As noted previously , when subjects consumed the LoBAG30 diet for 5 weeks, the fasting glucose concentration was decreased, as was the net integrated glucose area response. In the present study, the subjects ingested foods from the same 6-day rotating menu, but over 10 weeks. The area responses using the fasting value as baseline were the same at 5 and 10 weeks. These similar postprandial responses can be explained by the fact that the meals consumed at 5 and 10 weeks were very similar.
However, the fasting glucose concentration had decreased at 10 weeks when compared to the mean 5 week value. Consequently, the total 24 hr integrated glucose area response was significantly lower at 10 weeks compared to 5 weeks. Thus, a time-dependent metabolic adaptation had occurred in these subjects, but this was exclusively due to a decrease in the overnight fasting glucose concentration. Whether the fasting concentration would continue to decrease if the diet was continued for a longer period of time remains to be determined.
As indicated in the introduction, our previous studies were 5 weeks in duration because this represents the t1/2 for the glycated hemoglobin to reflect a decrease in glucose concentration to a new steady state if the change is instantaneous or very rapid . By extending the study to 10 weeks, we were interested in determining whether the %tGHb would indeed continue to decrease as expected. We also were interested in determining if the rate of change between weeks 5 and 10 would be greater than expected based on the reported t1/2. If so, this would support the concept that a metabolic change was taking place.
If the change in glycated hemoglobin is an exponential function, the expected decrease at 10 weeks would be 1/2 of that observed at 5 weeks. The %tGHb decreased from 10.0 at baseline to 8.7 at 5 weeks (Δ = 1.3). Thus the predicted value at 10 weeks would be 1/2 of 1.3 or a further decrease of 0.65, i.e. the 10-week value would be 8.05. The actual value was 7.5. Thus, the actual difference at 10 weeks (8.7 - 7.5 = 1.2) is 85% greater than the predicted decrease of 0.65 based on the t1/2. Indeed, the rate of change is linear from baseline to week 10 (r2 = 0.99). The reason the decrease in glycated hemoglobin was linear and lower than predicted, we suspect, is because there is a continuing metabolic adaptation in these subjects as a result of a continuing decrease in fasting glucose concentration over the period of the study. This assumes that the change in diet resulted in a very rapid decrease in post-prandial glucoses, but a slower change in the fasting glucose. The latter is likely to be due to a decrease in glucose appearance rate. This is a reasonable expectation, but has not been tested.
Of interest, one of the subjects chose to continue on a similar diet on his own. After 6 months, the %tGHb was continuing to decrease. The initial glycated hemoglobin was 11.2%; at 6 months it was 6.5%, and the final after one year was 6.0%. This occurred without weight loss.
In the present study, the fasting serum insulin concentration changed very little from baseline to week 5, confirming our previous report . The fasting concentration at 10 weeks also was similar. In addition, there was little difference in the postprandial insulin responses at baseline, 5 weeks, and 10 weeks. This is in spite of the fact that the glucose concentrations decreased. Our data suggest that insulin is more efficient in inhibiting glucose production and/or accelerating glucose disposition. The mechanism remains to be determined.
Dietary protein is known to stimulate an increase in circulating glucagon concentration acutely . Of some interest, the glucagon concentration increased progressively throughout the day and did not return to the baseline fasting value until the early morning hours. This pattern is similar to that noted earlier in normal young subjects . However, the return was somewhat delayed when the subjects were consuming the LoBAG30 diet. We did observe a progressive increase in postprandial glucagon area responses with the LoBAG30 diet . However, the increase did not reach statistical significance in either study. In addition, even though the protein content was increased in the LoBAG30 diet compared to the control diet, the total glucagon areas all remained very similar. The metabolic consequences, if any, of the sustained total circulating glucagon concentration is unclear. In any regard, it did not prevent a decrease in circulating glucose. It should be added that we have provided evidence that modest, physiologic elevations in glucagon concentration have little effect on net hepatic glucose output .
The fasting triacylglycerol concentration at 5 weeks was 25% less than at baseline. This is modestly less than the 40% decrease we reported previously , but the subjects in the current study started with a lower triacylglycerol concentration at baseline (~159 vs. 190 mg/dl). After 10 weeks on the LoBAG30 diet, the fasting concentration decreased an additional 12%, again indicating that a metabolic adaptation is occurring in these subjects. Dietary carbohydrate is known to raise the morning fasting triacylglycerol concentration . Our current data, and that reported previously, demonstrate that lowering the carbohydrate content also lowers the 24 hour integrated triacylglycerol concentration.
The NEFA concentration decreased when the insulin concentration increased, as expected. However, a threshold insulin concentration above which the NEFA decreased is not apparent. The general response is that when the insulin concentration increased after meals, the NEFA concentration decreased. As the insulin concentration decreased after the evening meal, the NEFA concentration increased (Figure 7).
In spite of the increased fat content of the LoBAG 30 diet, the lipid profile did not change.
An increase in fasting IGF-1 concentration between baseline and week 5 confirmed our previous data . Indeed, we previously reported that increasing the protein content of the diet from 15% to 30% of total food energy resulted in a 35% increase in IGF-1 concentration, regardless of the carbohydrate:fat ratio of the diet [21, 22, 16]. In the present study the increase at 5 weeks was 43%. Interestingly, the concentration increased another 7% by week 10, but the rate of increase was slowing, suggesting a near maximal effect of the diet had occurred at this time.
The serum albumin was increased at 5 weeks as we noted previously . However, in the present study, it returned toward baseline at 10 weeks. The difference between the increase at week 5 and the decrease at week 10 approached statistical significance (P = 0.02). There was no reason to suspect a decrease in plasma volume at 5 weeks, or an increase in plasma volume at 10 weeks to explain the changes in albumin concentration. Thus, the return toward baseline could represent another metabolic adaptation to the 30% protein diet.
When the subjects ingested the LoBAG30 diet for 5 weeks, the 24-hour urinary urea nitrogen increased, as expected. The mean increase was only ~70% and not 2-fold, as would be expected with a doubling of the protein content of the diet. Interestingly, following an additional 5 weeks on the diet, the urinary urea nitrogen excretion decreased very modestly, but this was statistically significant.
The protein balance, based on the urinary excretion of urea, was positive following 5 weeks on the LoBAG30 diet, and was similar to that in our previous study (30% vs 32%). At 10 weeks it was slightly less (25%) (Figure 13). The fate of the nitrogen not excreted in the urine remains unknown, but is consistent. The data indicate that nitrogen was being retained and/or that it was being lost from the gastrointestinal tract in the feces. Loses from any other site would not be sufficient for it to explain the difference. We did not attempt to determine changes in body composition in this study, but an exclusive gain in protein mass also would have to be very large in order to explain the difference.
The creatinine, and uric acid excretion were similar at 5 and 10 weeks, indicating that there was no further adaptation at 10 weeks.
Over the years the concept has developed that a high-protein, "Western" diet would result in a systemic acidosis, result in calcium leaching from bone and increase calcium excretion, all ultimately leading to a decrease in bone mass (Reviewed in ). Evidence to refute this concept was published by Bonjour . In the present study the urinary pH was unchanged, as noted by others , presumably because of ingestion of fruits and vegetables, which result in an increase in pH. Also, calcium excretion was not increased. Several studies subsequently have reported that an increase in dietary protein has a salutary effect on bone [25–31]. This has been attributed to an increase in IGF-1 (reviewed in ), as noted in the present study.
There was no significant difference in the serum NEFA, AAN, uric acid, urea nitrogen, albumin, prealbumin, TSH, Total T3, free T4, B12, folate, homocysteine, creatinine, growth hormone or renin between week 5 and week 10.
The literature on the effects of low carbohydrate diets is increasing rapidly. A Medline search of "low carbohydrate diet" or "Diet, carbohydrate-restricted", limited to humans and English language, yielded 7 papers for the year 2000: 32 for 2005: 81 for 2009. For 2010, through July Week 1, the yield was 26. The majority of this literature contains studies employing weight loss. Although a recent review reported that the beneficial effects of carbohydrate restriction do not require weight loss , the references cited [34–36], other than our work, all had different experimental designs or different primary outcome measures than those reported here. Nevertheless, the review points out that data are accumulating to suggest that carbohydrate restriction, without weight loss, potentially has beneficial health effects.
Our studies, including the current 10-week study differ from most in the literature in several respects. First, we make a conscious effort to keep subjects weight stable during the study. Second, we provide all of the food, and we use common foods, not synthetic diets. Also we do not use supplements. Finally, we emphasize keeping a pre-study activity level during the entire period of the study. By including all of these parameters in the experimental design, we have been able to provide sound scientific evidence that not only is blood glucose control improved, evidenced by a decrease in total glycated hemoglobin from 10.0 to 7.5, (Δ2.5%) in the present study, but a metabolic adaptation had occurred during that time that was the result of the dietary intervention.