Can the human body switch to a "starvation" mode?

Question

I've heard on multiple occasions from less than credible sources that dieting by eating less (usually implemented by skipping meals, not smaller meals) will cause the human body to enter some sort of "starvation" mode where it hoards calories because it thinks it might not get more calories in the future. Most people also mention it can cause an increase in weight. The people I hear it from say it like it's common sense and widely believed so they never have anything to support it. "That's just the way it is" they claim.

There's a lot of variables at play here and I'm no biologist or nutritionist but I'm interested in knowing if there's any research backing this claim. Is there a starvation mode that your body can fall in to? If so, what triggers it? Has there been any research into how the body reacts to fewer feeding times and overall less calories? What are the long term (6+ months?) affects of a diet like this if a person can still ensure proper nutrient intake aside from calories?

I'm not talking about extreme fasting or starvation diets. I'm thinking diets with 1000-1500 calories a day in a consistent fashion (taking in roughly the same amount of calories at roughly the same time of day). Although I would be interested in studies or knowledge outside of these limits.

Even if we ignore starvation as a dietary method, if a person stopped eating would there be a distinct difference in how their body processes fats or expends energy in the starvation process?

Answer 1

I'm going to approach this from a strictly physiological stance. With all of these diets available it can be very confusing. Ketogenic, Atkins, Paleo, etc.. The body uses glucose for energy. Many, many, many compounds we eat are complex and large. Our body utilizes each of these in a very efficient manner.

Short Answer: YES! We have a "starvation" mode, but it's called gluconeogenesis. When we stop eating, we use glycogen (huge chains of linked glucose) for energy. When glycogen is depleted we begin to indiscriminately (situationally) using proteins and lipids for energy. There are many molecules that contain the secret ingredient (carbon) that can be manipulated by enzymes to create the ever-important six-ringed molecule we need to live.

To be honest 1000-1500 is not really starving; depending on sex, activity level, BMR, etc. Depending on the makeup of those 1500 calories, the body will utilize the path of least resistance. If you take in ~600 calories from carbohydrates, you will use them. If you take in 100 calories from carbohydrates, you will use those and glycogen supplementation.

There are signaling pathways in the body that are purposefully designed to ensure a constant, steady supply of glucose in our bloodstream. Our brain alone uses about ~120g of carbohydrate per day. As long as we have carbon molecules available, our body will take them and rearrange them to make "fuel". That's why you see people lose weight or muscle mass -- because the body will literally eat muscle tissue to survive.

Answer 2

I found a pretty good article here that discusses starvation mode pretty intelligently and cites valid research. In short, yes, starvation mode exists, but it will not cause a person to gain weight, at least while being starved. However, once a person who has triggered their starvation reflex is allowed to eat normally and at will, they lose their sense of being satiated and not only gain their original weight back but put on much more. It takes several days of starvation (not just one or two) to trigger starvation mode. On average, people in the Minnesota study saw an average drop of metabolic rate of about 40%. This is why starvation diets are generally not recommended.

Answer 3

Starvation/fasting does lead to physiological changes that recently are being investigated as an adjunct to chemotherapy for cancer control.

The dietary recommendation for cancer patients receiving chemotherapy, as described by the American Cancer Society, is to increase calorie and protein intake. Yet, in simple organisms, mice, and humans, fasting--no calorie intake--induces a wide range of changes associated with cellular protection, which would be difficult to achieve even with a cocktail of potent drugs. In mammals, the protective effect of fasting is mediated, in part, by an over 50% reduction in glucose and insulin-like growth factor 1 (IGF-I) levels. Because proto-oncogenes function as key negative regulators of the protective changes induced by fasting, cells expressing oncogenes, and therefore the great majority of cancer cells, should not respond to the protective signals generated by fasting, promoting the differential protection (differential stress resistance) of normal and cancer cells. Preliminary reports indicate that fasting for up to 5 days followed by a normal diet, may also protect patients against chemotherapy without causing chronic weight loss. By contrast, the long-term 20 to 40% restriction in calorie intake (dietary restriction, DR), whose effects on cancer progression have been studied extensively for decades, requires weeks-months to be effective, causes much more modest changes in glucose and/or IGF-I levels, and promotes chronic weight loss in both rodents and humans. In this study, we review the basic as well as clinical studies on fasting, cellular protection and chemotherapy resistance, and compare them to those on DR and cancer treatment. Although additional pre-clinical and clinical studies are necessary, fasting has the potential to be translated into effective clinical interventions for the protection of patients and the improvement of therapeutic index.

http://www.ncbi.nlm.nih.gov/pubmed/21516129