eat carbohydrates insulin can go with "Plan A" and fill your muscles.

You get two other benefits from weight training. Growing your muscles increases your glycogen capacity. More sugar can be stored in muscles rather than fat. Second (and more importantly), the body gets a 36 hour metabolic benefit from heavy resistance training. When you workout to the point of being sore, you cause beneficial micro-trauma to your muscle tissues. The body repairs these sore and worn-out muscle fibers stronger than before (this is what makes your muscle stronger). But this takes time - about 36 hours - and is metabolically expensive. Repair and recuperation takes energy - which requires sugar.

You ever hear someone complain about having a low metabolism? Working out with weights - and forcing the body to regularly make these repairs - is what they can do to change that.

You don't get a similar metabolic boast from cardio exercise. You burn energy while doing cardio and its proven to be very valuable for your heart. But, when you are done and your heart rate returns to normal, that's it. Very little recuperation is needed and, therefore, you don't get the 36 hour benefit you get from heavy weights. That's why the TNT diet doesn't really emphasize cardio training. But I'm following the Body-for-Life recommendation - a brief but intense 20 minute cardio session three days a week on days I'm not lifting weights. For me that's Tuesday, Thursday, and Saturday. I'm doing this less for the weight it will help me lose (which is minimal), and more for heart-health.

Second, the TNT plan includes carbohydrate restriction. You might be wondering "if I'm not eating carbohydrate, how will I have energy for those tough workouts and the recovery?" You'll have the energy because you'll be forcing your stone age body to do a thing it hates - burn its precious fat stores. That's right, you'll be reclaiming the sugar that was stored as fat for use in your muscles. That's a good deal.

That's basically how it works. The TNT plan includes some weird exercises and its workout forms are not as user-friendly as the Body-for-Life forms. So, I've stuck with Body-for-Life on the exercise part of my plan with one major concession to TNT. TNT advocates whole body workouts 3 times a week in order to dump glycogen from all muscles as often as possible. Body-for-Life splits the upper and lower body. With Body-for-Life you would do upper body work on Monday and Friday with lower body work on Wednesday. The next week you reverse it. Lower body on Monday and Friday, upper body on Wednesday.

This means that half your body goes unworked from Friday to the following Wednesday - 120 hours. This is long past the needed 36 hour recovery time. So, I'm using the Body-for-Life forms, but I'm adding a single set of 12 repetitions with heavy weight for each major muscle group in the other half of the body. This turns the 45 minute Body-for-Life workout into a 1 hour workout. I still emphasize upper or lower on a particular day, but the whole body gets worked Monday, Wednesday, and Friday.

One final point. When you diet without weight training you lose weight both in fat and muscle. You can lose up to 3 pounds of muscle for every 10 pounds lost with dieting. This means that after the excess fat is lost with a successful diet, you will probably be weak and gaunt from muscle loss. I know because that's what happened to me with low carb dieting (without exercise) a few years back. I'm sure that's not a result that most people are hoping to achieve with their dieting efforts.

With low carb plus weight training you can actually build muscle while losing fat. The result: buff beats gaunt any day.