Someone really ought to figure out why it's so easy for some people to maintain a healthy weight, and so difficult for others
I've given a lot of thought and research to this question over the past couple years and I'm pretty confident that John Walker's explanation (as set forth in The Hacker's Diet) is the primary reason. Specifically, your body has a natural feedback system -- if you do not eat enough, you have the urge to eat more. If you eat too much, you have the urge to eat less. People who naturally gain weight have feedback systems which do not function properly in the modern environment. They don't get strong enough signals to stop eating so they have a tendency to over-eat every day and they slowly gain weight.
In your husband's case, when he pigs out, he has a natural urge to eat less which results in him maintaining balance. So he can eat whatever he feels like without gaining weight.
Eliezer and I have put together this first pass at a recipe for DIY ketogenic soylent--or, as he prefers to call it,The Mildly Surprising Super Ketonic Dietary Replacement Weight-Loss Fluid - It's Not Food, It's Dietary Replacement Fluid!(R) (I am not in full support of this particular preference...)
So let's play Make a Prediction. It seems more likely to be useful than "free associate with why soylent/ketosis is awesome/stupid". Imagine that Eliezer has around a week's worth of infinite willpower (that can only be spent on resisting food cravings etc.). Further, imagine your crystal ball shows you that a month from now, he hasn't lost any weight. What does it tell you about why?
Ketogenic Soylent v1
1 scoop Gold Standard whey protein
1½ tsp calcium citrate powder
1 tsp creatine
½ cup+1 tbsp ground flax
4 tbsp Hershey’s Special Dark cocoa powder
2 tbsp lecithin
⅗ tsp potassium citrate on day 1, increase gradually to 1tsp by day 5, don’t exceed 2tsp
½ tsp iodized salt
⅛ packet Emergen-C
⅕ cup olive oil
1¼ tbsp flaxseed oil
⅗ cup MCT oil
Sucralose to taste
We might need to add water to get the consistency right.
Pills
1 capsule vitamin D supplement/day
2 Opti-Women vitamins per day
1 capsule MSM
Totals Per Day
Calories: 1866
Fats: 193.5g
Carbs: 12g
Protein: 54g
Fiber: 25g
Biotin 250mcg
Calcium 150mg
Chromium 120mcg
Copper 2mg
Folic Acid 600mcg
Iodine 200mcg
Iron 18mg
Magnesium 75mg
Manganese 5mg
Methyl-Sulfonyl-Methane 1000mg
Molybdenum 70mcg
Niacin 20mg
Pantothenic Acid 20mg
Potassium: 5g
Riboflavin 20mg
Selenium 70mcg
Sodium 3g
Thiamin 20mg
Vitamin A 5,000IU
Vitamin B6 20mg
Vitamin B12 100mcg
Vitamin C 250mg
Vitamin D 1600IU
Vitamin E 100IU
Vitamin K 80mcg
Zinc 15mg