Submitted By: Mikael Jansson, Sweden

In your book NHE, you present some studies that points to a reduced conversion of T4 to T3 (probably through inhibition of diodenase?) on a strict low-carb diet. You also point to the importance of doing carb-loading twice weekly.

Now, I've been doing low-carb since September 2008, and while I have suspected low thyroid function for a couple of years now (typical hypothyroidism symptoms), I gave notice to it after doing a health check-up a month into the low-carb world. It did show some bad values, but nothing out of the roof. (slightly low T4, slightly elevated TSH).

Anyway. The list of studies pointing out to reduced thyroid function show that it will happen during calorie restriction, or the animal studies after trauma (which I assume is supposed to emulate high-intensity workout?).

From the titles alone I can't find evidence to the fact that reducing carbs to approx 20g/day, *ad libitum*, will lower thyroid function. Alas, I don't have access to the full text of the studies so I cannot verify it myself.

So, what I want to know is

a) how you came up with the twice-weekly carb loading, and
b) which studies point to reduced function.

Obviously, I don't want to know out of spite -- I mostly care for my (and well, others'!) health as there's a surge in low-carb interest here in Sweden. Well, except for maybe this passage, regarding carb loading, that I feel need clarification:

"Make sure you are not loading up on sugar. The carb-load should consist of mainly of pasta, potatoes, bread or other starches. Also, eat as much fruit as you desire during this phase."

Well, sugar/sackarose is half fructose, half glucose. You say that should be avoided. But you recommend fruit, which has the same composition as sugar/sackarose. So which is it?

Thank you for your letter. I will get back to you with a response to your question re: thyroid and diet. You also asked for a clarification on carb-loading. . .

My current opinion is that someone who is doing a lot of exercise, especially one trying to build muscle or training for athletic competition, should stick with  pasta, potatoes, bread, rice and beans, etc. and consume no fruit or small amounts when carb-loading. This will allow for a higher volume carb-load (more carbs/calories) and greater muscle glycogen repletion. For someone primarily interested in fat loss whose exercise routine consists of walking a few miles per week or light exercise at home, I would recommend that this person consume whatever they want at the carb-load by way of pasta, bread, potatoes, rice and beans, etc., or fresh fruit. Fruit contains many beneficial nutrients and the amount of fructose contained in fruit is relatively low due to high water/fiber content (compared with calorie-dense foods high in refined sugar). Accordingly, I would not advise someone in this category to restrict carb-load fruit consumption if that is what they wish to eat, as opposed to individuals whose objectives include muscle building or maximizing training capacity and performance. This illustrates the importance of prioritizing one’s goals before attempting to devise an optimal dietary plan - for which I’ve gained a greater appreciation since I wrote NHE based on my experiences and continuing research. Another difference is that an athlete or bodybuilder for whom fat loss is not a priority can more liberally partake of desserts as part of the carb-load. Where fat loss is a secondary or non-consideration, my dietary recommendations would differ significantly from recommendations applicable to one whose main purpose for dieting is fat loss.

¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

I will get back to you with some thoughts on carb-load frequency. You also asked about thyroid function in relation to diet. . .

Dietary quantity and composition affect hormone levels and appetite. For example, overeating increases and undereating decreases thyroid hormone levels. By this and other metabolic adjustments, the body seeks to avoid becoming "too fat" or "too lean" in reference to your genetically determined fat setpoint. Any diet that curtails appetite or food availability, consequently diminishing caloric intake, can cause a decline in T3 levels.

Diets high in processed starch and refined sugar reinforce carbohydrate cravings, whereas low carbohydrate diets have the opposite effect after you get beyond the first few days. For overeaters to normalize appetite and drop excess weight, eliminating carb cravings is often necessary. But where more generalized appetite suppression occurs and T3 levels fall to counteract a substantial reduction in caloric intake, the rate of fat loss slows and the probability of eventual weight regain increases. Continuously restrictive low-carb diets in which the fat/protein ratio is high are linked to this outcome. This is one reason why such diets tend to be more useful over a period of weeks to redirect metabolism away from reliance on glucose and mitigate false hunger associated with overconsumption of processed high-carb foods, than as a long-range solution to the problem of obesity.

The relationship between dietary content and thyroid is less well understood. At the micronutrient level, adequate intake of several minerals such as iodine is required for optimal thyroid function. At the macronutrient level, a limited number of studies have addressed the issue of direct carbohydrate-thyroid interaction. These studies furnish some support for the theory that very low carbohydrate diets can inhibit thyroid hormone production independent of the amount of calories consumed.

Where reducing carbohydrate results in healthier, more nutritious eating and more stable blood sugar levels (as with those consuming carbs in excess) the net effect may be to enhance thyroid output. Extreme blood sugar fluctuations can prompt adrenal release of cortisol to raise blood sugar levels. This hormone's adverse effect on both insulin sensitivity and thyroid activity, may help explain how stress and poor diet interact to cause unnatural weight gain by elevating both the urge to eat and the propensity to store fat.

Clin Endocrinol Metab 1976;42:197
Metabolism 1986;35:394
Am J Clin Nutr 1982;35:24

¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

The carbohydrate controversy is reducible to whether "a calorie is a calorie" in relation to body composition. In other words, does equivalency exist among a calorie of fat, carbohydrate, and protein? Is one equal to the other equal to the other? If the body processed energy the same way a machine does, maybe the answer would be yes. One flawed assumption underlying the misconception that a calorie equals a calorie regardless of source is that all incoming calories are either used or stored as energy. Calories also can be dissipated as heat or used to build or restore tissue, depending in part upon hormonal state. Qualitatively there's a difference as well, attributable the varying rates at which energy can be transferred from different substrates. For example, because carbs are a quicker energy source than fats, a given amount of carbohydrate calories conveys more useable energy during a 100-meter race. All other factors being equal, a carb-loaded runner will win every time against a carb-depleted one. The more challenging question is who wins when two athletes have maximum glycogen levels on the day of competition, but one carb-loaded from a low-glycogen state while the other carb-loaded continuously through all phases of training. For average folks this is an unnecessary theoretical exercise whereas for athletes the matter has added significance.

The optimal amount of carbohydrate varies based on individualized considerations. These include objectives (less fat, more muscle, better performance) and body size (specifically muscle mass which determines glycogen storage capacity). Based on practical experience twice weekly carb-loading is ideal for most purposes, and my recommendation is to modify magnitude rather than frequency to account for objectives and body size. The difference between three large high-carb meals and two medium-sized or one large meal can amount to several hundred grams of carbohydrate.

Given the escalating incidence of obesity in many countries, there is every reason to explore new solutions. Depending on the cause of obesity in a particular instance, and often there are multiple overlapping causes, a strict low-carb diet sometimes cures the condition. More often the person quits the diet and regains the weight. Compliance and long-term success rates are greater where carb limits are more permissive, one reason to advise occasional higher carb meals. For those eating a healthy balanced diet and not overweight, lowering carbohydrate intake may be only marginally beneficial. A lean individual consuming very few processed foods may have to decrease consumption of fruits and vegetables to conform to a strict low-carb diet. This is an example of circumstances in which restricting carbs could work against you. For most Americans, reducing carbohydrate intake leads to a more natural healthier diet and higher nutrient/calorie ratio.

Low-carb eating requires some planning and innovation to make the diet sustainable and keep down expense. High-carb foods generally cost less to produce. They are also more readily available, even as food manufacturers have diversified their product lines to serve better the health-minded public and low-carb consumers. Low-carb recipe books and support groups are increasingly accessible, and represent key resources for successfully maintaining a low-carb lifestyle.