As a child growing up in the '70s, I remember seeing a TV commercial  for a diet that promised you could "Eat all you want and still lose weight!"  I'm not sure what diet it was (back then it seemed like new diets came out every week), but to me this sounded like a great plan if I ever needed it in the future, as I was a skinny child with a huge appetite. 

A typical eating day for me as a child:
Breakfast: Large bowl of granola cereal with a few tablespoons of Nestle's Quik AND a few tablespoons of sugar plus  milk, with additional milk to drink. Or four donuts or a large stack of pancakes with butter and syrup on the weekends.
Lunch: Peanut butter and jelly sandwich, Twinkie or cookies,  chocolate milk
Snack: Cookies and  milk
Dinner: Large portion of meat or poultry, potatoes/rice/noodles, salad,  chocolate milk,  cookies or pie         
Snack: Cookies or ice cream

What a sweet tooth! I don't even want to think about counting up all those carbs and calories.

In addition, I remember being hungry a lot and often digging into my school lunchbox around 10:30 a.m. to sneak a few cookies or chips.  Somehow my body was able to  use all those calories as energy (and I wasn't even particularly athletic)  instead of storing them as fat...

...until shortly after I hit puberty at age 12.  All of a sudden, I began to put on weight. I was eating the same way I always had, but now I was growing wider as well as taller. In a panic, I began crash dieting to try to get my weight under control. But I could only limit myself to 500 calories per day for short periods of time, so I would inevitably binge. By the time I turned 14, I would hit my all-time-high weight of 160 pounds.

I'm 5'8", and although that may not sound like much, I'm pretty small framed and not very muscular, so it was a lot for me. Worse than that, though, was my dysfunctional relationship with food. For the rest of my teen years, I fluctuated between 130-160 pounds, alternately starving and eating the same way I had for the first 12 years of my life. 

Around age 18 I started reading about the importance of health and decided I really wanted to start eating better once and for all. I also realized that feeling good and being happy with the way I looked was more important to me than consuming junk food and  overeating.  I began to think of food as nourishment and made sure I consciously enjoyed every morsel rather than overindulging or depriving myself. I think some hormonal changes and brain chemistry shifts  occurred around this age as well. But by changing the way I thought about food and walking at least 30 minutes daily, I lost the weight. I've remained between 125-130 pounds consistently for more than 25 years.

What have I been eating all this time to keep my weight where I want it?  My approach has varied from simply reducing portion sizes of everything except vegetables to veganism (for ethical reasons) to pescatarian (eating eggs, dairy, and  fish but no meat) to, most recently, low carb.  Always lots of veggies, though, regardless of whatever else I was eating. With the exception of my vegan phase, I usually felt satisifed after meals and rarely hungry in between unless I went too long without eating.  I became more and more interested in health, striving to make each meal as nutrient-dense as possible.

                             
By the time I became a dietitian in 2008, a typical day looked something like this:
Breakfast: 6 oz  nonfat yogurt with 1 cup berries and 1 Tbsp walnuts, 1 slice whole wheat toast with peanut butter, coffee with sugar sub  
Lunch: peanut butter OR tuna salad sandwich made with light mayo on whole wheat bread, 2-3 cups salad with olive oil vinaigrette, a piece of fruit,  and diet iced tea
Dinner: 4 oz fish/tofu/beans; 1 cup brown rice/potatoes/whole grain pasta; 2-3 cups nonstarchy veggies, 2 Tbsp  olive oil ,  6 oz nonfat yogurt , 1 cup fruit,  nuts, water

My weight stayed the same, and I was happy eating this way and continued doing so until  February of 2011, when I found out that my  blood sugar was spiking after meals and I had  mild hypothyroidsim  to boot. Given soy's negative effect on thyroid function and the impact of carbohydrates on blood glucose, I realized I needed to make some dietary changes in order to address both issues.  I stopped eating soy, cut my carb intake gradually until I reached 60-80 grams per day, added humanely raised chicken and grass-fed beef, and continued eating lots of nonstarchy veggies.  Doing my own research online and in books prompted me to make additional changes like eating  whole eggs, using butter instead of margarine, and getting rid of sugar substitutes. My blood sugar improved, and I felt full and satisfied.  I've been eating this way for several months and plan to continue for the foreseable future.

My typical day now looks like this:
Breakfast: 4 oz sardines/salmon OR 2 eggs with 2 cups kale or spinach cooked in 1 tsp coconut oil; 1/2 cup fruit OR 1/2 cup sweet potato, 2 Brazil nuts,  green tea
Lunch:  Usually leftovers from dinner (same amount)  OR 1 cup plain Greek yogurt, 2 cups chopped raw vegetables, 4 olives, and 3 Tbsp guacamole;  1/2 cup fruit, 1 Tbsp almond butter with small square of dark chocolate, water
Dinner: 4-6 oz beef/fish/poultry, 2-3 cups raw or cooked nonstarchy vegetables with 1 Tbsp olive oil, 1/2  cup fruit, 1 Tbsp almond butter, water

Despite cutting carbs considerably, I didn't lose weight, nor did I want to. Unlike many who start a low-carbohydrate diet, my caloric intake didn't decrease. Restricting carbs often results in an intial large loss of water (proportional to the size of the individual) in addition to fat.  Those who eat low carb tend to automatically eat fewer calories since ketones reduce appetite and fat and protein are more filling and satiating than carbohydrates. In addition, lower insulin levels that occur with carbohydrate restriction antagonize fat gain. I'm definitely less hungry between meals, which I attribute in part to more stable blood sugar levels. But my caloric intake remains about  the same, 1500-1800 per day, sometimes a little more.

I follow a low-carbohydrate diet because it allows me to prevent blood glucose spikes, thereby decreasing my risk of progressing to diabetes. I think it's by far the best way to manage all types of blood sugar issues. I also believe the same diet can be extremely helpful for weight loss and maintenance. But I don't think it should be an "Eat as much protein and fat as you want as long as your carbs are low" plan.  I'm fairly confident that if I were to significantly increase my portion sizes of meat and add large amounts of fat to everything I eat, I would gain weight. For instance, coconut oil may contain medium-chain fatty acids that are easily burned for energy, but it also has 120 calories per tablespoon that will be stored instead if caloric needs have already been met.  Eating 16 oz of beef provides 
900-1300 calories depending on the fat content, and about 110 grams of protein.  In the liver, a portion of that protein will be converted to glucose,  which will be stored as fat unless needed for immediate use.

Below are suggested menus from Dr. Richard K. Bernstein  and Mark Sisson:

Dr. Berstein's own menu from Diabetes Solution:
Breakfast: 2 oz Nova Scotia salmon, 1 ounce cream cheese, 2 Bran-a-Crisp crackers
Lunch: Leek soup, 3 1/2 ounces hamburger meat
Dinner: 1 medium artichoke with melted butter, 4 1/2 ounces meat, fish or poultry
Total calories (estimated):   1200

Mark Sisson's plan for Ken Korg in The Primal Blueprint:
Breakfast: "Primal Omelet": 3-egg omelet with 1 ounce cream and 1 tablespoon cheddar cheese, chopped mushrooms, red onions, and red peppers (1/4 cup each); 2 ounces avocado, 2 Tbsp fresh salsa, 1/4 cup fresh blueberries, black coffee
Lunch:  "Primal Salad": 2 cups salad greens, 2 ounces each chopped onions, carrots, jicama, red peppers and cherry tomatoes, 3 ounces chicken, 1/3 ounce sesame seeds, 1/2 ounce walnuts, 2 Tbsp oil-based dressing
Dinner: 6 ounces broiled salmon, 6 ounces each steamed zucchini and asparagus with 1 Tbsp butter, 5-ounce glass red wine
Snacks: hard boiled egg, 17 macadamia nuts, 4-inch strip venison jerky
Total calories (as listed in book):  2045

Notice the portion sizes.  Dr. Bernstein is older, of short stature, and doesn't exercise much, so his caloric intake is too low for me and presumably most people reading this (I think he probably eats a little more than this, at least on occasion).  On the other hand, Mark Sisson is pretty athletic (He probably consumes around 3000 calories a day),  but he advises people to consume only 0.7-1.0 gram of protein per pound of lean body mass. He recommends 104 grams of protein for Ken based on 148 lbs of lean mass. The protein and fat portions at each meal are quite modest and appropriate. 

Do I believe that those who follow a low-carb diet develop a  "metabolic advantage" that allows them to eat large quantities of high-calorie but low-carb foods  and still lose weight, aside from the initial loss?  I'm not discounting it 100%, but at this point I don't see the empirical evidence for it.  Just a couple of examples that argue against this idea can be found on  the Bye Bye Carbs and Mark's Daily Apple websites. I've read many comments on other sites about people who stopped losing weight on low-carb plans. Calories do count, regardless of the macronutrients in question. 

I think many different ways of eating can result in weight loss and long-term maintenance. I know several vegans; some have been slim for years, while others remain heavy. There are some prominent low-carbers who are overweight or obese, while many others have found that carbohydrate restriction is the only way they can maintain their ideal weight. My mother's weight history and pictures are very similar to mine, although she has maintained her 30-lb loss even longer, excluding pregnancy, simply by keeping portion sizes in check and limiting sweets. 

As someone who has lost weight and been able to keep it off,  I feel that you can't really "eat all you want and still lose weight" -- unless "all you want" is fewer calories than you actually need. Unfortunately, it takes some discipline along with choosing whatever eating plan works for you. For everyone who is losing weight or maintaining weight lost on a low-carb plan, I couldn't be happier -- we need more success stories! But for those who are finding sustainable weight loss elusive, I recommend taking a look at the amount of calories you're consuming rather than adding another tablespoon of coconut oil to your meal.